WO2005071411A1

WO2005071411A1 - Methods for the identification of inhibitors of adenylosuccinate synthase and cyclic nucleotide phosphodiesterase as antibiotics

Info

Publication number: WO2005071411A1
Application number: PCT/US2005/000991
Authority: WO
Inventors: Matthew M. Tanzer; Jeffrey R. Shuster; Lisbeth Hamer; Todd M. Dezwaan; Sze-Chung C. Lo; Maria Victoris Montenegro-Chamorro; Blaise A. Darveaux; Sheryl A. Frank; Ryan W. Heiniger; Sanjoy K. Mahanty; Huaqin Pan; Amy S. Covington; Rex Tarpey; Kiichi Adachi
Original assignee: Icoria, Inc.
Priority date: 2004-01-13
Filing date: 2005-01-13
Publication date: 2005-08-04

Abstract

The present inventors discovered that adenylosuccinate synthase is essential for normal fungal pathogenicity. Specifically, inhibition of adenylosuccinate synthase expression in fungi results drastically reduces pathogenicity. The present inventors also discovered that a cyclic nucleotide phosphodiesterase is essential for normal fungal pathogenicity. Specifically, inhibition of PDE2 expression in Magnaportha grisea severely reduces pathogenicity of the fungus. Thus, adenylosuccinate synthase and PDE2 are useful targets in methods for identification of antibiotics, preferably antifungals and fungicides.

Description

METHODS FOR THE IDENTIFICATION OF INHIBITORS OF ADENYLOSUCCINATE SYNTHASE AND CYCLIC NUCLEOTIDE PHOSPHODIESTERASE AS ANTIBIOTICS FIELD OF THE INVENTION The invention relates generally to methods for the identification of antibiotics, including antifungals that affect the biosynthesis of purines.

BACKGROUND OF THE INVENTION Filamentous fungi are causal agents responsible for many serious pathogenic infections of plants and animals. Since fungi are eukaryotes, and thus more similar to their host organisms than, for example bacteria, the treatment of mfections by fungi poses special risks and challenges not encountered with other types of infections. One such fungus is Mαgnαporthe grisea, the fungus that causes rice blast disease, a significant threat to food supplies worldwide. Other examples of plant pathogens of economic importance include the pathogens in the genera Agαricus, Alternαriα, Anisogrαmmα, Anthracoidea, Antrodia, Apiognomonia, Apiosporina, Armillaria, Ascochyta, Aspergillus, Bipolaris, Bjerkandera, Botryosphaeria, Botrytis, Ceratobasidium, Ceratocystis, Cercospora, Cercosporidium, Cerotelium, Cerrena, Chondrostereum, Chiyphonectria, Chrysomyxa, Cladosporium, Claviceps, Cocliliobolus, Coleosporium, Colletotrichium, Colletotrichum, Corticium, Corynespora, Cronartium, Cryphonectria, Cryptosphaeria, Cyathus, Cymadothea, Cytospora, Daedaleopsis, Diaporthe, Didymella, Diplocarpon, Diplodia, Discohainesia, Discula, Dothistroma, Drechslera, Echinodontium, Elsinoe, Endocronartium, Endothia, Entyloma, Epichloe, Erysiphe, Exobasidium, Exserohilum, Fomes, Fomitopsis, Fusarium, Gaeumannomyces, Ganoderma, Gibberella,

Gloeocercospora, Gloeophyllum, Gloeoporus, Glomerella, Gnomoniella, Guignardia, Gymnosporangium, Helminthosporium, erpotrichia, Heterobasidion, Hirschioporus, Hypodermella, Inonotus, Irpex, Kabatiella, Kabatina, Laetiporus, Laetisaria, Lasiodiplodia, Laxitextum, Leptographium, Leptosphaeria, Leptosphaerulina, Leucytospora, Linospora, Lophodermella, Lophodermium, Macrophomina, Magnaporihe, Marssonina, Melampsora, Melampsorella, Meria, Microdochium, Microsphaera, Monilinia, Monochaetia, Morchella, Mycosphaerella, Myrothecium, Nectria, Nigrospora, Ophiosphaerella, Ophiostoma, Penicillium, Perenniporia, Peridermium, Pestalotia, Phaeocryptopus, Phaeolus, Phakopsora, Phellinus, Phialophora, Phoma, Phomopsis, Phragmidium, Phyllachora, Phyllactinia, Phyllosticta, Phymatotrichopsis, Pleospora, Podosphaera, Pseudopeziza, Pseudoseptoria, Puccinia, Pucciniastrum, Pyricularia, Rhabdocline, Rhizoctonia, Rhizopus, Rhizosphaera, Rhynchosporium, Rhytisma, Schizophyllum, Schizopora, Scirrhia, Sclerotinia,

Sclerotium, Scytinostroma, Septoria, Setosphaera, Sirococcus, Spaerotheca, Sphaeropsis, Sphaerotheca, Sporisorium, Stagonospora, Stemphylium, Stenocarpella, Stereum, Taphrina, Thielaviopsis, Tilletia, Trametes, Tranzschelia, Trichoderma, Tubakia, Typhula, Uncinula, Urocystis, Uromyces, Ustilago, Valsa, Venturia, Verticillium, Xylaria, and others. Related organisms are classified in the oomycetes classification and include the genera Albugo, Aphanomyces, Bremia, Peronospora, Phytophthora, Plasmodiophora, Plasmopara, Pseudoperonospora, Pythium, Sclerophthora, and others. Oomycetes are also significant plant pathogens and are sometimes classified along with the true fungi. Human diseases that are caused by filamentous fungi include life- threatening lung and disseminated diseases, often a result of infections by Aspergillus fumigαtus. Other fungal diseases in animals are caused by fungi in the genera Fusαrium, Blαstomyces, Microsporum, Trichophyton, Epidermophyton, Candida, Histoplamsa, Pneumocystis, Cryptococcus, other Aspergilli, and others. The control of fungal diseases in plants and animals is usually mediated by chemicals that inhibit the growth, proliferation, and/or pathogenicity of the fungal organisms. To date, there are less than twenty known modes-of-action for plant protection fungicides and human antifungal compounds. A pathogenic organism has been defined as an organism that causes, or is capable of causing disease. Pathogenic organisms propagate on or in tissues and may obtain nutrients and other essential materials from their hosts. A substantial amount of work concerning filamentous fungal pathogens has been performed with the human pathogen, Aspergillus fumigatus. Shibuya et al, 27 Microb. Pathog. 123 (1999) (PubMed Identifier (PMED): 10455003) have shown that the deletion of either of two suspected pathogenicity related genes encoding an alkaline protease or a hydrophobia (rodlet), respectively, did not reduce mortality of mice infected with these mutant strains. Smith et al, 62 Infect. Hnmun. 5247 (1994) (PMID: 7960101) showed similar results with alkaline protease and the ribotoxin restrictocin; Aspergillus fumigatus strains mutated for either of these genes were fully pathogenic to mice. Reichard et al, 35 J. Med. Net. Mycol. 189 (1997) (PMID: 9229335)) showed that deletion of the suspected pathogenicity gene encoding aspergillopepsin (PEP) in Aspergillus fumigatus had no effect on mortality in a guinea pig model system, whereas Aufauvre-Brown et al, 21 Fungal. Genet. Biol. 141 (1997) (PMID: 9073488) showed no effects of a chitin synthase mutation on pathogenicity. However, not all experiments produced negative results. Ergosterol is an important membrane component found in fungal organisms. Pathogenic fungi lacking key enzymes in the ergosterol biochemical pathway might be expected to be non- pathogenic since neither the plant nor animal hosts contain this particular sterol. Many antifungal compounds that affect the ergosterol biochemical pathway have been previously described. (United States Patent Νos. 4,920,109; 4,920,111 ; 4,920,112; 4,920,113; and 4,921,844; Hewitt, H. G. Fungicides in Crop Protection Cambridge, University Press(1998)). D'Enfert et al, 64 Infect. Immun. 4401 (1996) (PMID: 8926121)) showed that an Aspergillus fumigatus strain mutated in an orotidine

5'-phosphate decarboxylase gene was entirely non-pathogenic in mice, and Brown et αl. (Brown et αl., 36 Mol. Microbiol. 1371 (2000) (PMID: 10931287)) observed a non- pathogenic result when genes involved in the synthesis of para-aminobenzoic acid were mutated. Some specific target genes have been described as having utility for the screening of inhibitors of plant pathogenic fungi. U.S. Patent No. 6,074,830 to Bacot et αl., describe the use of 3,4-dihydroxy-2-butanone 4-phosphate synthase, and U.S. Patent No. 5,976,848 to Davis et αl. describes the use of dihydroorotate dehydrogenase for potential screening purposes. There are also a number of papers that report less clear results, showing neither full pathogenicity nor non-pathogenicity of mutants. For example, Hensel et αl. (Hensel, M. et αl., 258 Mol. Gen. Genet. 553 (1998) (PMfD: 9669338)) showed only moderate effects of the deletion of the are A transcriptional activator on the pathogenicity of Aspergillus fumigatus. Therefore, it is not currently possible to determine which specific growth materials may be readily obtained by a pathogen from its host, and which materials may not.

SUMMARY OF THE INVENTION The present inventors have discovered that in vivo disruption of the. gene encoding adenylosuccinate synthase (ADE12) in Magnaporthe grisea severely reduces the pathogenicity of the fungus. Thus, the present inventors have discovered that adenylosuccinate synthase is useful as a target for the identification of antibiotics, preferably fungicides. Accordingly, the present invention provides methods for the identification of compounds that inhibit adenylosuccinate synthase expression or activity. The methods of the invention are useful for the identification of antibiotics, preferably fungicides. The present inventors have further discovered that in vivo disruption of the gene encoding a cyclic nucleotide phosphodiesterase in Magnaporthe grisea eliminates the pathogenicity of the fungus. Thus, the present inventors have discovered that the cyclic nucleotide phosphodiesterase is useful as a target for the identification of antibiotics, preferably fungicides. Accordingly, the present invention provides methods for the identification of compounds that inhibit the cyclic nucleotide phosphodiesterase expression or activity. Methods of the invention are useful for the identification of antibiotics, preferably fungicides.

BRIEF DESCRIPTION OF THE FIGURES Figure 1. Diagram of the reversible reaction catalyzed by adenylosuccinate synthase (ADE12). The enzyme catalyzes the reversible interconversion of GTP (guanosine 5'-triphosphate), IMP (inosine monophosphate), and L-aspartate to GDP (guanosine 5'-diphosphate), phosphate, and N6-(l,2-dicarboxyethyl)-AMP (adenylosuccinate). This reaction is part of the purine biosynthesis pathway. Figure 2. Digital image showing the effect of ADE12 gene disruption on Magnaporthe grisea pathogenicity using whole plant infection assays. Rice variety CO39 was inoculated with wild-type strain Guyl 1, and transposon insertion strains Kl- 10 and Kl-14. Leaf segments were imaged at seven days post-inoculation. Figure 3. Digital image showing the effect of PDE2 gene disruptions on Magnaporthe grisea pathogenicity using whole plant infection assays. Rice variety CO39 was inoculated with wild-type strain Guyl 1 and cpgmra0048001a02 transposon insertion strains Kl-7 and Kl-9. Leaf segments were imaged at five days post- inoculation.

DETAILED DESCRIPTION OF THE INVENTION Unless otherwise indicated, the following terms are intended to have the following meanings in interpreting the present invention. As used herein, the term "ADE12" means a gene encoding adenylosuccinate synthase activity, referring to an enzyme that catalyses the reversible interconversion of GTP, IMP, and L-aspartate to GDP, phosphate, and N6-(l,2-dicarboxyethyl)-AMP. ADE12 or adenylosuccinate synthase is also used used herein to refer to the adenylosuccinate synthase polypeptide. By "fungal ADE12" or "fungal adenylosuccinate synthase" is meant an enzyme that can be found in at least one fungus, and that catalyzes the reversible interconversion of GTP, IMP, and L-aspartate to GDP, phosphate, and N6-(l ,2-dicarboxyethyI)-AMP. As used herein, the terms "adenylosuccinate synthase" and "adenylosuccinate synthase polypeptide" are synonymous with "the ADE12 gene product" and refer to an enzyme that catalyzes the reversible interconversion of GTP, IMP, and L-aspartate to GDP, phosphate, and N6-(l,2-dicarboxyethyl)-AMP. As used herein, the terms "cyclic nucleotide phosphodiesterase," "cyclic nucleotide-specific phosphodiesterase," "PDE2 polypeptide," "PDE2," and "PDE2 gene product" are used interchangeably and refer to a polypeptide that catalyzes the reversible inter-conversion of nucleoside 3',5'-cyclic phosphate and H₂O to nucleoside 5'-phosphate. Although the name of the protein and/or the name of the gene that encodes the protein may differ between species, the terms "cyclic nucleotide phosphodiesterase" and "PDE2 gene product" are intended to encompass any polypeptide that catalyzes the foregoing reaction. The term "antibiotic" refers to any substance or compound that when contacted with a living cell, organism, virus, or other entity capable of replication, results in a reduction of growth, viability, or pathogenicity of that entity. The term "antipathogenic", as used herein, refers to a mutant form of a gene, which inactivates a pathogenic activity of an organism on its host organism or substantially reduces the level of pathogenic activity, wherein "substantially" means a reduction at least as great as the standard deviation for a measurement, preferably a reduction by 50%, more preferably a reduction of at least one magnitude, i.e. to 10%. The pathogenic activity affected may be an aspect of pathogenic activity governed by the normal form of the gene, or the pathway the normal form of the gene functions on, or the organism's pathogenic activity in general. "Antipathogenic" may also refer to a cell, cells, tissue, or organism that contains the mutant form of a gene; a phenotype associated with the mutant form of a gene, and/or associated with a cell, cells, tissue, or organism that contain the mutant form of a gene. The term "binding" refers to a non-covalent or a covalent interaction, preferably non-covalent, that holds two molecules together. For example, two such molecules could be an enzyme and an inhibitor of that enzyme. Non-covalent interactions include hydrogen bonding, ionic interactions among charged groups, van der Waals interactions and hydrophobic interactions among nonpolar groups. One or more of these interactions can mediate the binding of two molecules to each other. The term "biochemical pathway" or "pathway" refers to a connected series of biochemical reactions normally occurring in a cell, or more broadly a cellular event such as cellular division or DNA replication. Typically, the steps in such a biochemical pathway act in a coordinated fashion to produce a specific product or products or to produce some other particular biochemical action. Such a biochemical pathway requires the expression product of a gene if the absence of that expression product either directly or indirectly prevents the completion of one or more steps in that pathway, thereby preventing or significantly reducing the production of one or more normal products or effects of that pathway. Thus, an agent specifically inhibits such a biochemical pathway requiring the expression product of a particular gene if the presence of the agent stops or substantially reduces the completion of the series of steps in that pathway. Such an agent, may, but does not necessarily, act directly on the expression product of that particular gene. As used herein, the term "conditional lethal" refers to a mutation permitting growth and/or survival only under special growth or environmental conditions. As used herein, the term "cosmid" refers to a hybrid vector, used in gene cloning, that includes a cos site (from the lambda bacteriophage). In some cases, the cosmids of the invention comprise drug resistance marker genes and other plasmid genes. Cosmids are especially suitable for cloning large genes or multigene fragments. "Fungi" (singular: fungus) refers to whole fungi, fungal organs and tissues (e.g., asci, hyphae, pseudohyphae, rhizoid, sclerotia, sterigmata, spores, sporodochia, sporangia, synnemata, conidia, ascostroma, cleistothecia, mycelia, perithecia, basidia and the like), spores, fungal cells and the progeny thereof. Fungi are a group of organisms (about 50,000 known species), including, but not limited to, mushrooms, mildews, moulds, yeasts, etc., comprising the kingdom Fungi. They can either exist as single cells or make up a multicellular body called a mycelium, which consists of filaments known as hyphae. Most fungal cells are multinucleate and have cell walls, composed chiefly of chitin. Fungi exist primarily in damp situations on land and, because of the absence of chlorophyll and thus the inability to manufacture their own food by photosynthesis, are either parasites on other organisms or saprotrophs feeding on dead organic matter. The principal criteria used in classification are the nature of the spores produced and the presence or absence of cross walls within the hyphae. Fungi are distributed worldwide in terrestrial, freshwater, and marine habitats. Some live in the soil. Many pathogenic fungi cause disease in animals and man or in plants, while some saprotrophs are destructive to timber, textiles, and other materials. Some fungi form associations with other organisms, most notably with algae to form lichens. As used herein, the term "fungicide," "antifungal," or "antimycotic" refers to an antibiotic substance or compound that kills or suppresses the growth, viability, or pathogenicity of at least one fungus, fungal cell, fungal tissue or spore. In the context of this disclosure, "gene" should be understood to refer to a unit of heredity. Each gene is composed of a linear chain of deoxyribonucleotides that can be referred to by the sequence of nucleotides forming the chain. Thus, "sequence" is used to indicate both the ordered listing of the nucleotides which form the chain, and the chain, itself, which has that sequence of nucleotides. "Sequence" is used in the similar way in referring to RNA chains, linear chains made of ribonucleotides. The gene may include regulatory and control sequences, sequences which can be transcribed into an RNA molecule, and may contain sequences with unknown function. The majority of the RNA transcription products are messenger RNAs (mRNAs), which include sequences which are translated into polypeptides and may include sequences which are not translated. It should be recognized that small differences in nucleotide sequence for the same gene can exist between different fungal strains, or even within a particular fungal strain, without altering the identity of the gene. As used in this disclosure, the terms "growth" or "cell growth" of an organism refer to an increase in mass, density, or number of cells of the organism. Some common methods for the measurement of growth include the determination of the optical density of a cell suspension, the counting of the number of cells in a fixed volume, the counting of the number of cells by measurement of cell division, the measurement of cellular mass or cellular volume, and the like. As used in this disclosure, the term "growth conditional phenotype" indicates that a fungal strain having such a phenotype exhibits a significantly greater difference in growth rates in response to a change in one or more of the culture parameters than an otherwise similar strain not having a growth conditional phenotype. Typically, a growth conditional phenotype is described with respect to a single growth culture parameter, such as temperature. Thus, a temperature (or heat-sensitive) mutant (i.e., a fungal strain having a heat-sensitive phenotype) exhibits significantly different growth, and preferably no growth, under non-permissive temperature conditions as compared to growth under permissive conditions. In addition, such mutants preferably also show intermediate growth rates at intermediate, or semi-permissive, temperatures. Similar responses also result from the appropriate growth changes for other types of growth conditional phenotypes. As used herein, the term "heterologous adenylosuccinate synthase" or "heterologous ADE12" means either a nucleic acid encoding a polypeptide or a polypeptide, wherein the polypeptide has at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% sequence identity or each integer unit of sequence identity from 50-100%) in ascending order to M. grisea adenylosuccinate synthase protein (SEQ ID NO:3) and at least 10%, 25%, 50%, 75%, 80%, 90%, 95%, or 99% activity or each integer unit of activity from 10-100% in ascending order of the activity of M. grisea adenylosuccinate synthase protein (SEQ ID NO:3). Examples of heterologous adenylosuccinate synthases include, but are not limited to, adenylosuccinate synthetase from Schizosaccharomyces pombe (fission yeast), adenylosuccinate synthase from Saccharomyces cerevisiae (baker's yeast), and adenylosuccinate synthase from Drosophila melanogaster (fruit fly). As used herein, the terms "heterologous PDE2" and "heterologous cyclic nucleotide phosphodiesterase" mean either a nucleic acid encoding a polypeptide or a polypeptide, wherein the polypeptide has at least 38%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% sequence identity or each integer unit of sequence identity from 38-100% in ascending order to M. grisea cyclic nucleotide phosphodiesterase protein (SEQ ID NO:5) and at least 10%, 25%, 50%, 75%, 80%, 90%, 95%, or 99% activity or each integer unit of activity from 10-100% in ascending order of the activity of M. grisea cyclic nucleotide phosphodiesterase protein (SEQ ID NO:5). Examples of heterologous cyclic nucleotide phosphodiesterases include, but are not limited to, PDE2 from Neurospora crassa (EAA27561.1; SEQ ID NO:7), Candida albicans (AAM89252.1; SEQ ID NO:8), Trypanosoma brucei (AAG23160.1; SEQ ID NO:9), Trypanosoma cruzi (AAP49573.1; SEQ ID NO:10), Dictyostelium discoideumand (AAB03508.1; SEQ ID NO:ll), and Saccharomyces cerevisiae (CAA99689.1; SEQ ID NO:12). As used herein, the term "His-Tag" refers to an encoded polypeptide consisting of multiple consecutive histidine amino acids. As used herein, the terms "hph," "hygromycin B phosphotransferase," and "hygromycin resistance gene" refer to a hygromycin phosphotransferase gene or gene product. As used herein, the term "imperfect state" refers to a classification of a fungal organism having no demonstrable sexual life stage. The term "inhibitor," as used herein, refers to a chemical substance that inactivates enzymatic activity or substantially reduces the level of enzymatic activity. The term "adenylosuccinate synthase inhibitor," as used herein, refers to a chemical substance that inactivates the enzymatic activity of adenylosuccinate synthase or substantially reduces the level of enzymatic activity, wherein "substantially" means a reduction at least as great as the standard deviation for a measurement, preferably a reduction by 50%, more preferably a reduction of at least one magnitude, i.e. to 10%. The adenylosuccinate synthase inhibitor may function by interacting directly with the enzyme, a cofactor of the enzyme, the substrate of the enzyme, or any combination thereof. The term "phosphodiesterase inhibitor," as used herein, refers to a chemical substance that interferes with PDE2 function, such as interfering with and/or inactivating or substantially reducing the enzymatic activity of cyclic nucleotide phosphodiesterase, wherein "substantially" means a reduction at least as great as the standard deviation for a measurement, preferably a reduction to 50% activity, more preferably a reduction of at least one magnitude, i.e. to 10% activity. The inhibitor may function by interacting directly with the enzyme, a cofactor of the enzyme, the substrate of the enzyme, or any combination thereof. A polynucleotide may be "introduced" into a fungal cell by any means known to those of skill in the art, including transfection, transformation or transduction, transposable element, electroporation, particle bombardment, infection and the like. The introduced polynucleotide may be maintained in the cell stably if it is incorporated into a non-chromosomal autonomous replicon or integrated into the fungal chromosome. Alternatively, the introduced polynucleotide may be present on an extra-chromosomal non-replicating vector and be transiently expressed or transiently active. As used herein, the term "knockout" or "gene disruption" refers to the creation of organisms carrying a null mutation (a mutation in which there is no active gene product), a partial null mutation or mutations, or an alteration or alterations in gene regulation by interrupting a DNA sequence through insertion of a foreign piece of DNA. Usually the foreign DNA encodes a selectable marker. As used herein, the term "mutant form" of a gene refers to a gene which has been altered, either naturally or artificially, changing the base sequence of the gene. The change in the base sequence may be of several different types, including changes of one or more bases for different bases, deletions, and/or insertions, such as by a transposon. In contrast, a normal form of a gene (wild-type) is a form commonly found in natural populations of an organism. Commonly a single form of a gene will predominate in natural populations. In general, such a gene is suitable as a normal form of a gene, however, other forms which provide similar functional characteristics may also be used as a normal gene. In particular, a normal form of a gene does not confer a growth conditional phenotype on the strain having that gene, while a mutant form of a gene suitable for use in these methods does provide such a growth conditional phenotype. The term "NAD(P)" is herein used to mean either "NAD" or "N ADP" and, similarly, the term "NAD(P)H" is herein used to mean "NADH" or "NADPH." As used herein, the term "Ni-NTA" refers to nickel sepharose. As used herein, a "normal" form of a gene (wild-type) is a form commonly found in natural populations of an organism. Commonly a single form of a gene will predominate in natural populations. In general, such a gene is suitable as a normal form of a gene, however, other forms which provide similar functional characteristics may also be used as a normal gene. In particular, a normal form of a gene does not confer a growth conditional phenotype on the strain having that gene, while a mutant form of a gene suitable for use in these methods does provide such a growth conditional phenotype. As used herein, the term "one form" of a gene is synonymous with the term "gene," and a "different form" of a gene refers to a gene that has greater than 49% sequence identity and less than lO0%> sequence identity with the first form. As used herein, the term "pathogenicity" refers to a capability of causing disease and or degree of capacity to cause disease. The term is applied to parasitic microorganisms in relation to their hosts. As used herein, "pathogenicity," "pathogenic," and the like, encompass the general capability of causing disease as well as various mechanisms and structural and or functional deviations from normal used in the art to describe the causative factors and/or mechanisms, presence, pathology, and/or progress of disease, such as virulence, host recognition, cell wall degradation, toxin production, infection hyphae, penetration peg production, appressorium production, lesion formation, sporulation, and the like. The "percent (%) sequence identity" between two polynucleotide or two polypeptide sequences is determined according to either the BLAST program (Basic Local Alignment Search Tool; (Altschul, S.F. et al, 215 J. Mol. Biol. 403 (1990) (PMID: 2231712)) or using Smith Waterman Alignment (T.F. Smith & M. S. Waterman (1981) 147 J. Mol. Biol. 195 (1981) (PMID: 7265238)). It is understood that for the purposes of determining sequence identity when comparing a DNA sequence to an RNA sequence, a thymine nucleotide is equivalent to a uracil nucleotide. By "polypeptide" is meant a chain of at least two amino acids joined by peptide bonds. The chain may be linear, branched, circular or combinations thereof. The polypeptides may contain amino acid analogs and other modifications, including, but not limited to glycosylated or phosphorylated residues. As used herein, the term "proliferation" is synonymous to the term "growth." As used herein, "semi-permissive conditions" are conditions in which the relevant culture parameter for a particular growth conditional phenotype is intermediate between permissive conditions and non-permissive conditions. Consequently, in semi-permissive conditions an organism having a growth conditional phenotype will exhibit growth rates intermediate between those shown in permissive conditions and non-permissive conditions. In general, such intermediate growth rate may be due to a mutant cellular component which is partially functional under semi-permissive conditions, essentially fully functional under permissive conditions, and is non-functional or has very low function under non-permissive conditions, where the level of function of that component is related to the growth rate of the organism. An intermediate growth rate may also be a result of a nutrient substance or substances that are present in amounts not sufficient for optimal growth rates to be achieved. "Sensitivity phenotype" refers to a phenotype that exhibits either hypersensitivity or hyposensitivity. The term "specific binding" refers to an interaction between adenylosuccinate synthase or PDE2 and a molecule or compound, wherein the interaction is dependent upon the primary amino acid sequence and/or the tertiary conformation of adenylosuccinate synthase. An "adenylosuccinate synthase ligand" is an example of specific binding. "Transform," as used herein, refers to the introduction of a polynucleotide (single or double stranded DNA, RNA, or a combination thereof) into a living cell by any means. Transformation may be accomplished by a variety of methods, including, but not limited to, electroporation, polyethylene glycol mediated uptake, particle bombardment, agrotransformation, and the like. This process may result in transient or stable expression of the transformed polynucleotide. By "stably transformed" is meant that the sequence of interest is integrated into a replicon in the cell, such as a chromosome or episome. Transformed cells encompass not only the end product of a transformation process, but also the progeny thereof which retain the polynucleotide of interest. For the purposes of the invention, "transgenic" refers to any cell, spore, tissue or part, that contains all or part of at least one recombinant polynucleotide. In many cases, all or part of the recombinant polynucleotide is stably integrated into a chromosome or stable extra-chromosomal element, so that it is passed on to successive generations. As used herein, the term "Tween 20" means sorbitan mono-9-octadecenoate poly(oxy- 1 , 1 -ethanediyl). As used in this disclosure, the term "viability" of an organism refers to the ability of an organism to demonstrate growth under conditions appropriate for the organism, or to demonstrate an active cellular function. Some examples of active cellular functions include respiration as measured by gas evolution, secretion of proteins and or other compounds, dye exclusion, mobility, dye oxidation, dye reduction, pigment production, changes in medium acidity, and the like.

ADE12 The present inventors have discovered that disruption of the ADE12 gene and/or gene product reduces the pathogenicity oϊ Magnaporthe grisea. Thus, the inventors are the first to demonstrate that adenylosuccinate synthase is a target for antibiotics, preferably antifungals. Accordingly, the invention provides methods for identifying compounds that inhibit ADE12 gene expression or biological activity of its gene product(s). Such methods include ligand-binding assays, assays for enzyme activity, cell-based assays, and assays for ADE12 gene expression. The compounds identified by the methods of the invention are useful as antibiotics. Thus, in one embodiment, the invention provides a method for identifying a test compound as a candidate for an antibiotic, comprising contacting an adenylosuccinate synthase polypeptide with a test compound and detecting the presence or absence of binding between the test compound and the adenylosuccinate synthase polypeptide, wherein binding indicates that the test compound is a candidate for an antibiotic. The adenylosuccinate synthase polypeptides of the invention have the amino acid sequence of a naturally occurring adenylosuccinate synthase found in a fungus, animal, plant, or microorganism, or have an amino acid sequence derived from a naturally occurring sequence. Preferably the adenylosuccinate synthase is a fungal adenylosuccinate synthase. A cDNA encoding M. grisea adenylosuccinate synthase protein is set forth in SEQ ID NO.T, an M. grisea ADE12 genomic DNA is set forth in SEQ ID NO:2, and an M. grisea adenylosuccinate synthase polypeptide is set forth in SEQ ID NO:3. In one embodiment, the adenylosuccinate synthase is a Magnaporthe adenylosuccinate synthase. Magnaporthe species include, but are not limited to, Magnaporthe rhizophila, Magnaporthe salvinii, Magnaporthe grisea, Magnaporthe oryzae and Magnaporthe poae and the imperfect states of Magnaporthe in the genus Pyricularia. Preferably, the Magnaporthe adenylosuccinate synthase is from Magnaporthe grisea. In one embodiment, the invention provides a polypeptide consisting essentially of SEQ ID NO:3. For the purposes of the present invention, a polypeptide consisting essentially of SEQ ID NO:3 has at least 50% sequence identity with M. grisea adenylosuccinate synthase (SEQ ID NO:3) and at least 10% of the activity of SEQ ID NO:3. A polypeptide consisting essentially of SEQ ID NO:3 has at least 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% sequence identity with SEQ ID NO:3 and at least 25%>, 50%, 75%, or 90% of the activity of M. grisea adenylosuccinate synthase. In various embodiments, the adenylosuccinate synthase can be from Powdery Scab (Spongospora subterranea), Grey Mould (Botrytis cinerea), White Rot (Armillaria mellea), Heartrot Fungus (Ganoderma adspersum), Brown-Rot (Piptoporus betulinus), Corn Smut (Ustilago maydis), Heartrot (Polyporus squamosui , Gray Leaf Spot (Cercospora zeae-maydis), Honey Fungus (Armillaria gallica), Root rot (Armillaria luteobubalina), Shoestring Rot (Armillaria ostoyae), Banana A ithracnose Fungus (Colletotrichum musae), Apple-rotting Fungus (Moniliniafructigena), Apple-rotting Fungus (Penicillium expansum), Clubroot Disease (Plasmodiophora brassicae), Potato Blight (Phytophthora infestans), Root pathogen (Heterobasidion annosum), Take-all Fungus (Gaeumannomyces graminis), Dutch Elm Disease (Op iostoma ulmi), Bean Rust (Uromyces appendiculatus), Northern Leaf Spot (Cochliobolus carbonum), Milo Disease (Periconia circinata), Southern Corn Blight (Cochliobolus heterostrophus), Leaf Spot (Cochliobolus lunata), Brown Stripe (Cochliobolus stenospilus), Panama disease

(Fusarium oxysporum), Wheat Head Scab Fungus (Fusarium graminearum), Cereal Foot Rot (Fusarium culmorum), Potato Black Scurf (Rhizoctonia solani), Wheat Black Stem Rust (Puccinia graminis), White mold (Sclerotinia sclerotiorum), and the like. Fragments of an adenylosuccinate synthase polypeptide are useful in the methods of the invention. In one embodiment, the adenylosuccinate synthase fragments include an intact or nearly intact epitope that occurs on the biologically active wild-type adenylosuccinate synthase. The fragments comprise at least 1O consecutive amino acids of an adenylosuccinate synthase. The fragments comprises at least 15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100, 125, 150, 175, 200, 225, 250, 300, 325, 350, 375, 400, or at least 423 consecutive amino acid residues of an adenylosuccinate synthase. In one embodiment, the fragment is from a Magnaporthe adenylosuccinate synthase. In one embodiment, the fragment contains an amino acid sequence conserved among fungal adenylosuccinate synthases. Polypeptides having at least 50% sequence identity with M. grisea adenylosuccinate synthase (SEQ 3D NO:3) protein are also useful in the methods of the invention. In one embodiment, the sequence identity is at least 55%, 60%, 65%>, 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98% or 99%, or any integer from 50-100% sequence identity in ascending order with M. grisea adenylosuccinate synthase (SEQ ID NO:3) protein. In addition, it is preferred that polypeptides of the invention have at least 10% of the activity of M. grisea adenylosuccinate synthase (SEQ ID NO:3) protein. Adenylosuccinate synthase polypeptides of the invention have at least 10%, 15%, 20%o, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85% or at least 90% of the activity of M. grisea adenylosuccinate synthase (SEQ ID NO:3) protein. Thus, in another embodiment, the invention provides a method for identifying a test compound as a candidate for a fungicide, comprising: contacting a test compound with at least one polypeptide selected from the group consisting of: a polypeptide consisting essentially of SEQ ID NO:3, a polypeptide having at least ten consecutive amino acids of anM grisea adenylosuccinate synthase (SEQ ID NO:3) protein, a polypeptide having at least 50% sequence identity with an M. grisea adenylosuccinate synthase (SEQ ID NO:3) protein and at least 10% of the activity of an M grisea adenylosuccinate synthase (SEQ ID NO:3) protein; and a polypeptide consisting of at least 50 amino acids having at least 50% sequence identity with an M. grisea adenylosuccinate synthase (SEQ ID NO:3) protein and at least 10% of the activity of an M. grisea adenylosuccinate synthase (SEQ LD NO:3) protein; and detecting the presence and/or absence of binding between the test compound and the polypeptide, wherein binding indicates that the test compound is a candidate for an antibiotic. Any technique for detecting the binding of a ligand to its target may be used in the methods of the invention. For example, the ligand and target are combined in a buffer. Many methods for detecting the binding of a ligand to its target are known in the art, and include, but are not limited to, the detection of an immobilized ligand-target complex or the detection of a change in the properties of a target when it is bound to a ligand. For example, in one embodiment, an array of immobilized candidate ligands is provided. The immobilized ligands are contacted with an adenylosuccinate synthase protein or a fragment or variant thereof, the unbound protein is removed and the bound adenylosuccinate synthase is detected. In a preferred embodiment, bound adenylosuccinate synthase is detected using a labeled binding partner, such as a labeled antibody. In an alternate preferred embodiment, adenylosuccinate synthase is labeled prior to contacting the immobilized candidate ligands. Preferred labels include fluorescent or radioactive moieties. Preferred detection methods include fluorescence correlation spectroscopy (FCS) and FCS-related confocal nanofluorimetric methods. Once a compound is identified as a candidate for an antibiotic, it can be tested for the ability to inhibit adenylosuccinate synthase enzymatic activity. The compounds can be tested using either in vitro or cell based assays. Alternatively, a compound can be tested by applying it directly to a fungus or fungal cell, or expressing it therein, and monitoring the fungus or fungal cell for changes or decreases in growth, development, viability, pathogenicity, or alterations in gene expression. Thus, in one embodiment, the invention provides a method for determining whether a compound identified as an antibiotic candidate by an above method has antifungal activity, further comprising: contacting a fungus or fungal cells with the antifungal candidate and detecting a decrease in the growth, viability, or pathogenicity of the fungus or fungal cells. By decrease in growth, is meant that the antifungal candidate causes at least a

10%) decrease in the growth of the fungus or fungal cells, as compared to the growth of the fungus or fungal cells in the absence of the antifungal candidate. By a decrease in viability is meant that at least 20% of the fungal cells, or portion of the fungus contacted with the antifungal candidate are nonviable. Preferably, the growth or viability will be decreased by at least 40%. More preferably, the growth or viability will be decreased by at least 50%, 75% or at least 90% or more. Methods for measuring fungal growth and cell viability are known to those skilled in the art. By decrease in pathogenicity, is meant that the antifungal candidate causes at least a 10% decrease in the disease caused by contact of the fungal pathogen with its host, as compared to the disease caused in the absence of the antifungal candidate. Preferably, the disease will be decreased by at least 40%). More preferably, the disease will be decreased by at least 50%>, 75%> or at least 90% or more. Methods for measuring fungal disease are well known to those skilled in the art, and include such metrics as lesion formation, lesion size, sporulation, respiratory failure, and/or death. The ability of a compound to inhibit adenylosuccinate synthase activity can be detected using in vitro enzymatic assays in which the disappearance of a substrate or the appearance of a product is directly or indirectly detected. Adenylosuccinate synthase catalyzes the reversible interconversion of GTP, IMP, and L-aspartate to GDP, phosphate, and N6-(l,2-dicarboxyethyl)-AMP (see Figure 1). Methods for detection of GTP, IMP, L-aspartate, GDP, phosphate, and/or N6-(l,2-dicarboxyethyl)-AMP include spectrophotometry, fluorimetry, mass spectroscopy, thin layer chromatography (TLC) and reverse phase HPLC. Thus, the invention provides a method for identifying a test compound as a candidate for an antibiotic, comprising: contacting GTP, IMP, and L-aspartate with an adenylosuccinate synthase in the presence and absence of a test compound or contacting GDP, phosphate, and N6-(l,2-dicarboxyethyl)-AMP with an adenylosuccinate synthase in the presence and absence of a test compound; and determining a concentration for at least one of GTP, IMP, L-aspartate, GDP, phosphate, and or N6-(l,2-dicarboxyethyι)~ AMP in the presence and absence of the test compound, wherein a change in the concentration for any of the above substances indicates that the test compound is a candidate for an antibiotic. Enzymatically active fragments of M. grisea adenylosuccinate synthase set forth in SEQ ID NO: 3 are also useful in the methods of the invention. For example, an enzymatically active polypeptide comprising at least 50 consecutive amino acid residues and at least 10% of the activity of M. grisea adenylosuccinate synthase set forth in SEQ ID NO: 3 are useful in the methods of the invention. In addition, enzymatically active polypeptides having at least 10% of the activity of SEQ ID NO:3 and at least 50%, 60%>, 70%, 80%, 90%, 95%, 96%, 97%, 98% or 99% sequence identity with SEQ ID NO:3 are useful in the methods of the invention. Most preferably, the enzymatically active polypeptide has at least 50% sequence identity with SEQ ID NO:3 and at least 25%, 75% or at least 90%> of the activity thereof. Thus, the invention provides a method for identifying a test compound as a candidate for an antibiotic, comprising: contacting GTP, IMP, and L-aspartate or GDP, phosphate, and N6-(l,2-dicarboxyethyl)-AMP with a polypeptide selected from the group consisting of: a polypeptide consisting essentially of SEQ ID NO:3, a polypeptide having at least 50% sequence identity with the M. grisea adenylosuccinate synthase set forth in SEQ ID NO:3 and having at least 10% of the activity thereof, a polypeptide comprising at least 50 consecutive amino acids of M. grisea adenylosuccinate synthase set forth in SEQ ID NO: 3 and having at least 10%> of the activity thereof, and a polypeptide consisting of at least 50 amino acids and having at least 50% sequence identity with M. grisea adenylosuccinate synthase set forth in SEQ ID NO: 3 and having at least 10% of the activity thereof; contacting GTP, IMP, and L-aspartate or GDP, phosphate, and N6-(l,2-dicarboxyethyι)-AMP with the polypeptide and a test compound; and determining a concentration for at least one of GTP, IMP, L-aspartate, GDP, phosphate, and/or N6-(l,2-dicarboxyethyl)-AMP in the presence and absence of the test compound, wherein a change in concentration for any of the above substances indicates that the test compound is a candidate for an antibiotic. For the in vitro enzymatic assays, adenylosuccinate synthase protein and derivatives thereof may be purified from a fungus or may be recombinantly produced in and purified from an archael, bacterial, fungal, or other eukaryotic cell culture. Preferably these proteins are produced using an E. coli, yeast, or filamentous fungal expression system. An example of a method for the purification of an adenylosuccinate synthase polypeptide is described in Lipps and Krauss (1999) Biochem J. 341 :537-43. Other methods for the purification of adenylosuccinate synthase proteins and polypeptides are known to those skilled in the art. As an alternative to in vitro assays, the invention also provides cell based assays. In one embodiment, the invention provides a method for identifying a test compound as a candidate for an antibiotic, comprising: a) measuring the expression or activity of an adenylosuccinate synthase in a cell, cells, tissue, or an organism in the absence of a test compound; b) contacting the cell, cells, tissue, or organism with the test compound and measuring the expression or activity of the adenylosuccinate synthase in the cell, cells, tissue, or organism; and c) comparing the expression or activity of the adenylosuccinate synthase in steps (a) and (b), wherein an altered expression or activity in the presence of the test compound indicates that the compound is a candidate for an antibiotic. Expression of adenylosuccinate synthase can be measured by detecting the adenylosuccinate synthase primary transcript or mRNA, adenylosuccinate synthase polypeptide, or adenylosuccinate synthase enzymatic activity. Methods for detecting the expression of RNA and proteins are known to those skilled in the art. (See, e.g., Current Protocols in Molecular Biology, Ausubel et al, eds., Greene Publishing & Wiley- Interscience, New York, (1995)). The method of detection is not critical to the present invention. Methods for detecting adenylosuccinate synthase RNA include, but are not limited to amplification assays such as quantitative reverse transcriptase-PCR, and/or hybridization assays such as Northern analysis, dot blots, slot blots, in-situ hybridization, transcriptional fusions using an adenylosuccinate synthase promoter fused to a reporter gene, DNA assays, and microarray assays. Methods for detecting protein expression include, but are not limited to, immunodetection methods such as Western blots, ELISA assays, polyacrylamide gel electrophoresis, mass spectroscopy, and enzymatic assays. Also, any reporter gene system may be used to detect adenylosuccinate synthase protein expression. For detection using gene reporter systems, a polynucleotide encoding a reporter protein is fused in frame with adenylosuccinate synthase, so as to produce a chimeric polypeptide. Methods for using reporter systems are known to those skilled in the art. Chemicals, compounds or compositions identified by the above methods as modulators of adenylosuccinate synthase expression or activity can then be used to control fungal growth. Diseases such as rusts, mildews, and blights spread rapidly once established. Fungicides are thus routinely applied to growing and stored crops as a preventive measure, generally as foliar sprays or seed dressings. For example, compounds that inhibit fungal growth can be applied to a fungus or expressed in a fungus, in order to prevent fungal growth. Thus, the invention provides a method for inhibiting fungal growth, comprising contacting a fungus with a compound identified by the methods of the invention as having antifungal activity. Antifungals and antifungal adenylosuccinate synthase inhibitor candidates identified by the methods of the invention can be used to control the growth of undesired fungi, including ascomycota, zygomycota, basidiomycota, chytridiomycota, and lichens. Examples of undesired fungi include, but are not limited to Powdery Scab (Spongospora subterranea), Grey Mould (Botiγtis cinerea), White Rot (Armillaria mellea), Heartrot Fungus (Ganoderma adspersum), Brown-Rot (Piptoporus betulinus), Corn Smut (Ustilago maydis), Heartrot (Polyporus squamosus), Gray Leaf Spot (Cercospora zeae- maydis), Honey Fungus (Armillaria gallica), Root rot (Armillaria luteobubalina), Shoestring Rot (Armillaria ostoyae), Banana Anthracnose Fungus (Colletotrichum musae), Apple-rotting Fungus (Monilinia fructigena), Apple-rotting Fungus (Penicillium expansum), Clubroot Disease (Plasmodiophora brassicae), Potato Blight (Phytophthora infestans), Root pathogen (Heterobasidion annosum), Take-all Fungus (Gaeumannomyces graminis), Dutch Elm Disease (Ophiostoma ulmi), Bean Rust (Uromyces appendiculatus), Northern Leaf Spot (Cochliobolus carbonuin), Milo Disease (Periconia circinata), Southern Corn Blight (Cochliobolus heterostrophus), Leaf Spot (Cochliobolus lunata), Brown Stripe (Cochliobolus stenospilus), Panama disease (Fusarium oxysporum), Wheat Head Scab Fungus (Fusarium graminearum), Cereal Foot Rot (Fusarium culmorum), Potato Black Scurf (Rhizoctonia solani), Wheat Black Stem Rust (Puccinia graminis), White mold (Sclerotinia sclerotiorum), diseases of animals such as infections of lungs, blood, brain, skin, scalp, nails or other tissues (Aspergillus fumigatus Aspergillus sp. Fusraium sp., Trichophyton sp., Epidermophyton sp., and Microsporum sp., and the like). Also provided in the invention are methods of screening for an antibiotic by determining the in vivo activity of a test compound against two separate fungal organisms, wherein the fungal organisms comprise a first form of an adenylosuccinate synthase and a second form of the adenylosuccinate synthase, respectively, h the methods of the invention, at least one of the two forms of the adenylosuccinate synthase has at least 10%» of the activity of the polypeptide set forth in SEQ TO NO: 3. The methods comprise comparing the growth of the two organisms in the presence of the test compound relative to their respective controls without test compound. A difference in growth between the two organisms in the presence of the test compound indicates that the test compound is a candidate for an antibiotic. The forms of an adenylosuccinate synthase useful in the methods of the invention are selected from the group consisting of: a nucleic acid encoding SEQ ID NO:3, a nucleic acid encoding a polypeptide consisting essentially of SEQ ID NO:3, SEQ ID NO:l or SEQ ID NO:2, SEQ ID NO:l or SEQ ID NO:2 comprising a mutation either reducing or abolishing adenylosuccinate synthase protein activity, a heterologous adenylosuccinate synthase, and a heterologous adenylosuccinate synthase comprising a mutation either reducing or abolishing adenylosuccinate synthase protein activity. Any combination of two different forms of the adenylosuccinate synthase genes listed above are useful in the methods of the invention, with the limitation that at least one of the forms of the adenylosuccinate synthase has at least 10% of the activity of the polypeptide set forth in SEQ ID NO:3. Thus, in one embodiment, the invention provides a method for identifying a test compound as a candidate for an antibiotic, comprising: providing an organism having a first form of an adenylosuccinate synthase; providing an organism having a second form of the adenylosuccinate synthase; and determining the growth of the organism having the first form of the adenylosuccinate synthase and the growth of the organism having the second form of the adenylosuccinate synthase in the presence of the test compound, wherein a difference in growth between the two organisms in the presence of the test compound indicates that the test compound is a candidate for an antibiotic. It is recognized in the art that the optional determination of the growth of the organism having the first form of the adenylosuccinate synthase and the growth of the organism having the second form of the adenylosuccinate synthase in the absence of any test compounds is performed to control for any inherent differences in growth as a result of the different genes. Growth and/or proliferation of an organism are measured by methods well known in the art such as optical density measurements, and the like. In a preferred embodiment, the organism is Magnaporthe grisea. In another embodiment, the invention provides a method for identifying a test compound as a candidate for an antibiotic, comprising: providing an organism having a first form of an adenylosuccinate synthase; providing a comparison organism having a second form of the adenylosuccinate synthase; and determining the pathogenicity of the organism having the first form of the adenylosuccinate synthase and the organism having the second form of the adenylosuccinate synthase in the presence of the test compound, wherein a difference in pathogenicity between the two organisms in the presence of the test compound indicates that the test compound is a candidate for an antibiotic. In an optional embodiment of the inventon, the pathogenicity of the organism having the first form of the adenylosuccinate synthase and the organism having the second form of the adenylosuccinate synthase in the absence of any test compounds is determined to control for any inherent differences in pathogenicity as a result of the different genes.

Pathogenicity of an organism is measured by methods well known in the art such as lesion number, lesion size, sporulation, and the like. In a preferred embodiment the organism is Magnaporthe grisea. In one embodiment of the invention, the first form of an adenylosuccinate synthase is SEQ ID NO:l or SEQ ID NO:2, and the second form of the adenylosuccinate synthase is an adenylosuccinate synthase that confers a growth conditional phenotype (i.e. an adenine requiring phenotype) and/or a hypersensitivity or hyposensitivity phenotype on the organism. In a related embodiment of the invention, the second form of the adenylosuccinate synthase is SEQ ID NO.T comprising a transposon insertion that reduces activity. In a related embodiment of the invention, the second form of an adenylosuccinate synthase is SEQ ID NO: 1 comprising a transposon insertion that abolishes activity. In a related embodiment of the invention, the second form of the adenylosuccinate synthase is SEQ ID NO:2 comprising a transposon insertion that reduces activity. In a related embodiment of the invention, the second form of the adenylosuccinate synthase is SEQ ID NO:2 comprising a transposon insertion that abolishes activity. In a related embodiment of the invention, the second form of the adenylosuccinate synthase is Schizosaccharomyces pombe adenylosuccinate synthase. In a related embodiment of the invention, the second form of the adenylosuccinate synthase is Drosophila melanogaster adenylosuccinate synthase. In a related embodiment of the invention, the second form of the adenylosuccinate synthase is Saccharomyces cerevisiae adenylosuccinate synthase. In another embodiment of the invention, the first form of an adenylosuccinate synthase is Schizosaccharomyces pombe adenylosuccinate synthase and the second form of the adenylosuccinate synthase is Schizosaccharomyces pombe adenylosuccinate synthase comprising a transposon insertion that reduces activity. In a related embodiment of the invention, the second form of the adenylosuccinate synthase is Schizosaccharomyces pombe adenylosuccinate synthase comprising a transposon insertion that abolishes activity. In another embodiment of the invention, the first form of an adenylosuccinate synthase is Drosophila melanogaster adenylosuccinate synthase and the second form of the adenylosuccinate synthase is Drosophila melanogaster adenylosuccinate synthase comprising a transposon insertion that reduces activity. In a related embodiment of the invention, the second form of the adenylosuccinate synthase is Drosophila melanogaster adenylosuccinate synthase comprising a transposon insertion that abolishes activity. In yet another embodiment of the invention, the first form of an adenylosuccinate synthase is Saccharomyces cerevisiae adenylosuccinate synthase and the second form of the adenylosuccinate synthase is Saccharomyces cerevisiae adenylosuccinate synthase comprising a transposon insertion that reduces activity. In a related embodiment of the invention, the second form of the adenylosuccinate synthase is Saccharomyces cerevisiae adenylosuccinate synthase comprising a transposon insertion that abolishes activity. Conditional lethal mutants and/or antipathogenic mutants identify particular biochemical and/or genetic pathways given that at least one identified target gene is present in that pathway. Knowledge of these pathways allows for the screening of test compounds as candidates for antibiotics as adenylosuccinate synthase inhibitors of the substrates, products, proteins and/or enzymes of the pathway. The invention provides methods of screening for an antibiotic by determining whether a test compound is active against the purine biosynthetic pathway on which adenylosuccinate synthase functions. Pathways known in the art are found at the Kyoto Encyclopedia of Genes and Genomes and in standard biochemistry texts (See, e.g. Lehninger et al, Principles of Biochemistry. New York, Worth Publishers (1993)). Thus, in one embodiment, the invention provides a method for screening for test compounds acting against the biochemical and/or genetic pathway or pathways in which adenylosuccinate synthase functions, comprising: providing an organism having a first form of a gene in the purine biosynthetic pathway; providing an organism having a second form of the gene in the purine biosynthetic pathway; and determining the growth of the two organisms in the presence of a test compound, wherein a difference in growth between the organism having the first form of the gene and the organism having the second form of the gene in the presence of the test compound indicates that the test compound is a candidate for an antibiotic. It is recognized in the art that the optional determination of the growth of the organism having the first form of the gene and the organism having the second form of the gene in the absence of any test compounds is performed to control for any inherent differences in growth as a result of the different genes. Growth and/or proliferation of an organism are measured by methods well known in the art such as optical density measurements, and the like. In a preferred embodiment, the organism is Magnaporthe grisea. The forms of a gene in the purine biosynthetic pathway useful in the methods of the invention include, for example, wild-type and mutated genes encoding phosphoribosylglycinamide formyltransferase and adenylosuccinate lyase from any organism, preferably from a fungal organism, and most preferrably from M. grisea. The forms of a mutated gene in the purine biosynthetic pathway comprise a mutation either reducing or abolishing protein activity. In one example, the form of a gene in the purine biosynthetic pathway comprises a transposon insertion. Any combination of a first form of a gene in the purine biosynthetic pathway and a second form of the gene listed above are useful in the methods of the invention, with the limitation that one of the forms of a gene in the purine biosynthetic pathway has at least 10% of the activity of the corresponding M. grisea gene. In another embodiment, the invention provides a method for screening for test compounds acting against the biochemical and/or genetic pathway or pathways in which adenylosuccinate synthase functions, comprising: providing an organism having a first form of a gene in the purine biosynthetic pathway; providing an organism having a second form of the gene in the purine biosynthetic pathway; and determining the pathogenicity of the two organisms in the presence of the test compound, wherein a difference in pathogenicity between the organism having the first form of the gene and the organism having the second form of the gene in the presence of the test compound indicates that the test compound is a candidate for an antibiotic. In an optional embodiment of the inventon, the pathogenicity of the two organisms in the absence of any test compounds is determined to control for any inherent differences in pathogenicity as a result of the different genes. Pathogenicity of an organism is measured by methods well known in the art such as lesion number, lesion size, sporulation, and the like. In a preferred embodiment the organism is Magnaporthe grisea. Thus, in an alternate embodiment, the invention provides a method for screening for test compounds acting against the biochemical and/or genetic pathway or pathways in which adenylosuccinate synthase functions, comprising: providing paired growth media containing a test compound, wherein the paired growth media comprise a first medium and a second medium and the second medium contains a higher level of adenine than the first medium; inoculating the first and the second medium with an organism; and determining the growth of the organism, wherein a difference in growth of the organism between the first and the second medium indicates that the test compound is a candidate for an antibiotic. In one embodiment of the invention, the growth of the organism is determined in the first and the second medium in the absence of any test compounds to control for any inherent differences in growth as a result of the different media. Growth and/or proliferation of the organism are measured by methods well known in the art such as optical density measurements, and the like. In a preferred embodiment, the organism is Magnaporthe grisea . One embodiment of the invention is directed to the use of multi-well plates for screening of antibiotic compounds. The use of multi-well plates is a format that readily accommodates multiple different assays to characterize various compounds, concentrations of compounds, and fungal organisms in varying combinations and formats. Certain testing parameters for the screening method can sigmficantly affect the identification of growth adenylosuccinate synthase inhibitors, and thus can be manipulated to optimize screening efficiency and/or reliability. Notable among these factors are variable sensitivities of different mutants, increasing hypersensitivity with increasingly less permissive conditions, an apparent increase in hypersensitivity with increasing compound concentration, and other factors known to those in the art.

PDE2 The present inventors have discovered that disruption of the PDE2 gene in Magnaporthe grisea drastically reduces pathogenicity of the fungus. Thus, the inventors demonstrate that the PDE2 gene product is a target for antibiotics, preferably fungicides. Accordingly, the invention provides methods for identifying compounds that inhibit PDE2 gene expression or biological activity of its gene product(s). Such methods include ligand-binding assays, assays for enzyme activity, cell-based assays, and assays for PDE2 gene expression. The compounds identified by the methods of the invention are useful as antibiotics. Thus, in one embodiment, the invention provides a method for identifying a test compound as a candidate for an antibiotic, comprising contacting a PDE2 polypeptide with a test compound and detecting the presence or absence of binding between the test compound and the PDE2 polypeptide, wherein binding indicates that the test compound is a candidate for an antibiotic. PDE2 polypeptides of the invention have the amino acid sequence of naturally occurring PDE2 polypeptides found in a fungus, animal, plant, or microorganism, or have an amino acid sequence derived from a naturally occurring sequence. Preferably the PDE2 is a fungal PDE2. A cDNA encoding M. grisea PDE2 protein is set forth in SEQ ED NO:4 and aM. grisea PDE2 polypeptide is set forth in SEQ ID NO: 5. The genomic DNA encoding the M. grisea PDE2 protein is set forth in SEQ ID NO:6. In one embodiment, the PDE2 is a Magnaporthe PDE2. Magnaporthe species include, but are not limited to, Magnaporthe rhizophila, Magnaporthe salvinii, Magnaporthe grisea, Magnaporthe oryzae and Magnaporthe poae and the imperfect states of Magnaporthe in the genus Pyricularia. Preferably, the Magnaporthe PDE2 is from Magnaporthe grisea. In one embodiment, the invention provides a polypeptide consisting essentially of SEQ ID NO:5. For the purposes of the present invention, a polypeptide consisting essentially of SEQ ID NO:5 has at least 90% sequence identity with M. grisea PDE2 (SEQ ID NO:5) and at least 10% of the activity of SEQ ID NO:5. A polypeptide consisting essentially of SEQ ID NO:5 has at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% sequence identity with SEQ ID NO:5 and at least 25%, 50%, 75%, or 90% of the activity of M. grisea PDE2. Examples of polypeptides consisting essentially of SEQ ID NO:5 include, but are not limited to, polypeptides having the amino acid sequence of SEQ ID NO:5 with the exception that one or more of the amino acids are substituted with structurally similar amino acids providing a conservative amino acid substitution. Conservative amino acid substitutions are well known to those of skill in the art. Examples of polypeptides consisting essentially of SEQ ED NO: 5 include polypeptides having 1, 2, or 3 conservative amino acid substitutions relative to SEQ ID NO:5. Other examples of polypeptides consisting essentially of SEQ ID NO:5 include polypeptides having the sequence of SEQ ED NO:5, but with truncations at either or both the 3' and the 5' end. For example, polypeptides consisting essentially of SEQ ID NO: 5 include polypeptides having 1, 2, or 3 amino acids residues removed from either or both 3' and 5' ends relative to SEQ DD NO:5. In various embodiments, the PDE2 gene product can be from Powdery Scab (Spongospora subterranea), Grey Mould (Botrytis cinerea), White Rot (Armillaria mellea), Heartrot Fungus (Ganodenna adspersum), Brown-Rot (Piptoporus betulinus), Corn Smut (Ustilago maydis), Heartrot (Polyporus squamosus), Gray Leaf Spot (Cercospora zeae-maydis), Honey Fungus (Armillaria gallica), Root rot (Armillaria luteobubalina), Shoestring Rot (Armillaria ostoyae), Banana Anthracnose Fungus (Colletotrichum musae), Apple-rotting Fungus (Monilinia fructigena), Apple-rotting Fungus (Penicillium expansum), Clubroot Disease (Plasmodiophora brassicae), Potato Blight (Phytophthora infestans), Root pathogen (Heterobasidion annosum), Take-all Fungus (Gaeumannomyces graminis), Dutch Elm Disease (Ophiostoma ulmi), Bean Rust (Uromyces appendiculatus), Northern Leaf Spot (Cochliobolus carbonum), Milo Disease (Periconia circinata), Southern Corn Blight (Cochliobolus heterostrophus), Leaf Spot (Cochliobolus lunata), Brown Stripe (Cochliobolus stenospilus), Panama disease

(Fusarium oxysporum), Wheat Head Scab Fungus (Fusarium graminearum), Cereal Foot Rot (Fusarium culmorum), Potato Black Scurf (Rhizoctonia solani), Wheat Black Stem Rust (Puccinia graminis), White mold (Sclerotinia sclerotiorum), and the like. Fragments of a PDE2 polypeptide are useful in the methods of the invention. In one embodiment, the PDE2 fragments include an intact or nearly intact epitope that occurs on biologically active wild-type PDE2. For example, the fragments comprise at least 10 consecutive amino acids of PDE2 set forth in SEQ ED NO: 5. The fragments comprise at least 15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100, 125, 150, 175, 200, 225, 250, 300, 325, 350, 375, 400, 425, 450, 475, 500, 525, 550, 575, 600, 625, 650, 675, 700, 725, 750, 775, 800, 825, 850, 875 or at least 890 consecutive amino acids residues of PDE2 set forth in SEQ DD NO: 5. Fragments of heterologous PDE2's are also useful in the methods of the invention. For example, polypeptides having at least 50%, 60%>, 70%, 80%, 90%, 95%, 96%, 97%, 98% or 99% sequence identity with at least 50 consecutive amino acid residues of SEQ DD NO:5 are useful in the methods of the invention. In one embodiment, the fragment is from a Magnaporthe PDE2. In an alternate embodiment, the fragment contains an amino acid sequence conserved among fungal PDE2's. Particularly useful fragments of the invention are fragments of PDE2 proteins that include an intact or nearly intact epitope present at a non-membrane spanning region of a biologically active PDE2 protein. For example, such a fragment comprises at least 10 consecutive amino acids occurring at a non-membrane spanning region of the PDE2 protein set forth in SEQ ID NO:5. Procedures for identifying non-membrane spanning regions of proteins, such as the PDE2 proteins of the invention, based on analysis of the polypeptide sequence for conserved signal-processing and membrane spanning sequences are known to those of ordinary skill in the art. Polypeptides having at least 38%> sequence identity with M. grisea PDE2 (SEQ DD NO:5) protein are also useful in the methods of the invention. In one embodiment, the sequence identity is at least 38%, 39%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98% or 99%, or any integer from 38-100% sequence identity in ascending order with M. grisea PDE2 (SEQ ID NO:5) protein. In addition, it is preferred that polypeptides of the invention have at least 10% of the activity of M. grisea PDE2 (SEQ DD NO:5) protein. PDE2 polypeptides of the invention have at least 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%o, 80%>, 85% or at least 90% of the activity of M. grisea PDE2 (SEQ ID NO:5) protein. Thus, in another embodiment, the invention provides a method for identifying a test compound as a candidate for a fungicide, comprising: contacting a test compound with at least one polypeptide selected from the group consisting of: a polypeptide consisting essentially of SEQ ID NO:5, a polypeptide having at least ten consecutive amino acids of SEQ DD NO:5, a polypeptide having at least 38% sequence identity with SEQ DD NO:5 and at least 10%. of the activity of SEQ DD NO:5, and a polypeptide consisting of at least 50 amino acids having at least 50% sequence identity with SEQ ED NO:5 and at least 10% of the activity of SEQ DD NO:5, and detecting the presence and/or absence of binding between the test compound and the polypeptide, wherein binding indicates that the test compound is a candidate for an antibiotic. Any technique for detecting the binding of a ligand to its target may be used in the methods of the invention. For example, the ligand and target are combined in a buffer. Many methods for detecting the binding of a ligand to its target are known in the art, and include, but are not limited to, the detection of an immobilized ligand-target complex or the detection of a change in the properties of a target when it is bound to a ligand. In a preferred embodiment, bound PDE2 is detected using a labeled binding partner, such as a labeled antibody. In an alternate preferred embodiment, PDE2 is labeled prior to contacting the immobilized candidate ligands. Preferred labels include fluorescent or radioactive moieties. Preferred detection methods include fluorescence correlation spectroscopy (FCS), FCS-related confocal nanofluorimetric methods, and liquid scintillation counting. In another embodiment of the invention, compounds are identified as candidates for antibiotics by their ability to inhibit PDE2 enzymatic activity. The compounds are tested using either in vitro or cell based assays. Alternatively, a compound can be tested by applying it directly to a fungus or fungal cell, or expressing it therein, and monitoring the fungus or fungal cell for changes or decreases in growth, development, viability, pathogenicity, or alterations in gene expression. Thus, in one embodiment, the invention provides a method for determining whether a compound identified as an antibiotic candidate by an above method has antifungal activity, further comprising: contacting a fungus or fungal cells with the antifungal candidate and detecting a decrease in the growth, viability, or pathogenicity of the fungus or fungal cells. By decrease in growth, is meant that the antifungal candidate causes at least a 10% decrease in the growth of the fungus or fungal cells, as compared to the growth of the fungus or fungal cells in the absence of the antifungal candidate. By a decrease in viability is meant that at least 20% of the fungal cells, or portion of the fungus contacted with the antifungal candidate are nonviable. Preferably, the growth or viability will be decreased by at least 40%. More preferably, the growth or viability will be decreased by at least 50%>, 75% or at least 90% or more. Methods for measuring fungal growth and cell viability are known to those skilled in the art. By decrease in pathogenicity, is meant that the antifungal candidate causes at least a 10% decrease in the disease caused by contact of the fungal pathogen with its host, as compared to the disease caused in the absence of the antifungal candidate. Preferably, the disease will be decreased by at least 40%. More preferably, the disease will be decreased by at least 50%, 75% or at least 90% or more. Methods for measuring fungal disease are well known to those skilled in the art, and include such metrics as lesion formation, lesion size, sporulation, respiratory failure, and/or death. The ability of a compound to inhibit PDE2 activity can be detected using in vitro enzymatic assays in which the disappearance of a substrate or the appearance of a product is directly or indirectly detected. PDE2 proteins catalyze the inter-conversion of nucleoside 3',5'-cyclic phosphate and H₂O to nucleoside 5'-phosphate. Methods for measuring the progression of a PDE2 enzymatic reaction and/or a change in the concentration of one or more reactants are known to those of ordinary skill in the art and include, for example, incubating a radiolabeled nucleoside 3 ',5 '-cyclic phosphate with a PDE2 enzyme under conditions suitable for enzyme activity; precipitating the radiolabeled nucleoside 5'-phosphate product that is formed; and quantifying the amount of product formed using a scintillation counter. Thus, the invention provides a method for identifying a test compound as a candidate for an antibiotic, comprising: contacting a nucleoside 3',5'-cyclic phosphate substrate such as cAMP with a PDE2 enzyme in the presence and absence of a test compound; and comparing the concentration for the substrate and/or a nucleoside 5'- phosphate product in the presence and absence of the test compound, wherein a difference in the presence of the test compound, relative to the absence, for any of the above reactants indicates that the test compound is a candidate for an antibiotic. Active fragments of M. grisea PDE2 set forth in SEQ ID NO:5 are also useful in the methods of the invention. For example, an active polypeptide comprising at least 50 consecutive amino acid residues set forth in SEQ DD NO:5 and results in at least 10% of the activity of M. grisea PDE2 are useful in the methods of the invention. In addition, fragments of heterologous PDE2's are also useful in the methods of the invention. Active polypeptides having at least 10% of the activity of SEQ ED NO:5 and at least

50%, 60%, 70%,, 80%>, 90%, 95%, 96%, 97%, 98% or 99% sequence identity with at least 50 consecutive amino acid residues of SEQ DD NO:5 are useful in the methods of the invention. Most preferably, the active polypeptide has at least 50%> sequence identity with at least 50 consecutive amino acid residues of SEQ DD NO:5 and at least 25%>, 75%> or at least 90% of the activity thereof. Thus, the invention provides a method for identifying a test compound as a candidate for an antibiotic, comprising: contacting a nucleoside 3 ',5 '-cyclic phosphate substrate such as cAMP with a PDE2 polypeptide selected from the group consisting of: a polypeptide consisting essentially of SEQ DD NO:5, a polypeptide having at least 38% sequence identity with the M. grisea PDE2 set forth in SEQ ID NO:5 and having at least 10%) of the activity thereof, a polypeptide comprising at least 50 consecutive amino acids of M. grisea PDE2 set forth in SEQ ID NO:5 and having at least 10% of the activity thereof, and a polypeptide consisting of at least 50 amino acids and having at least 50% sequence identity with grisea PDE2 set forth in SEQ DD NO:5 and having at least 10%) of the activity thereof, contacting the nucleoside 3',5'-cyclic phosphate substrate such as cAMP and the PDE2 polypeptide with a test compound, and comparing the concentration for either or both of the substrate and a nucleoside 5 '-phosphate product in the presence and absence of the test compound, wherein a difference in concentration in the presence of the test compound, relative to the absence, for any of the above reactants indicates that the test compound is a candidate for an antibiotic. For in vitro enzymatic assays, a PDE2 protein and derivatives thereof are isolated from a fungus or may be recombinantly produced in and isolated from an archael, bacterial, fungal, or other eukaryotic cell culture. Preferably these proteins are produced using an E. coli, yeast, or filamentous fungal expression system using methods known to those skilled in the art. The invention also provides cell-based assays. In one embodiment, the invention provides a method for identifying a test compound as a candidate for an antibiotic, comprising: a) measuring the expression or activity of a PDE2 in a cell, cells, tissue, or an organism in the absence of a test compound; b) contacting the cell, cells, tissue, or organism with the test compound and measuring the expression or activity of the PDE2 in the cell, cells, tissue, or organism; and c) comparing the expression or activity of the PDE2 in steps (a) and (b), wherein an altered expression or activity in the presence of the test compound indicates that the compound is a candidate for an antibiotic. Expression of PDE2 can be measured by detecting the PDE2 primary transcript or mRNA, PDE2 polypeptide, or enzymatic activity of PDE2 polypeptide. Methods for detecting the expression of RNA and proteins are known to those skilled in the art. (Current Protocols in Molecular Biology, Ausubel et al, eds., Greene Publishing & Wiley-Interscience, New York, (1995)). The method of detection is not critical to the present invention. Methods for detecting PDE2 RNA include, but are not limited to, amplification assays such as quantitative reverse transcriptase-PCR, and/or hybridization assays such as Northern analysis, dot blots, slot blots, in-situ hybridization, transcriptional fusions using a PDE2 promoter fused to a reporter gene, DNA assays, and microarray assays. Methods for detecting protein expression include, but are not limited to, immunodetection methods such as Western blots, ELISA assays, polyacrylamide gel electrophoresis, mass spectroscopy, and enzymatic assays. Also, any reporter gene system may be used to detect PDE2 protein expression. For detection using gene reporter systems, a polynucleotide encoding a reporter protein is fused in frame with PDE2 coding region so as to produce a chimeric polypeptide. Methods for using reporter systems are known to those skilled in the art. Chemicals, compounds, or compositions identified by the above methods as modulators of PDE2 expression or activity can then be used to control fungal growth. Diseases such as rusts, mildews, and blights spread rapidly once established. Fungicides are thus routinely applied to growing and stored crops as a preventive measure, generally as foliar sprays or seed dressings. For example, compounds that inhibit fungal growth can be applied to a fungus or expressed in a fungus to prevent fungal growth. Thus, the invention provides a method for inhibiting fungal growth, comprising contacting a fungus with a compound identified by the methods of the invention as having antifungal activity. Antifungals and antifungal inhibitor candidates identified by the methods of the invention can be used to control the growth of undesired fungi, including ascomycota, zygomycota, basidiomycota, chytridiomycota, and lichens. Examples of undesired fungi include, but are not limited to Powdery Scab (Spongospora subterranea), Grey Mould (Botrytis cinerea), White Rot (Armillaria mellea), Heartrot Fungus (Ganoderma adspersum), Brown-Rot (Piptoporus betulinus), Corn Smut (Ustilago maydis), Heartrot (Polyporus squamosus), Gray Leaf Spot (Cercospora zeae-maydis), Honey Fungus (Armillaria gallica), Root rot (Armillaria luteobubalina), Shoestring Rot (Armillaria ostoyae), Banana Anthracnose Fungus (Colletotrichum musae), Apple-rotting Fungus (Monilinia fructigena), Apple-rotting Fungus (Penicillium expansum), Clubroot Disease {Plasmodiophora brassicae), Potato Blight (Phytophthora infestans), Root pathogen (Heterobasidion annosum), Take-all Fungus (Gaeumannomyces graminis), Dutch Elm Disease (Ophiostoma ulmi), Bean Rust (Uromyces appendiculatus), Northern Leaf Spot (Cochliobolus carbonum), Milo Disease (Periconia circinata), Southern Corn Blight (Cochliobolus heterostrophus), Leaf Spot (Cochliobolus lunata), Brown Stripe (Cochliobolus stenospilus), Panama disease (Fusarium oxysporum), Wheat Head Scab Fungus (Fusarium graminearum), Cereal Foot Rot (Fusarium culmorum), Potato Black Scurf (Rhizoctonia solani), Wheat Black Stem Rust (Puccinia graminis), White mold (Sclerotinia sclerotiorum), diseases of animals such as infections of lungs, blood, brain, skin, scalp, nails or other tissues (Aspergillus fumigatus, Aspergillus sp., Fusraium sp., Trichophyton sp., Epidermophyton sp., and Microsporum sp., and the like). Also provided in the invention are methods of screening for an antibiotic by determining the in vivo activity of a test compound against two separate fungal organisms, wherein the fungal organisms comprise a first form of a PDE2 and a second form of the PDE2, respectively. In the methods of the invention, at least one of the two forms of the PDE2 has at least 10%> of the activity of the polypeptide set forth in SEQ ED NO:5. The methods comprise comparing the growth of the two organisms in the presence of the test compound relative to their respective controls without the test compound. A difference in growth between the two organisms in the presence of the test compound indicates that the test compound is a candidate for an antibiotic. Forms of a PDE2 useful in the methods of the invention are selected from the group consisting of: a nucleic acid encoding SEQ ED NO:5; a nucleic acid encoding a polypeptide consisting essentially of SEQ DD NO:5; the nucleic acid set forth in SEQ ID NO:6; the nucleic acid set forth in SEQ ID NO:6 comprising a mutation either reducing or abolishing PDE2 protein activity; a nucleic acid encoding a heterologous PDE2; and a nucleic acid encoding a heterologous PDE2 comprising a mutation either reducing or abolishing PDE2 protein activity. Any combination of two different forms of the PDE2 genes listed above are useful in the methods of the invention, with the caveat that at least one of the forms of the PDE2 has at least 10%> of the activity of the polypeptide set forth in SEQ DD NO:5. Thus, in one embodiment, the invention provides a method for identifying a test compound as a candidate for an antibiotic, comprising: providing an organism having a first form of a PDE2; providing an organism having a second form of the PDE2; and determining the growth of the organism having the first form of the PDE2 and the growth of the organism having the second form of the PDE2 in the presence of the test compound, wherein a difference in growth between the two organisms in the presence of the test compound indicates that the test compound is a candidate for an antibiotic. It is recognized in the art that the optional determination of the growth of the organism having the first form of the PDE2 and the growth of the organism having the second form of the PDE2 in the absence of any test compounds is performed to control for any inherent differences in growth as a result of the different genes. Growth and/or proliferation of an organism are measured by methods well known in the art such as optical density measurements, and the like. In a preferred embodiment, the organism is Magnaporthe grisea. In another embodiment, the invention provides a method for identifying a test compound as a candidate for an antibiotic, comprising: providing an organism having a first form of a PDE2; providing a comparison organism having a second form of the PDE2; and determining the pathogenicity of the organism having the first form of the PDE2 and the organism having the second form of the PDE2 in the presence of the test compound, wherein a difference in pathogenicity between the two organisms in the presence of the test compound indicates that the test compound is a candidate for an antibiotic. In an alternate embodiment of the invention, the pathogenicity of the organism having the first form of the PDE2 and the organism having the second form of the PDE2 in the absence of any test compounds is determined to control for any inherent differences in pathogenicity as a result of the different genes. Pathogenicity of an organism is measured by methods well known in the art such as lesion number, lesion size, sporulation, and the like. In a preferred embodiment the organism is Magnaporthe grisea. One embodiment of the invention is directed to the use of multi-well plates for screening of antibiotic compounds. The use of multi-well plates is a format that readily accommodates multiple different assays to characterize various compounds, concentrations of compounds, and fungal organisms in varying combinations and formats. Certain testing parameters for the screening method can significantly affect the identification of growth inhibitors, and thus can be manipulated to optimize screening efficiency and/or reliability. Notable among these factors are variable sensitivities of different mutants, increasing hypersensitivity with increasingly less permissive conditions, an apparent increase in hypersensitivity with increasing compound concentration, and other factors known to those in the art.

EXPERIMENTAL Example 1 Construction of Plasmids with a Transposon Containing a Selectable Marker- Construction of Sif transposon: Sif was constructed using the GPS3 vector from the GPS-M mutagenesis system from New England Biolabs, Inc. (Beverly, MA) as a backbone. This system is based on the bacterial transposon Tn7. The following manipulations were done to GPS3 according to Sambrook et al, Molecular Cloning, a Laboratory Manual Cold Spring Harbor Laboratory Press (1989). The kanamycin resistance gene (npt) contained between the Tn7 arms was removed by EcoRV digestion. The bacterial hygromycin B phosphotransferase (hph) gene (Gritz & Davies, 25 Gene 179 (1983) (PMID: 6319235)) under control of the Aspergillus nidulans trpC promoter and terminator (Mullaney et al. , 199 Mol. Gen. Genet. 37 (1985) (PMID: 3158796)) was cloned by a Hpal/EcoRV blunt ligation into the Tn7 arms of the GPS3 vector yielding pSifl. Excision of the ampicillin resistance gene (bla) from pSifl was achieved by cutting pSifl with Xmnl and Bgll followed by a T4 DNA polymerase treatment to remove the 3' overhangs left by the Bgll digestion and religation of the plasmid to yield pSif. Top 10F' electrocompetent E. coli cells (Invitrogen) were transformed with ligation mixture according to manufacturer's recommendations. Transformants containing the Sif transposon were selected on LB agar (Sambrook et al, supra) containing 50 μg/ml of hygromycin B (Sigma Chem. Co., St. Louis, MO). Example 2 Construction of a Fungal Cosmid Library Cosmid libraries were constructed in the pcosKA5 vector (Hamer et al, 98 Proc. Nat'l. Acad. Sci. USA 5110 (2001) (PMED: 11296265)) as described in Sambrook et al. Cosmid libraries were quality checked by pulsed-field gel electrophoresis, restriction digestion analysis, and PCR identification of single genes.

Example 3 Construction of Cosmids with Transposon Insertion into Fungal Genes Sif Transposition into a Cosmid: Transposition of Sif into the cosmid framework was carried out as described by the GPS-M mutagenesis system (New England Biolabs, Inc.). Briefly, 2 μl of the 10X GPS buffer, 70 ng of supercoiled pSIF, 8-12 μg of target cosmid DNA were mixed and taken to a final volume of 20 μl with water. 1 μl of transposase (TnsABC) was added to the reaction and incubated for 10 minutes at 37°C to allow the assembly reaction to occur. After the assembly reaction, 1 μl of start solution was added to the tube, mixed well, and incubated for 1 hour at 37°C followed by heat inactivation of the proteins at 75°C for 10 minutes. Destruction of the remaining untransposed pSif was completed by PIScel digestion at 37°C for 2 hours followed by a 10 minute incubation at 75°C to inactivate the proteins. Transformation of ToplOF' electrocompetent cells (Invitrogen) was done according to manufacturers recommendations. Sif-containing cosmid transformants were selected by growth on LB agar plates containing 50 μg/ml of hygromycin B (Sigma Chem. Co.) and 100 μg/ml of Ampicillin (Sigma Chem. Co.). Example 4 High Throughput Preparation and Verification of Transposon Insertion into the M. srisea APE 12 Gene E. coli strains containing cosmids with transposon insertions were picked to 96 well growth blocks (Beckman Co.) containing 1.5 ml of TB (Terrific Broth, Sambrook et al, supra) supplemented with 50 μg/ml of ampicillin. Blocks were incubated with shaking at 37°C overnight. E. coli cells were pelleted by centrifugation and cosmids were isolated by a modified alkaline lysis method (Marra et al, 1 Genome Res. 1072 (1997) (PMID: 9371743)). DNA quality was checked by electrophoresis on agarose gels. Cosmids were sequenced using primers from the ends of each transposon and commercial dideoxy sequencing kits (Big Dye Terminators, Perkin Elmer Co.). Sequencing reactions were analyzed on an ABI377 DNA sequencer (Perkin Elmer Co.). The DNA sequences adjacent to the site of the transposon insertion were used to search DNA and protein databases using the BLAST algorithms (Altschul et al, supra). A single insertion of SIF into the Magnaporthe grisea ADE12 gene was chosen for further analysis. This construct was designated cpgmra0052001h08 and it contains the SIF transposon insertion approximately between amino acids 300 and 301 relative to the Schizosaccharomyces pombe homolog.

Example 5 Preparation of ADE12 Cosmid DNA and Transformation of Masnaporthe grisea Cosmid DNA from the ADE12 transposon tagged cosmid clone was prepared using QIAGEN Plasmid Maxi Kit (Qiagen), and digested by PI-PspI (New England Biolabs, Inc.). Fungal electro-transformation was performed essentially as described (Wu et al, 10 MPMI 700 (1997)). Briefly, M. grisea strain Guy 11 was grown in complete liquid media (Talbot et al, 5 Plant Cell 1575 (1993) (PMID: 8312740)) shaking at 120 rpm for 3 days at 25°C in the dark. Mycelia was harvested and washed with sterile H₂0 and digested with 4 mg/ml beta-glucanase (InterSpex) for 4-6 hours to generate protoplasts. Protoplasts were collected by centrifugation and resuspended in 20% sucrose at a concentration of 2x10⁸ protoplasts/ml. 50 μl of protoplast suspension was mixed with 10-20 μg of the cosmid DNA and pulsed using a Gene Pulser II instrument (BioRad) set with the following parameters: 200 ohm, 25μF, and 0.6kV. Transformed protoplasts were regenerated in complete agar media (Talbot et al, supra) with the addition of 20% sucrose for one day, then overlayed with CM agar media containing hygromycin B (250 ug/ml) to select transformants. Transformants were screened for homologous recombination events in the target gene by PCR (Hamer et al, supra). Two independent strains were identified and are hereby referred to as Kl -10 and Kl-14, respectively. Example 6 Effect of Transposon Insertion on Magnaporthe pathogenicity The target fungal strains, Kl-10 and Kl-14, obtained in Example 5 and the wild- type strain, Guyl 1, were subjected to a pathogenicity assay to observe infection over a 1- week period. Rice infection assays were performed using Indian rice cultivar CO39 essentially as described in Valent et al. (Valent et al, 127 Genetics 87 (1991) (PMID: 2016048)). All three strains were grown for spore production on complete agar media. Spores were harvested and the concentration of spores adjusted for whole plant inoculations. Two-week-old seedlings of cultivar CO39 were sprayed with 12 ml of conidial suspension (5 x 10⁴ conidia per ml in 0.01% Tween-20 solution). The inoculated plants were incubated in a dew chamber at 27°C in the dark for 36 hours, and transferred to a growth chamber (27 °C 12 hours/21 °C 12 hours at 70%> humidity) for an additional 5.5 days. Leaf samples were taken at 3, 5, and 7 days post-inoculation and examined for signs of successful infection (i.e. lesions). Figure 2 shows the effects of ADE12 gene disruption on Magnaporthe infection at seven days post-inoculation.

Example 7 Cloning. Expression, and Purification of Adenylosuccinate Synthase Protein The following is a protocol to obtain a purified adenylosuccinate synthase protein. Cloning and expression strategies: An adenylosuccinate synthase cDNA gene is cloned into E. coli (pET vectors- Novagen), Baculovirus (Pharmingen) and Yeast (Invitrogen) expression vectors containing His/fusion protein tags, and the expression of recombinant protein is evaluated by SDS-PAGE and Western blot analysis.

Extraction: Extract recombinant protein from 250 ml cell pellet in 3 ml of extraction buffer by sonicating 6 times, with 6 second pulses at 4°C. Centrifuge extract at 15000xg for 10 minutes and collect supernatant. Assess biological activity of the recombinant protein by activity assay. Purification: Purify recombinant protein by Ni-NTA affinity chromatography (Qiagen). Purification protocol (perform all steps at 4°C): • Use 3 ml Ni-beads • Equilibrate column with the buffer • Load protein extract • Wash with the equilibration buffer • Elute bound protein with 0.5 M imidazole Other methods for purifying adenylosuccinate synthase protein are described in. Lipps and Krauss (1999) Biochem J. 341 :537-43 and Ryzhova et al. (1998) Biochemistry (Mosc) 63(6):650-6.

Example 8 Assays for Measuring Binding of Test Compounds to Adenylosuccinate Synthase The following is a protocol to identify test compounds that bind to the adenylosuccinate synthase protein. • Purified full-length adenylosuccinate synthase polypeptide with a His/fusion protein tag (Example 7) is bound to a HISGRAB Nickel Coated Plate (Pierce, Rockford, IL) following manufacturer's instructions. • Buffer conditions are optimized (e.g. ionic strength or pH, Shoolingin- Jordan et al. (1997) Methods Enzymol 281: 309 - 16 (PMID: 9250995)) for binding of radiolabeled GTP, IMP, L-aspartate, GDP, phosphate, or N6-(l,2- dicarboxyethyl)-AMP to the bound adenylosuccinate synthase protein. • Screening of test compounds is performed by adding test compound and radioactive GTP, IMP, L-aspartate, GDP, phosphate, or N6-(l,2- dicarboxyethyl)-AMP to the wells of the HISGRAB plate containing bound adenylosuccinate synthase protein. • The wells are washed to remove excess labeled ligand and scintillation fluid (SCTNTINERSE, Fisher Scientific) is added to each well. • The plates are read in a microplate scintillation counter. • Candidate compounds are identified as wells with lower radioactivity as compared to control wells with no test compound added. Additionally, a purified polypeptide comprising 10-50 amino acids from the M. grisea adenylosuccinate synthase is screened in the same way. A polypeptide comprising 10-50 amino acids is generated by subcloning a portion of the ADE12 gene into a protein expression vector that adds a His-Tag when expressed (see Example 7). Oligonucleotide primers are designed to amplify a portion of the ADE12 gene using the polymerase chain reaction amplification method. The DNA fragment encoding a polypeptide of 10 - 50 amino acids is cloned into an expression vector, expressed in a host organism and purified as described in Example 7 above. Test compounds that bind adenylosuccinate synthase are further tested for antibiotic activity. M. grisea is grown as described for spore production on oatmeal agar media (Talbot et al. , supra). Spores are harvested into minimal media to a concentration of 2 x 10⁵ spores/ml and the culture is divided. Id. The test compound is added to one culture to a final concentration of 20-100 μg/ml. Solvent only is added to the second culture. The plates are incubated at 25°C for seven days and optical density measurements at 590 nm are taken daily. The growth curves of the solvent control sample and the test compound sample are compared. A test compound is an antibiotic candidate if the growth of the culture containing the test compound is less than the growth of the control culture. Test compounds that bind adenylosuccinate synthase are further tested for antipathogenic activity. M. grisea is grown as described for spore production on oatmeal agar media (Talbot et al, supra). Spores are harvested into water with 0.01% Tween 20 to a concentration of 5x10⁴ spores/ml and the culture is divided. Id. The test compound is added to one culture to a final concentration of 20-100 μg/ml. Solvent only is added to the second culture. Rice infection assays are performed using Indian rice cultivar CO39 essentially as described in Valent et al, supra). Two-week-old seedlings of cultivar CO39 are sprayed with 12 ml of conidial suspension. The inoculated plants are incubated in a dew chamber at 27°C in the dark for 36 hours, and transferred to a growth chamber (27 °C 12 hours/21 °C 12 hours at 70% humidity) for an additional 5.5 days. Leaf samples are examined at 5 days post-inoculation to determine the extent of pathogenicity as compared to the control samples. Alternatively, antipathogenic activity can be assessed using an excised leaf pathogenicity assay. Spore suspensions are prepared in water only to a concentration of 5x10⁴ spores/ml and the culture is divided. The test compoimd is added to one culture to a final concentration of 20-100 μg/ml. Solvent only is added to the second culture. Detached leaf assays are performed by excising 1 cm segments of rice leaves from Indian rice cultivar CO39 and placing them on 1%> agarose in water. 10 μl of each spore suspension is place on the leaf segments and the samples are incubated at 25°C for 5 days in the dark. Leaf samples are examined at 5 days post-inoculation to determine the extent of pathogenicity as compared to the control samples.

Example 9 Assays for Testing Inhibitors or Candidates for Inhibition of Adenylosuccinate Synthase Activity The enzymatic activity of adenylosuccinate synthase is determined in the presence and absence of candidate compounds in a suitable reaction mixture, such as described by Lipps and Krauss (1999) supra. Candidate compounds are identified by a decrease in products or a lack of a decrease in substrates in the presence of the compound, with the reaction proceeding in either direction. Candidate compounds are additionally determined in the same manner using a polypeptide comprising a fragment of the M. grisea adenylosuccinate synthase. The adenylosuccinate synthase polypeptide fragment is generated by subcloning a portion of the ADE12 gene into a protein expression vector that adds a His-Tag when expressed (see Example 7). Oligonucleotide primers are designed to amplify a portion of the

ADE12 gene using polymerase chain reaction amplification method. The DNA fragment encoding the adenylosuccinate synthase polypeptide fragment is cloned into an expression vector, expressed and purified as described in Example 7 above. Test compounds identified as inhibitors of adenylosuccinate synthase activity are further tested for antibiotic activity and antipathogenic activity as described in Example Example 10 Assays for Testing Compounds for Alteration of Adenylosuccinate Synthase Gene Expression Magnaporthe grisea fungal cells are grown under standard fungal growth conditions that are well known and described in the art. Wild-type M. grisea spores are harvested from cultures grown on complete agar or oatmeal agar media after growth for 10-13 days in the light at 25°C using a moistened cotton swab. The concentration of spores is determined using a hemacytometer and spore suspensions are prepared in a minimal growth medium to a concentration of 2x10⁵ spores per ml. 25 ml cultures are prepared to which test compounds will be added at various concentrations. A culture with no test compound present is included as a control. The cultures are incubated at 25°C for 3 days after which test compound or solvent only control is added. The cultures are incubated an additional 18 hours. Fungal mycelia is harvested by filtration through Miracloth (CalBiochem, La Jolla, CA), washed with water, and frozen in liquid nitrogen. Total RNA is extracted with TRIZOL Reagent using the methods provided by the manufacturer (Life Technologies, Rockville, MD). Expression is analyzed by Northern analysis of the RNA samples as described (Sambrook et al, supra) using a radiolabeled fragment of the ADE12 gene as a probe. Test compounds resulting in an altered level of ADE12 mRNA relative to the untreated control sample are identified as candidate antibiotic compounds. Test compounds identified as inhibitors of adenylosuccinate synthase activity are further tested for antibiotic activity and antipathogenic activity as described in Example 8.

Example 11 In Vivo Cell Based Assay Screening Protocol with a Fungal Strain Containing a Mutant Form of Adenylosuccinate Synthase that Lacks Activity The effect of test compounds on the growth of wild-type fungal cells and mutant fungal cells having a mutant ADE12 gene is measured and compared as follows. Magnaporthe grisea fungal cells containing a mutant form of the ADE12 gene that lacks activity, for example a ADE12 gene containing a transposon insertion, are grown under standard fungal growth conditions that are well known and described in the art. Magnaporthe grisea spores are harvested from cultures grown on complete agar medium containing adenine (Sigma) after growth for 10-13 days in the light at 25°C using a moistened cotton swab. The concentration of spores is determined using a hemacytometer and spore suspensions are prepared in a minimal growth medium containing adenine to a concentration of 2x10⁵ spores per ml. Approximately 4 l0⁴ spores are added to each well of 96-well plates to which a test compound is added (at varying concentrations). The total volume in each well is 200μl. Wells with no test compound present (growth control), and wells without cells are included as controls (negative control). The plates are incubated at 25°C for seven days and optical density measurements at 590nm are taken daily. Wild-type cells are screened under the same conditions. The effect of each of the test compounds on the mutant and wild-type fungal cells is measured against the growth control and the percent of inhibition is calculated as the OD₅₉₀ (fungal strain plus test compound)/OD₅₉₀ (growth control) x 100. The percent of growth inhibition in the presence of the test compound on the mutant and wild-type fungal strains are compared. Compounds that show differential growth inhibition between the mutant and the wild-type cells are identified as potential antifungal compounds. Similar protocols may be found in Kirsch & DiDomenico, 26 Biotechnology 177 (1994) (PMDD: 7749303)). Test compounds that produce a differential growth response between the mutant and wild-type fungal strains are further tested for antipathogenic activity as described in Example 8.

Example 12 In Vivo Cell Based Assay Screening Protocol with a Fungal Strain Containing a Mutant Form of Adenylosuccinate Synthase with Reduced Activity The effect of test compounds on the growth of wild-type fungal cells and mutant fungal cells having a mutant ADE12 gene is measured and compared as follows. Magnaporthe grisea fungal cells containing a mutant form of the ADE12 gene resulting in reduced activity, such as a the transposon insertion mutation of cpgmra0052001h08 or a promoter truncation mutation that reduces expression, are grown under standard fungal growth conditions that are well known and described in the art. A promoter truncation is made by deleting a portion of the promoter upstream of the transcription start site using standard molecular biology techniques that are well known and described in the art (Sambrook et al. , supra). The mutant and wild-type Magnaporthe grisea spores are harvested from cultures grown on complete agar medium containing adenine (Sigma) after growth for 10-13 days in the light at 25°C using a moistened cotton swab. The concentration of spores is determined using a hemacytometer and spore suspensions are prepared in a minimal growth medium to a concentration of 2x10⁵ spores per ml. Approximately 4xl0⁴ spores are added to each well of 96-well plates to which a test compoimd is added (at varying concentrations). The total volume in each well is 200μl. Wells with no test compound present (growth control), and wells without cells are included as controls (negative control). The plates are incubated at 25°C for seven days and optical density measurements at 590nm are taken daily. Wild-type cells are screened under the same conditions. The effect of each test compound on the mutant and wild-type fungal strains is measured against the growth control and the percent of inhibition is calculated as the OD₅₉₀ (fungal strain plus test compound)/OD5₉₀ (growth control) x 100. The percent growth inhibition as a result of each of the test compounds on the mutant and wild-type cells is compared. Compounds that show differential growth inhibition between the mutant and the wild-type cells are identified as potential antifungal compounds. Similar protocols may be found in Kirsch & DiDomenico, supra Test compounds that produce a differential growth response between the mutant and wild-type fimgal strains are further tested for antipathogenic activity as described in Example 8. Example 13 In Vivo Cell Based Assay Screening Protocol with a Fungal Strain Containing a Mutant Form of a Purine Biosynthetic Gene that Lacks Activity The effect of test compounds on the growth of wild-type fungal cells and mutant fungal cells having a mutant form of a gene in the purine biosynthetic pathway is measured and compared as follows. Magnaporthe grisea fungal cells containing a mutant form of a gene that lacks activity in the purine biosynthetic pathway (e.g. phosphoribosylglycinamide formyltransferase or adenylosuccinate lyase encoding gene having a transposon insertion) are grown under standard fungal growth conditions that are well known and described in the art. Magnaporthe grisea spores are harvested from cultures grown on complete agar medium containing adenine (Sigma) after growth for 10-13 days in the light at 25°C using a moistened cotton swab. The concentration of spores is determined using a hemacytometer and spore suspensions are prepared in a minimal growth medium containing adenine to a concentration of 2x10⁵ spores per ml. Approximately 4xl0⁴ spores or cells are harvested and added to each well of 96- well plates to which growth media is added in addition to an amount of test compound (at varying concentrations). The total volume in each well is 200μl. Wells with no test compound present, and wells without cells are included as controls. The plates are incubated at 25°C for seven days and optical density measurements at 590nm are taken daily. Wild-type cells are screened under the same conditions. The effect of each compound on the mutant and wild-type fungal strains is measured against the growth control and the percent of inhibition is calculated as the OD₅₅1₀ (fungal strain plus test compound) / OD₅₉o (growth control) x 100. The percent of growth inhibition as a result of each of the test compounds on the mutant and the wild- type cells are compared. Compounds that show differential growth inhibition between the mutant and the wild-type cells are identified as potential antifungal compounds. Similar protocols may be found in Kirsch & DiDomenico, supra. Test compounds that produce a differential growth response between the mutant and wild-type fungal strains are further tested for antipathogenic activity as described in Example 8. Example 14 In Vivo Cell Based Assay Screening Protocol with a Fungal Strain Containing a Mutant Form of a Purine Biosynthetic Gene with Reduced Activity The effect of test compounds on the growth of wild-type fungal cells and mutant fungal cells having a mutant form of a gene in the purine biosynthetic pathway is measured and compared as follows. Magnaporthe grisea fungal cells containing a mutant form of a gene resulting in reduced protein activity in the purine biosynthetic pathway (e.g. phosphoribosylglycinamide formyltransferase or adenylosuccinate lyase gene having a promoter truncation that reduces expression), are grown under standard fungal growth conditions that are well known and described in the art. Mutant and wild- type Magnaporthe grisea spores are harvested from cultures grown on complete agar medium containing adenine (Sigma) after growth for 10-13 days in the light at 25°C using a moistened cotton swab. The concentration of spores is determined using a hemacytometer and spore suspensions are prepared in a minimal growth medium to a concentration of 2x10⁵ spores per ml. Approximately 4xl0⁴ spores or cells are harvested and added to each well of 96- well plates to which growth media is added in addition to an amount of test compound (at varying concentrations). The total volume in each well is 200μl. Wells with no test compound present, and wells without cells are included as controls. The plates are incubated at 25° C for seven days and optical density measurements at 590 nm are taken daily. Wild-type cells are screened under the same conditions. The effect of each compound on the mutant and wild-type fungal strains is measured against the growth control and the percent of inhibition is calculated as the OD₅₉₀ (fungal strain plus test compound) / OD₅₉₀ (growth control) x 100. The percent of growth inhibition as a result of each of the test compounds on the mutant and wild-type cells are compared. Compounds that show differential growth inhibition between the mutant and the wild- type cells are identified as potential antifungal compounds. Similar protocols may be found in Kirsch & DiDomenico, supra. Test compounds that produce a differential growth response between the mutant and wild-type fungal strains are further tested for antipathogenic activity as described in Example 8. Example 15 In Vivo Cell Based Assay Screening Protocol with a Fungal Strain Containing a Heterologous Adenylosuccinate Synthase Gene. The effect of test compounds on the growth of wild-type fungal cells and fungal cells lacking a functional endogenous adenylosuccinate synthase gene and containing a heterologous adenylosuccinate synthase gene is measured and compared as follows. Wild-type M. grisea fungal cells and M. grisea fungal cells lacking an endogenous adenylosuccinate synthase gene and containing a heterologous adenylosuccinate synthase gene from Schizosaccharomyces pombe (Genbank Accession No. Q02787), having 59% sequence identity, are grown under standard fungal growth conditions that are well known and described in the art. A M. grisea strain carrying a heterologous adenylosuccinate synthase gene is made as follows. A M. grisea strain is made with a nonfunctional endogenous adenylosuccinate synthase gene, such as one containing a transposon insertion in the native gene that abolishes protein activity. A construct containing a heterologous adenylosuccinate synthase gene is made by cloning a heterologous adenylosuccinate synthase gene, such as from Schizosaccharomyces pombe, into a fungal expression vector containing a trpC promoter and terminator (e.g. Carroll et al, 41 Fungal Gen. News Lett. 22 (1994) (describing pCB 1003) using standard molecular biology techniques that are well known and described in the art (Sambrook et al, supra). The vector construct is used to transform the M. grisea strain lacking a functional endogenous adenylosuccinate synthase gene. Fungal transformants containing a functional adenylosuccinate synthase gene are selected on minimal agar medium lacking adenine, as only transformants carrying a functional adenylosuccinate synthase gene grow in the absence of adenine. Wild-type strains of M. grisea and strains containing a heterologous form of adenylosuccinate synthase are grown under standard fungal growth conditions that are well known and described in the art. M. grisea spores are harvested from cultures grown on complete agar medium after growth for 10 - 13 days in the light at 25° C using a moistened cotton swab. The concentration of spores is determined using a hemacytometer and spore suspensions are prepared in a minimal growth medium to a concentration of 2x10⁵ spores per ml. Approximately 4xl0⁴ spores or cells are harvested and added to each well of 96- well plates to which growth media is added in addition to an amount of test compound (at varying concentrations). The total volume in each well is 200 μl. Wells with no test compound present, and wells without cells are included as controls. The plates are incubated at 25° C for seven days and optical density measurements at 590 nm are taken daily. The effect of each compound on the wild-type and heterologous fungal strains is measured against the growth control and the percent of inhibition is calculated as the OD₅₉o (fungal strain plus test compound) / OD₅₉o (growth control) x 100. The percent of growth inhibition as a result of each of the test compounds on the wild-type and heterologous fungal strains are compared. Compounds that show differential growth inhibition between the wild-type and heterologous strains are identified as potential antifungal compounds with specificity to the native or heterologous adenylosuccinate synthase gene products. Similar protocols may be found in Kirsch & DiDomenico, supra. Test compounds that produce a differential growth response between the strain containing a heterologous gene and strain containing a fungal gene are further tested for antipathogenic activity as described in Example 8.

Example 16 Pathway Specific In Vivo Assay Screening Protocol Compounds are tested as candidate antibiotics as follows. Magnaporthe grisea fungal cells are grown under standard fungal growth conditions that are well known and described in the art. Wild-type M. grisea spores are harvested from cultures grown on oatmeal agar media after growth for 10-13 days in the light at 25°C using a moistened cotton swab. The concentration of spores is determined using a hemocytometer and spore suspensions are prepared in a minimal growth medium and a minimal growth medium containing adenine (Sigma) to a concentration of 2xl0⁵ spores per ml. The minimal growth media contains carbon, nitrogen, phosphate, and sulfate sources, and magnesium, calcium, and trace elements (for example, the inoculating fluid may consist of 0.05% Phytagel, 0.03% Pluronic F68, 1% glucose, 23.5 mM NaNO₃, 6.7 mM KC1, 3.5 mM Na₂SO₄, 11.0 mM KH₂PO₄, 0.1 mM MgCl₂, 1.0 mM CaCl₂ and trace elements, pH adjusted to 6.0 with NaOH. Final concentrations of trace elements in this case would be: 7.6 μM ZnCl₂, 2.5 μM MnCl₂4H₂0, 1.8 μM FeCl₂4H₂O, 0.71 μM CoCl₂6H₂O, 0.64 μM CuCl₂2H₂O, 0.62 μM Na₂MoO , 18 μM H₃BO₃). Spore suspensions are added to each well of a 96-well microtiter plate (approximately 4xl0⁴ spores/well). For each well containing a spore suspension in minimal media, an additional well is present containing a spore suspension in minimal medium containing adenine. Test compounds are added to wells containing spores in minimal media and mimmal media containing adenine. The total volume in each well is 200μl. Both minimal media and adenine containing media wells with no test compound are provided as controls. The plates are incubated at 25°C for seven days and optical density measurements at 590nm are taken daily. A compound is identified as a candidate for an antibiotic acting against the adenine biosynthetic pathway when the observed growth in the well containing minimal media is less than the observed growth in the well containing adenine as a result of the addition of the test compound. Similar protocols may be found in Kirsch & DiDomenico, supra.

Example 17 High Throughput Preparation and Verification of Transposon Insertion into the M. srisea PDE2 Gene E. coli strains containing cosmids with transposon insertions were picked to 96 well growth blocks (Beckman Co.) containing 1.5ml of TB (Terrific Broth, Sambrook et al, supra) supplemented with 50μg/ml of ampicillin. Blocks were incubated with shaking at 37°C overnight. E. coli cells were pelleted by centrifugation and cosmids were isolated by a modified alkaline lysis method (Marra et al, 1 Genome Res. 1072 (1997) (PMDD: 9371743)). DNA quality was checked by electrophoresis on agarose gels. Cosmids were sequenced using primers from the ends of each transposon and commercial dideoxy sequencing kits (Big Dye Terminators, Perkin Elmer Co.). Sequencing reactions were analyzed on an ABI377 DNA sequencer (Perkin Elmer Co.). The DNA sequences adjacent to the site of the transposon insertion were used to search DNA and protein databases using the BLAST algorithms (Altschul et al, supra). A construct having the SIF transposon insertion into the Magnaporthe grisea PDE2 gene was chosen for further analysis and designated cpgmra0048001a02.

Example 18 Preparation of PDE2 Cosmid DNA and Transformation of Magnaporthe grisea Cosmid DNA from the PDE2 transposon tagged cosmid clone was prepared using QIAGEN Plasmid Maxi Kit (Qiagen), and digested by PI-PspI (New England Biolabs, Inc.). Fungal electro-transformation was performed essentially as described (Wu et al, 10 MPMI100 (1997)). Briefly, M. grisea strain Guy 11 was grown in complete liquid media (Talbot et al, 5 Plant Cell 1575 (1993) (PMID: 8312740)) shaking at 120rpm for 3 days at 25°C in the dark. Mycelia was harvested and washed with sterile H₂O and digested with 4mg/ml beta-glucanase (InterSpex) for 4-6hr to generate protoplasts. Protoplasts were collected by centrifugation and resuspended in 20% sucrose at a concentration of 2x10⁸ protoplasts/ml. 50μl of protoplast suspension was mixed with 10- 20μg of the cosmid DNA and pulsed using a Gene Pulser II instrument (BioRad) set with the following parameters: 200ohm, 25μF, and 0.6kV. Transformed protoplasts were regenerated in complete agar media (Talbot et al, supra) with the addition of 20%> sucrose for one day, then overlayed with CM agar media containing hygromycin B (250 μ.g/ml) to select transformants. Transformants were screened for homologous recombination events in the target gene by PCR (Hamer et al, supra). Two independent strains were identified and are hereby referred to as Kl-7 and Kl-9. Example 19 Effect of Transposon Insertion into PDE2 on Magnaporthe Pathogenicity The target fungal strains, Kl-9, and Kl-7, obtained in Example 18 and the wild- type strain, Guyl 1 , were subjected to a pathogenicity assay to observe infection over a 1- week period. Rice infection assays were performed using Indica rice cultivar CO39 essentially as described in Valent et al. (Valent et al., 127 Genetics 87 (1991) (PMED: 2016048)). All three strains were grown for spore production on complete agar media. Spores were harvested and the concentration of spores adjusted for whole plant inoculations. Two-week-old seedlings of cultivar CO39 were sprayed with 12ml of conidial suspension (5 x 10⁴ conidia per ml in 0.01% Tween-20 solution). The inoculated plants were incubated in a dew chamber at 27°C in the dark for 36hr, and transferred to a growth chamber (27°C, 12hr / 21°C, 12hrs at 70% humidity) for an additional 5.5 days. Leaf samples were taken at 3, 5, and 7 days post-inoculation and examined for signs of successful infection (i.e. lesions). Figure 3 shows the effects of PDE2 gene disruption on Magnaporthe infection at five days post-inoculation. Example 20 Cloning, Expression, and Isolation of Recombinant PDE2 The following is a protocol to obtain an isolated PDE2 protein or protein fragment. Cloning and expression strategies: A PDE2 encoding nucleic acid is cloned into E. coli (pET vectors-Novagen),

Baculovirus (Pharmingen) and Yeast (Invitrogen) expression vectors containing His/fusion protein tags, and the expression of recombinant protein is evaluated by SDS- PAGE and Western blot analysis. Extraction: Extract recombinant protein from 250 ml cell pellet in 3 ml of an extraction buffer by sonicating 6 times, with 6 second pulses at 4°C. Centrifuge extract at 15000xg for lOmin and collect supernatant. Assess biological activity of the recombinant protein using an activity assay such as the following ZnSO4/Ba(OH)2 precipitation method of Schilling et al. (216 Anal. Biochem. 154-8 (1994)) as described by Zoraghi et al. 276 J. Biol. Chem. 11559-66 (2001), herein incorporated by reference in its entirety: ■ Incubate the recombinant PDE2 enzyme in 50mM HEPES, pH 7.5, 0.5mM EDTA, lOmM MgCl₂, and [³H]cAMP (50,000 dpm/reaction) in a total volume of 100μl for 20min at 30°C. ■ Quench reaction with 50μl of 21.5mM ZnCl₂ followed by 50μl of 9mM Ba(OH)₂ and incubate on ice for 30min. ^■ Filter precipitate through GF-C glass fiber filters and wash filters 3 times with ImM NaOH, lOOmM NaCl. ■ Dry filters and count amount of product formed in liquid scintillation fluid (4g/liter omnifluor in toluene). Isolation: Isolate recombinant protein by Ni-NTA affinity chromatography (Qiagen). Purification protocol (perform all steps at 4°C): ^■ Use 3ml Ni-beads ■ Equilibrate column with the buffer ■ Load protein extract ^■ Wash with the equilibration buffer ■ Elute bound protein with 0.5M imidazole Assess biological activity of the recombinant protein by activity assay. Example 21 Assays for Screening Test Compounds for Binding/Inhibition of Isolated PDE2 Polypeptide The following are protocols to identify test compounds that bind/inhibit isolated PDE2 protein. Assay 1 : ^■ Test compounds are immobilized on a supportive medium. ■ Radioactively labeled PDE2 polypeptide is prepared by expressing the PDE2 polypeptide as described in Example 20 in the presence of radioactively labeled methionine (³⁵S-methionine, Amersham). ^■ Screening for inhibitors is performed by incubating the radioactively labeled PDE2 polypeptide with the immobilized test compounds. ^■ The wells are washed to remove excess labeled polypeptide and scintillation fluid (SCΓNTIVERSE, Fisher Scientific) is added to each well. ^■ The plates are read in a microplate scintillation counter. Candidate compounds are identified as wells with higher radioactivity as compared to control wells with no test compound added. Assay 2: ■ Incubate the recombinant PDE2 enzyme in the presence and absence of test compounds in 50mM HEPES, pH 7.5, 0.5mM EDTA, lOmM MgCl₂, and [³H]cAMP (50,000 dpm reaction) in a total volume of lOOμl for 20min at 30°C. ^■ Quench reactions with 50μl of 21.5mM ZnCl₂ followed by 50μl of 9mM Ba(OH)₂ and incubate on ice for 30min. ^■ Filter precipitate through GF-C glass fiber filters and wash filters 3 times with ImM NaOH, lOOmM NaCl. ^■ Dry filters and count amount of product formed in liquid scintillation fluid (4g/liter omnifluor in toluene). A decrease in measured scintillation in the presence relative to the absence of a test compound indicates that the compound is a candidate antibiotic. Additionally, an isolated polypeptide comprising 10-50 amino acids from the M. grisea PDE2 is screened in the same way. A polypeptide comprising 10-50 amino acids is generated by subcloning a portion of the PDE2 encoding nucleic acid into a protein expression vector that adds a His-Tag when expressed (see Example 20). Oligonucleotide primers are designed to amplify a portion of the PDE2 coding region using the polymerase chain reaction amplification method. The DNA fragment encoding a polypeptide of 10-50 amino acids is cloned into an expression vector, expressed in a host organism and isolated as described in Example 20 above. Test compounds that bind and/or inhibit the PDE2 polypeptide are further tested for antipathogenic activity. M. grisea is grown as described for spore production on oatmeal agar media (Talbot et al, supra). Spores are harvested into water with 0.01% Tween 20 to a concentration of 5x10⁴ spores/ml and the culture is divided. Id. The test compound is added to one culture to a final concentration of 20-100 μg/ml. Solvent only is added to the second culture. Rice infection assays are performed using Indica rice cultivar CO39 essentially as described in Valent et al, supra). Two-week-old seedlings of cultivar CO39 are sprayed with 12ml of conidial suspension. The inoculated plants are incubated in a dew chamber at 27°C in the dark for 36 hours, and transferred to a growth chamber (27°C 12 hours/21°C 12 hours at 70% humidity) for an additional 5.5 days. Leaf samples are examined at 5 days post-inoculation to determine the extent of pathogenicity as compared to the control samples. Alternatively, antipathogenic activity can be assessed using an excised leaf pathogenicity assay. Spore suspensions are prepared in water only to a concentration of 5xl0⁴ spores/ml and the culture is divided. The test compound is added to one culture to a final concentration of 20-100μg/ml. Solvent only is added to the second culture. Detached leaf assays are performed by excising 1 cm segments of rice leaves from Indica rice cultivar CO39 and placing them on 1%> agarose in water. lOμl of each spore suspension is place on the leaf segments and the samples are incubated at 25°C for 5 days in the dark. Leaf samples are examined at 5 days post-inoculation to determine the extent of pathogenicity as compared to the control samples.

Example 22 Assays for Testing Compounds for Alteration of PDE2 Gene Expression Magnaporthe grisea fungal cells are grown under standard fungal growth conditions that are well known and described in the art. Wild-type M. grisea spores are harvested from cultures grown on complete agar or oatmeal agar media after growth for 10-13 days in the light at 25°C using a moistened cotton swab. The concentration of spores is determined using a hemacytometer and spore suspensions are prepared in a minimal growth medium to a concentration of 2x10⁵ spores per ml. 25ml cultures are prepared to which test compounds will be added at various concentrations. A culture with no test compound present is included as a control. The cultures are incubated at 25°C for 3 days after which test compound or solvent only control is added. The cultures are incubated an additional 6hr. Fungal mycelia is harvested by filtration through Miracloth (CalBiochem, La Jolla, CA), washed with water, and frozen in liquid nitrogen. Total RNA is extracted with TRIZOL Reagent using the methods provided by the manufacturer (Life Technologies, Rockville, MD). Expression is analyzed by Northern analysis of the RNA samples as described (Sambrook et al, supra) using a radiolabeled fragment of the PDE2 encoding nucleic acid as a probe. Test compounds resulting in an altered level of PDE2 mRNA relative to the untreated control sample are identified as candidate antibiotic compounds. Test compounds identified as inhibitors of PDE2 gene expression are further tested for antibiotic activity by measuring the effect of the test compound on Magnaporthe grisea growth and/or pathogenicity as described above in Example 21. Example 23 In Vivo Cell Based Assay Screening Protocol with a Fungal Strain Containing a Mutant Form of PDE2 with Reduced or No Activity The effect of test compounds on the growth and/or pathogenicity of wild-type and mutant fungal cells having a mutant PDE2 gene is measured and compared as follows. Magnaporthe grisea fungal cells containing a mutant form of the PDE2 gene that lacks activity, for example a PDE2 gene containing a transposon insertion, are grown under standard fungal growth conditions that are well known and described in the art. The effect of test compounds on the pathogenicity of wild-type fungal cells and mutant fungal cells having a mutant PDE2 gene is measured and compared as follows. Magnaporthe grisea fungal cells containing a mutant form of the PDE2 gene that lacks activity, for example a PDE2 gene containing a transposon insertion, are grown under standard fungal growth conditions that are well known and described in the art. Magnaporthe grisea spores are harvested from cultures grown on complete agar medium after growth for 10- 13 days in the light at 25°C using a moistened cotton swab. The concentration of spores for each strain is determined using a hemacytometer and spore suspensions are prepared to a concentration of 5xl0⁴ spores per ml in 0.01%> Tween 20. Spore suspensions for each strain are divided and test compounds (at varying concentrations) are added one of the suspensions. Solvent only is added to the second suspension. Two-week-old rice seedlings of cultivar CO39 are sprayed with the spore suspensions until just before runoff. The inoculated plants are incubated in a dew chamber at 27°C in the dark for 36 hours, and transferred to a growth chamber (27 °C 12 hours/21 °C 12 hours at 70% humidity) for an additional 5.5 days. Leaf samples are examined for signs of successful infection (i.e. formation of lesions) at 3, 5, and 7 days post-inoculation. The effect of each of the test compounds on pathogenicity for the mutant and wild-type strains relative to the solvent controls is compared. Compounds that show differential degrees of pathogenicity between the mutant and the wild-type strains relative to the solvent controls (e.g. differences in lesion number, lesion size, or the competency of a lesion to sporulate) are identified as potential fungicidal compounds. For example, a reduction in the pathogenicity of the wild-type strain but not the mutant strain in the presence relative to the absence of the test compound suggests that the target of the test compound is the PDE2 gene product. The effect of test compounds on the growth of wild-type fungal cells and mutant fungal cells having a mutant PDE2 gene is measured and compared as follows. Magnaporthe grisea spores are harvested from cultures grown on complete agar medium after growth for 10-13 days in the light at 25°C using a moistened cotton swab. The concentration of spores is determined using a hemacytometer and spore suspensions are prepared in a minimal growth medium to a concentration of 2xl0⁵ spores per ml. Approximately 4 l0⁴ spores are added to each well of 96-well plates to which a test compound is added (at varying concentrations). The total volume in each well is 200μl. Wells with no test compound present (growth control), and wells without cells are included as controls (negative control). The plates are incubated at 25°C for seven days and optical density measurements at 590nm are taken daily. Wild-type cells are screened under the same conditions. The effect of each of the test compounds on the mutant and wild-type fungal cells is measured against the growth control and the percent of inhibition is calculated as the OD₅₉o (fungal strain plus test compound)/OD₅₉o (growth control) x 100. The percent of growth inhibition in the presence of the test compound on the mutant and wild-type fungal strains are compared. Compounds that show differential growth inhibition between the mutant and the wild-type cells are identified as potential antifungal compounds. Similar protocols may be found in Kirsch & DiDomenico, 26 Biotechnology 177 (1994) (PMID: 7749303)). Published references and patent publications cited herein are incorporated by reference as if terms incorporating the same were provided upon each occurrence of the individual reference or patent document. While the foregoing describes certain embodiments of the invention, it will be understood by those skilled in the art that variations and modifications maybe made that will fall within the scope of the invention. The foregoing examples are intended to exemplify various specific embodiments of the invention and do not limit its scope in any manner.

Claims

CLAIMSWhat is claimed is:

1. A method for identifying a test compound as a candidate for an antibiotic, comprising: a) contacting a polypeptide with a test compound, wherein said polypeptide is selected from the group consisting of: i) a non-fungal adenylosuccinate synthase polypeptide; ii) a fimgal adenylosuccinate synthase polypeptide, iii) a Magnaporthe adenylosuccinate synthase polypeptide; iv) SEQ DD NO:3; v) a polypeptide consisting essentially of SEQ DD NO:3; vi) a polypeptide having at least ten consecutive amino acids of SEQ DD NO:3; vii) a polypeptide having at least 50%> sequence identity with SEQ DD NO:3 and at least 10% of the activity of SEQ ID NO:3; and viii) a polypeptide consisting of at least 50 amino acids having at least 50% sequence identity with SEQ DD NO:3 and at least 10% of the activity of SEQ ID NO:3; and b) detecting the presence or absence of binding between the test compound and the adenylosuccinate synthase polypeptide, wherein binding indicates that the test compound is a candidate for an antibiotic.

2. A method for identifying a test compound as a candidate for an antibiotic, comprising: a) contacting a polypeptide with a test compound, wherein said polypeptide is selected from the group consisting of: i) non-fungal PDE2 polypeptide; ii) a fungal PDE2 polypeptide; iii) a Magnaporthe PDE2 polypeptide; iv) SEQ DD NO:5; v) a polypeptide consisting essentially of SEQ ID NO:5 ; vi) a polypeptide having at least ten consecutive amino acids ofSEQ DD NO:5; vii) a polypeptide having at least 38% sequence identity with SEQ DD NO:3 and at least 10% of the activity of SEQ ED NO:5; and viii) a polypeptide consisting of at least 50 amino acids having at least 50% sequence identity with SEQ DD NO:5 and at least 10% of the activity of SEQ ED NO:5; and b) detecting the presence or absence of binding between the test compound and the PDE2 polypeptide, wherein binding indicates that the test compound is a candidate for an antibiotic.

3. A method for identifying a test compound as a candidate for an antibiotic, comprising: a) contacting GTP, IMP, and L-aspartate with a polypeptide in the presence and absence of a test compound or contacting GDP, phosphate, and N6-(l,2- dicarboxyethyl)-AMP with an adenylosuccinate synthase in the presence and absence of a test compound, wherein said polypeptide is selected from the group consisting of: i) a non-fungal adenylosuccinate synthase polypeptide; ii) a fungal adenylosuccinate synthase polypeptide; iii) a Magnaporthe adenylosuccinate synthase polypeptide; iv) SEQ ID NO:3; v) a polypeptide consisting essentially of SEQ DD NO:3 ; vi) a polypeptide having at least ten consecutive amino acids of SEQ DD NO:3; vii) a polypeptide having at least 50% sequence identity with SEQ DO NO:3 and at least 10% of the activity of SEQ ID NO:3; and viii) a polypeptide consisting of at least 50 amino acids having at least 50% sequence identity with SEQ DD NO:3 and at least 10% of the activity of SEQ f NO:3; and b) determining a concentration for at least one of GTP, IMP, L- aspartate, GDP, phosphate, and/or N6-(l,2-dicarboxyethyl)-AMP in the presence and absence of the test compound, wherein a change in the concentration for any of GTP, IMP, L-aspartate, GDP, phosphate, and/or N6-(l,2-dicarboxyethyl)-AMP indicates that the test compound is a candidate for an antibiotic.

4. A method for identifying a test compound as a candidate for an antibiotic, comprising: a) contacting a nucleoside 3',5'-cyclic phosphate substrate with a polypeptide in the presence and absence of a test compound, under conditions suitable for the polypeptide to convert the nucleoside 3',5'-cyclic phosphate substrate to a nucleoside 5'-phosphate product, wherein said polypeptide is selected from the group consisting of: i) a non-fungal PDE2 polypeptide; ii) a fungal PDE2 polypeptide; iii) a Magnaporthe PDE2 polypeptide; iv) SEQ D NO:5; v) a polypeptide consisting essentially of SEQ DD NO:5; vi) a polypeptide having at least ten consecutive amino acids ofSEQ DD NO:5; vii) a polypeptide having at least 38%> sequence identity with SEQ ID NO:5 and at least 10% of the activity of SEQ TD NO:5; and viii) a polypeptide consisting of at least 50 amino acids having at least 50% sequence identity with SEQ ED NO:5 and at least 10% of the activity of SEQ ID NO:5; and b) comparing the concentration of the nucleoside 3',5'-cyclic phosphate substrate and/or the nucleoside 5'-phosphate product in the presence and absence of the test compound, wherein a difference in the presence of the test compound, relative to the absence indicates that the test compound is a candidate for an antibiotic.

5. The method of claim 4, wherein the nucleoside 3',5'-cyclic phosphate substrate is cAMP.

6. A method for identifying a test compound as a candidate for an antibiotic, comprising: a) measuring the expression of an adenylosuccinate synthase in an organism, or a cell or tissue thereof, in the presence and absence of a test compound; and b) comparing the expression of the adenylosuccinate synthase in the presence and absence of the test compound, wherein an altered expression in the presence of the test compound indicates that the test compound is a candidate for an antibiotic.

7. A method for identifying a test compound as a candidate for an antibiotic, comprising: a) measuring the expression of a PDE2 in an organism, or a cell or tissue thereof, in the presence and absence of a test compound; and b) comparing the expression of a PDE2 in the presence and absence of the test compound, wherein an altered expression in the presence of the test compound indicates that the test compound is a candidate for an antibiotic.

8. The method of claim 6 or 7, wherein the organism is a fungus.

9. The method of claim 6 or 7, wherein the organism is Magnaporthe.

10. The method of claim 6 or 7, wherein the adenylosuccinate synthase is

SEQ ΓD NO-.3.

11. The method of claim 6 or 7, wherein expression is measured by at least one of the following methods: detecting expressed mRNA, detecting expressed polypeptide, and detecting expressed polypeptide enzyme activity.

12. A method for identifying a test compound as a candidate for an antibiotic comprising: a) providing a fungal organism having a first form of an adenylosuccinate synthase; b) providing a fungal organism having a second form of the adenylosuccinate synthase, wherein one of the first or the second form of the adenylosuccinate synthase has at least 10% of the activity of SEQ ID NO:3; and c) determining the growth of the organism having the first form of the adenylosuccinate synthase and the organism having the second form of the adenylosuccinate synthase in the presence of a test compound, wherein a difference in growth between the two organisms in the presence of the test compound indicates that the test compound is a candidate for an antibiotic.

13. The method of claim 12, wherein the fungal organism having the first form of the adenylosuccinate synthase and the fungal organism having the second form of the adenylosuccinate synthase axe Magnaporthe; wherein the first form of the adenylosuccinate synthase is selected from the group consisting of: fungal adenylosuccinate synthases, SEQ DD NO:l, and SEQ DD NO:2; and wherein the second form of the adenylosuccinate synthase is selected from the group consisting of: fungal adenylosuccinate synthases, SEQ DD NO:l, and SEQ DD NO:2, a heterologous adenylosuccinate synthase, SEQ DD NO:l comprising a transposon insertion that reduces or abolishes adenylosuccinate synthase activity, and SEQ DD NO:2 comprising a transposon insertion that reduces or abolishes adenylosuccinate synthase activity.

14. A method for identifying a test compound as a candidate for an antibiotic, comprising: a) providing a fungal organism having a first form of an adenylosuccinate synthase; b) providing a fungal organism having a second form of the adenylosuccinate synthase, wherein one of the first or the second form of the adenylosuccinate synthase has at least 10% of the activity of SEQ DD NO:3; and c) determining the pathogenicity of the organism having the first form of the adenylosuccinate synthase and the organism having the second form of a adenylosuccinate synthase in the presence of a test compound, wherein a difference in pathogenicity between the two organisms in the presence of the test compound indicates that the test compound is a candidate for an antibiotic.

15. The method of claim 14, wherein the fungal organism having the first form of the adenylosuccinate synthase and the fungal organism having the second form of the adenylosuccinate synthase are Magnaporthe; wherein the first form of the adenylosuccinate synthase is selected from the group consisting of: fungal adenylosuccinate synthases, SEQ DD NO:l, and SEQ DD NO:2; and wherein the second form of the adenylosuccinate synthase is selected from the group consisting of: fungal adenylosuccinate synthases, SEQ ED NOT, and SEQ TD NO:2, a heterologous adenylosuccinate synthase, SEQ DD NOT comprising a transposon insertion that reduces or abolishes adenylosuccinate synthase activity, and SEQ DD NO:2 comprising a transposon insertion that reduces or abolishes adenylosuccinate synthase activity.

16. A method for identifying a test compound as a candidate for an antibiotic, comprising: a) providing a fungal organism having a first form of a gene in the purine biosynthetic pathway; b) providing a fungal organism having a second form of said gene in the purine biosynthetic pathway, wherein one of the first or the second form of the gene has at least 10% of the activity of a corresponding Magnaportha grisea gene; and c) determining the growth of the organism having the first form of the gene and the organism having the second form of the gene in the presence of a test compound, wherein a difference in growth between the two organisms in the presence of the test compound indicates that the test compound is a candidate for an antibiotic.

17. The method of claim 16, wherein the fungal organism having the first form of the gene and the fungal organism having the second form of the gene are Magnaporthe and i) the first form of the gene in the purine biosynthetic pathway is Magnaporthe grisea phosphoribosylglycinamide formyltransferase and the second form of the gene is selected from the group consisting of: a heterologous phosphoribosylglycinamide formyltransferase and Magnaporthe grisea phosphoribosylglycinamide formyltransferase comprising a transposon insertion that reduces or abolishes phosphoribosylglycinamide formyltransferase protein activity or ii) the first form of the gene in the purine biosynthetic pathway is Magnaporthe grisea adenylosuccinate lyase and the second form of the gene is selected from the group consisting of: a heterologous adenylosuccinate lyase and Magnaporthe grisea adenylosuccinate lyase comprising a transposon insertion that reduces or abolishes adenylosuccinate lyase protein activity.

18. A method for identifying a test compound as a candidate for an antibiotic, comprising: a) providing a fungal organism having a first form of a gene in the purine biosynthetic pathway; b) providing a fungal organism having a second form of said gene in the purine biosynthetic pathway, wherein one of the first or the second form of the gene has at least 10% of the activity of a corresponding Magnaportha grisea gene; and c) determining the pathogenicity of the organism having the first form of the gene and the organism having the second form of the gene in the presence of a test compound, wherein a difference in pathogenicity between the organism and the comparison organism in the presence of the test compound indicates that the test compound is a candidate for an antibiotic.

19. The method of claim 18, wherein the fungal organism having the first form of the gene and the fungal organism having the second form of the gene are Magnaporthe and i) the first form of the gene in the purine biosynthetic pathway is Magnaporthe grisea phosphoribosylglycinamide formyltransferase, and the second form of the gene is selected from the group consisting of: a heterologous phosphoribosylglycinamide formyltransferase and Magnaporthe grisea phosphoribosylglycinamide formyltransferase comprising a transposon insertion that reduces or abolishes phosphoribosylglycinamide formyltransferase protein activity or ii) the first form of the gene in the purine biosynthetic pathway is Magnaporthe grisea adenylosuccinate lyase and the second form of the gene is selected from the group consisting of: a heterologous adenylosuccinate lyase and Magnaporthe grisea adenylosuccinate lyase comprising a transposon insertion that reduces or abolishes adenylosuccinate lyase protein activity.

20. A method for identifying a test compound as a candidate for an antibiotic, comprising: a) providing paired growth media containing a test compound, wherein the paired growth media comprise a first medium and a second medium and the second medium contains a higher level of adenine than the first medium; b) inoculating the first and the second medium with an organism; and c) determining the growth of the organism, wherein a difference in growth of the organism between the first and second medium indicates that the test compound is a candidate for an antibiotic.

21. The method of claim 20, wherein the organism is a fungus.

22. The method of claim 20, wherein the organism is Magnaporthe.

23. A method for identifying a test compound as a candidate for an antibiotic, comprising: a) providing a fungal organism having a first form of a PDE2; b) providing a fungal organism having a second form of the PDE2, wherein one of the first or the second form of the PDE2 has at least 10% of the activity of SEQ DD NO:5; and c) determining the growth of the organism having the first form of the PDE2 and the organism having the second form of the PDE2 in the presence of a test compound, wherein a difference in growth between the two organisms in the presence of the test compound indicates that the test compound is a candidate for an antibiotic.

24. The method of claim 23, wherein the fungal organism having the first form of the PDE2 and the fungal organism having the second form of the PDE2 are Magnaporthe and the first and the second form of the PDE2 are fungal PDE2's.

25. The method of claim 23, wherein the first form of the PDE2 is SEQ ED NO:5.

26. The method of claim 23, wherein the fungal organism having the first form of the PDE2 and the fungal organism having the second form of the PDE2 are Magnaporthe and the first form of the PDE2 is SEQ BD NO:5.

27. The method of claim 23, wherein the fungal organism having the first form of the PDE2 and the fungal organism having the second form of the PDE2 are Magnaporthe, the first form of the PDE2 is SEQ ED NO:5, and the second form of the PDE2 is a heterologous PDE2.

28. The method of claim 23, wherein the fungal organism having the first form of the PDE2 and the fungal organism having the second form of the PDE2 are Magnaporthe, the first form of the PDE2 is SEQ DD NO:5, and the second form of the PDE2 is SEQ ED NO: 5 comprising a transposon insertion that reduces or abolishes PDE2 activity.

29. A method for identifying a test compound as a candidate for an antibiotic, comprising: a) providing a fungal organism having a first form of a PDE2; b) providing a fungal organism having a second form of the PDE2, wherein one of the first or the second form of PDE2 has at least 10% of the activity of SEQ HD NO:5; and c) determining the pathogenicity of the organism having the first form of the PDE2 and the organism having the second form of the PDE2 in the presence of a test compound, wherein a difference in pathogenicity between the two organisms in the presence of the test compound indicates that the test compound is a candidate for an antibiotic.

30. The method of claim 29, wherein the fungal organism having the first form of the PDE2 and the fungal organism having the second form of the PDE2 are Magnaporthe and the first and the second form of the PDE2 are fungal PDE2's.

31. The method of claim 29, wherein the first form of the PDE2 is SEQ DD NO:5.

32. The method of claim 29, wherein the fungal organism having the first form of the PDE2 and the fungal organism having the second form of the PDE2 are Magnaporthe and the first form of the PDE2 is SEQ ED NO:5.

33. The method of claim 29, wherein the fungal organism having the first form of the PDE2 and the fungal organism having the second form of the PDE2 are Magnaporthe, the first form of the PDE2 is SEQ DD NO:5, and the second form of the PDE2 is a heterologous PDE2.

34. The method of claim 29, wherein the fungal organism having the first form of the PDE2 and the fungal organism having the second form of the PDE2 are Magnaporthe, the first form of the PDE2 is SEQ ED NO:5, and the second form of the PDE2 is SEQ DD NO:5 comprising a transposon insertion that reduces or abolishes PDE2 activity.