US20050119306A1

US20050119306A1 - Alkaloid that inhibits biosynthesis of mycotoxins and method for screening for mycotoxin inhibitors

Info

Publication number: US20050119306A1
Application number: US10/943,331
Authority: US
Inventors: Frances Trail; Raymond Hammerschmidt; John Linz; Haixin Xu; Luis Velasquez; Seanna Annis
Original assignee: Michigan State University MSU
Current assignee: Michigan State University MSU
Priority date: 1999-09-28
Filing date: 2004-09-17
Publication date: 2005-06-02
Also published as: US7041678B2; US20050026953A1; US6825216B1

Abstract

The present invention provides an alkaloid compound that is an inhibitor of mycotoxin biosynthesis. The alkaloid is an alkenyl piperidine amide wherein the alkenyl is a C18 alkenyl with one or more double bonds. The alkaloid inhibits transcription of the fungus genes nor-1, tri5, and ipnA. The present invention further provides a method for identifying compounds, which inhibit biosynthesis of aflatoxin in Aspergillus spp. and deoxynivalenol in Gibberella spp. without inhibiting growth of the fungus in vitro.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional application Ser. No. 60/156,381, which was filed Sep. 28, 1999.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

None.

BACKGROUND OF THE INVENTION

(1) Field of the Invention
The present invention relates to an alkaloid compound that inhibits biosynthesis of particular products of secondary metabolism. In particular, the present invention relates to an alkenyl piperidine amide wherein the alkenyl is a C18 alkenyl with one or more double bonds, which can be isolated from Piper nigrum, that inhibits transcription of fungus genes nor-1, tri5, ver-1, verA, fas-1a, omt-1, alfR and ipnA. The present invention further relates to a method for identifying compounds that inhibit the biosynthesis of mycotoxins in fungi. In particular, a method for identifying compounds that inhibit biosynthesis of aflatoxin in Aspergillus spp. and deoxynivalenol in Gibberella spp.
(2) Description of Related Art
Mycotoxins are a group of structurally heterogeneous secondary metabolites produced by a diverse group of fungal plants pathogens. Infestation of crops and food commodities by mycotoxin producing fungi is a serious problem in view of the immunosuppressive, carcinogenic, cytotoxic, and teratogenic effects of the compounds in humans and animals. One of the most economically important mycotoxins worldwide is aflatoxin, a polyketide produced by several Aspergillus spp. Aflatoxin is the best studied of the mycotoxins and much of the molecular biology of the biosynthetic pathway has been determined in Aspergillus flavus, Aspergillus parasiticus, and Aspergillus nidulans. Aspergillus flavus produces aflatoxin B1 and aflatoxin B2 whereas Aspergillus parasiticus produces in addition aflatoxin G1 and G2. Aspergillus nidulans, which is not considered to be an agricultural threat, has been used as a model genetic system for studies of aflatoxin biosynthesis because it produces sterigmatocystin, an aflatoxin precursor. The genes for aflatoxin biosynthesis are clustered in all three species. The molecular biology of aflatoxin biosynthesis is reviewed by Trail et al., in Microbiol. 141: 755-765 (1995). Aspergillus flavus and Aspergillus parasiticus are weak pathogens of corn, cotton, peanut, and nut crops: their effect is limited to a slight reduction in crop yield. However, the significant consequence of crops infected with either of these fungi is contamination by aflatoxin, which is produced under certain conditions during the infection. Traditional control strategies such as breeding crops for resistance to the fungi or chemical treatments of crops to prevent infection by the fungi have not been effective.
Aflatoxin is a secondary metabolite that appears to be the most potent naturally occurring carcinogen known (Council for Agricultural Science and technology (CAST), 1989). It is suspected of being responsible for the high incidence of human liver cancer in many areas of the world (Eaton and Gallagher, Ann. Rev. Pharmacol. Toxicol. 34: 135-139 (1994)). Aflatoxin is introduced into the food chain by preharvest and postharvest contamination of foods and feeds. Also, products from animals that have been fed aflatoxin contaminated feed may also become contaminated. Currently, the U.S. Food and Drug Administration limits the allowable amount of aflatoxin in food to 20 ppb, with slightly higher levels allowed in feeds. Because the level of aflatoxin in products destined for human consumption is strictly regulated in the U.S., aflatoxin contamination is primarily of economic importance. However, even though aflatoxin levels in foods is limited to 20 ppb, the effect of chronic exposure to low levels of aflatoxin on human health is unknown. Thus, some European countries require the presence of aflatoxin in foods intended for human consumption to be 0 ppb. In areas of the world where regulations do not exist, aflatoxin is a serious health problem (CAST, 1989).
Approaches to control of aflatoxin have been broadly grouped into preharvest and postharvest strategies. Proper grain storage can greatly reduce contamination postharvest, and some decontamination methods, while costly, are used, e.g., ammoniation. However, most research efforts at control of aflatoxin has been directed at the preharvest elimination of infection and contamination, since the ability to control preharvest contamination would reduce the need for postharvest elimination. Preharvest methods have included agricultural practices such as irrigation strategies designed to eliminate stress to crops associated with drought, which appears to increase production of aflatoxin by the fungus. Other methods include using regionally adapted varieties of crop plants. However, these methods have been expensive to implement and have not been completely effective. Chemical control methods have also been ineffective at controlling infection by these fungi.
The development of host plants that are resistant to Aspergillus infection and aflatoxin contamination has not been as successful as have programs for breeding resistance to other pathogens. In general, the resistant varieties that have been made are unstable from growing season to growing season and from region to region. Also, screening plants for resistance to colonization by Aspergillus spp. and aflatoxin contamination has been difficult. In corn, and frequently in cotton, inoculation methods have been difficult, often requiring wounding the plant to introduce the fungus, which may overwhelm the plants natural resistance reactions making it difficult to evaluate the plants resistance mechanisms (Cotty, Plant Dis. 73: 489-492 (1989)).
Methods have been developed for inhibiting mycotoxin production in crops. For example, U.S. Pat. No. 5,942,661 to Keller discloses a method of inhibiting mycotoxin production by introducing into the plant a gene encoding a lipoxygenase pathway enzyme of the mycotoxin. The method may produce transgenic plants that are substantially resistant to mycotoxin contamination. Mycotoxin resistance is further increased by introducing into the plant antisense genes for the 9-hyperoxide fatty acid producing lipoxygenases. However, reducing aflatoxin contamination by making transgenic plants resistant to aflatoxin production is expensive and time consuming, and since transformation efficiencies varies from plant species to plant species, the method may not be successful for all plant species. Furthermore, the long-term effect of introducing transgenic plants into the environment is unknown.
Since traditional methods for controlling fungal infection and/or production of aflatoxin by breeding, chemicals, or transgenic plants have not been completely effective, there is a need for an inexpensive and effective method for either controlling infection of crops by fungi such as Aspergillus spp. or Gibberella spp., or controlling the biosynthesis and accumulation of mycotoxins such as aflatoxin or deoxynivalenol in plants infected with fungi such as Aspergillus spp or Gibberella spp., respectively. There is also a need for a rapid and inexpensive method for identification of chemicals or compounds in natural extracts that inhibit production of mycotoxins such as aflatoxin and deoxynivalenol.

SUMMARY OF THE INVENTION

The present invention provides a substantially pure alkaloid compound that inhibits the biosynthesis of particular products of secondary metabolism. In particular, the present invention provides an alkenyl piperidine amide, which can be isolated from Piper nigrum. The alkenyl piperidine amide inhibits transcription from the nor-1 promoter, the tri5 promoter, ver-1 promoter, the verA promoter, the omt-1 promoter, the fas-1a promoter, alfR promoter, the ipnA promoter, and mutant thereof. In a preferred embodiment the alkenyl piperidine amide inhibits at least transcription of the nor-1 promoter of Aspergillus parasiticus, the tri5 promoter of Gibberella pulicaris or the ver-1 promoter of Aspergillus nidulans without killing the fungus in vitro. In a preferred embodiment, the alkenyl is a C18 alkenyl with one or more double bonds. Preferably, the C18 alkenyl has two to four double bonds.
The compound of the present invention is useful for inhibiting biosynthesis of a mycotoxin by a fungus growing on a plant material. In particular, the compound inhibits the biosynthesis of aflatoxin and deoxynivalenol. Thus, the present invention provides a formulation which comprises as an active ingredient an alkenyl piperidine amide wherein the alkenyl is a C18 alkenyl with one or more double bonds, or its salt, or its ester, associated with one or more acceptable carriers, excipients or vehicles therefore. Preferably, the C18 alkenyl has two to four double bonds.
Thus, the present invention provides the use of a compound, which is an alkenyl piperidine amide, wherein the alkenyl is a C18 alkenyl with one or more double bonds for treatment of plant material to inhibit mycotoxin production by a fungus. Further, the present invention provides for use of the above compound for the preparation of a composition for treatment of a plant material to inhibit mycotoxin production by a fungus. In particular, wherein the plant material is selected from the group consisting of seeds, nuts, and animal feeds.
The present invention further provides a method for inhibiting mycotoxin biosynthesis by a fungus in a plant material comprising applying an effective amount of an alkenyl piperidine amide in a carrier to the plant material wherein the compound inhibits biosynthesis of the mycotoxin. In a preferred embodiment, the alkenyl is a C18 alkenyl with one or more double bonds, preferably, two to four double bonds. In the method, the alkenyl piperidine amide can be provided in the carrier at a concentration between about 1 and 100 μg/ml. In a preferred application, the plant material is selected from the group consisting of seeds, nuts, grains, and animal feeds.
Because the alkaloid of the present invention is able to inhibit the biosynthesis of mycotoxins, it would be useful to provide transgenic plants that are able to synthesize the alkaloid of the present invention. Therefore, the present invention further provides a transgenic plant that contains DNA comprising genes that encode enzymes involved in biosynthesis of the alkenyl piperidine amide wherein the compound synthesized by the transgenic plant inhibits biosynthesis of the mycotoxin by the fungus. In a preferred embodiment, transgenic plant produces an alkenyl piperidine amide wherein the alkenyl is a C18 alkenyl with one or more double bonds, preferably, two to four double bonds.
The present invention further provides a method for identifying compounds that inhibit biosynthesis of a product of secondary metabolism such as a mycotoxin in a fungus. In particular, a method is provided for identifying compounds that inhibit biosynthesis of aflatoxin by Aspergillus spp. and biosynthesis of deoxynivalenol by Gibberella spp.
Therefore, the present invention provides a method for determining whether a compound inhibits biosynthesis of a secondary metabolite comprising providing a culture of a transgenic fungus comprising a reporter gene operably linked to a promoter for a gene involved in the biosynthesis of the secondary metabolite, providing to the culture the compound to be determined, incubating the culture containing the extract under conditions that cause biosynthesis of the secondary metabolite, and measuring expression of the reporter gene wherein absence of expression of the reporter gene indicates that the compound inhibits biosynthesis of the secondary metabolite.
The present invention further provides a method for identifying and isolating a compound in a material that inhibits the biosynthesis of a secondary metabolite of a fungus comprising providing an extract of the material, separating the material into fractions or compounds by a chromatography method, providing a spore suspension of a transgenic fungus comprising a reporter gene operatively linked to a promoter that is the same as the promoter that controls transcription of a gene involved in biosynthesis of the secondary metabolite, adding the spore suspension to the separated compounds, allowing the spores to germinate and grow fungi, and detecting expression of the reporter gene wherein absence of expression of the reporter gene identifies the fractions or compounds that inhibits biosynthesis of the secondary metabolite. In a preferred embodiment of the method, the chromatography is thin layer chromatography (TLC) using TLC plates. Preferably, to grow the fungi the TLC plates are incubated in a dark moist atmosphere at 30° C. for a time sufficient for the fungi to cover the plate. It is further preferable that the fungi be lysed by freeze-thawing to release the reporter expression product.
In a preferred embodiment of the present invention, the secondary metabolite is a mycotoxin, preferably selected from the group consisting of aflatoxin, deoxynivalenol, or sterigmatocystin. In practicing the present invention, it is preferable that the transgenic fungus be a fungus selected from the group consisting of Aspergillus parasiticus, Aspergillus nidulans, Aspergillus versicolor, Aspergillus flavus, Gibberella pulicaris, and Gibberella zeae. In the present invention, it is preferable that the reporter gene be operably linked to a promoter that is selected from the group consisting of nor-1 promoter, ver-1 promoter, verA promoter, fas-1a promoter, omt-1 promoter, alfR promoter, ipnA promoter, tri5 promoter, and mutant thereof. In an embodiment further still, it is preferable that the reporter gene be selected from the group consisting of a gene encoding β-glucuronidase, a gene encoding β-galactosidase, a gene encoding luciferase, and a gene encoding fluorescence green protein. In an embodiment further still, a transgenic fungus is provided that comprises a reporter gene operatively linked to a constitutive promoter, which provides a control for the method. Preferably, the constitutive promoter is the promoter for the benA gene or mutant thereof.
Therefore, it is an object to provide a compound that inhibits transcription of one or more genes encoding proteins involved in secondary metabolism of fungi. In particular, compounds that inhibit genes involved in biosynthesis of mycotoxins.
It is also an object of the present invention to provide a method for determining whether an extract comprises compounds that inhibit transcription of one or more genes encoding a protein involved in secondary metabolism of fungi. In particular, compounds that inhibit genes involved in biosynthesis of mycotoxins.
Further still, it is an object of the present invention to provide a method for identifying and purifying compounds that inhibit transcription of one or more genes encoding a protein involved in secondary metabolism of fungi. In particular, compounds that inhibit genes involved in biosynthesis of mycotoxins.
These and other objects of the present invention will become increasingly apparent with reference to the following drawings and preferred embodiments.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a TLC plate with resolved crude pepper extract were coated with Aspergillus parasiticus G5 (nor-1-GUS fusion) in lane A or GAPN2 (benA-GUS fusion in lane B. Plates were subsequently coated with the GUS substrate X-gluc. The blue areas indicate GUS activity and the white areas indicate inhibition of GUS activity by the underlying resolved compounds. Fungal growth was seen in the area corresponding to compound Cp2. An unidentified fungitoxic compound inhibited fungal growth in both strains (arrow). The TLC solvent front is indicated by the arrowhead.
FIG. 2A shows the effect of various concentrations of Cp2 on aflatoxin biosynthesis in Aspergillus parasiticus. Samples were taken 48 hours after the addition of Cp2 to GMS medium containing the fungus and the aflatoxin components resolved on a TLC plate. The TLC plate was visualized under UV light. Lane 1, control; lane 2, extract from fungus grown in medium containing 2.6 μg/ml Cp2; lane 3, extract from fungus grown in medium containing 26 μg/ml Cp2; lane 4, extract from fungus grown in medium containing 39 μg/ml Cp2; lane 5, extract from fungus grown in medium containing 52 μg/ml Cp2; lane 6, extract from fungus grown in medium containing 78 μg/ml Cp2.
FIG. 2B shows the effect of various concentrations of Cp2 on accumulation of nor-1 transcripts in wild-type Aspergillus parasiticus SU-1. The lower panel is an agarose gel of RNA extracted from SU-1 grown at various concentrations of Cp2. The upper panel is a Northern analysis of the same gel probed with a labeled nucleotide probe to the nor-1 gene. Lanes 1-3 are samples analyzed 36 hours after Cp2 was added to the medium and lanes 4-8 are samples analyzed 48 hours after Cp2 was added to the medium. Lane 1, 36 hour control; lane 2, extract from fungus grown for 36 hours in medium containing 39 μg/ml Cp2; lane 3, extract from fungus grown for 36 hours in medium containing 52 μg/ml Cp2; Lane 4, 48 hour control; lane 5, extract from fungus grown for 48 hours in medium containing 2.6 μg/ml Cp2; lane 6, extract from fungus grown for 48 hours in medium containing 26 μg/ml Cp2; lane 7, extract from fungus grown for 48 hours in medium containing 52 μg/ml Cp2.
FIG. 3A shows the effect of Cp2 and piperine on the expression of GUS operably linked to the ipnA promoter in a transgenic Aspergillus nidulans 48, 72, and 96 hours after addition to the medium. The GUS substrate was MUG (4-methylumbelliferyl glucuronide) and the samples were visualized using UV light.
FIG. 3B shows the effect of Cp2 and piperine on expression of GUS operably linked to the tri5 promoter in a transgenic Gibberella zeae 96 hours after addition to the medium. The GUS substrate was MUG and the samples were visualized using UV light.
FIG. 4A shows a schematic diagram of a restriction enzyme map of plasmid pAPGUSN.
FIG. 4B shows a schematic diagram of a restriction enzyme map of plasmid pAPGUSNN. Plasmid pPAPGUSNNA contains the niaD gene in a clockwise orientation, and PAPGUSSNB contains the niaD gene in a counter-clockwise orientation.
FIG. 4C shows a schematic diagram of a restriction enzyme map of plasmid pHD6-6.
FIG. 4D shows a schematic diagram of a restriction enzyme map of plasmid pGAP2.
FIG. 5A shows time course assays on the expression of aflatoxin biosynthesis and GUS activity in parental Aspergillus parasiticus strain C2N.
FIG. 5B shows time course assays on the expression of aflatoxin biosynthesis and GUS activity in strain G5 containing plasmid pAPGUSN.
FIG. 6 shows GUS expression under the control of the nor-1 promoter in mycelia growing in peanut testa. Aspergillus parasiticus C2N was the fungal strain transformed with pAPGUSN, and the peanut pods comprising the testa were stained with X-Gluc.
FIG. 7A shows transformed fungus strain G5 (containing pAPGUSN) grown without the addition of a pepper extract. Mycelia were harvested, macerated, and extracts were used to detect GUS activity using the GUS substrate MUG. The photograph was taken under UV illumination. Shown are three independent experiments without the pepper extract.
FIG. 7B shows transformed fungus strain G5 grown with the addition of a pepper extract. Mycelia were harvested, macerated, and extracts were used to detect GUS activity using the GUS substrate MUG. The photograph was taken under UV illumination. Shown are three independent experiments with the pepper extract.
FIG. 8 shows the genomic DNA of SEQ ID NO:1, which encodes the nor-1 gene from Aspergillus parasiticus (GenBank accession M27801). The gene spans nucleotides 269-1501 and the exons consists of nucleotides 269-317, 424-961, 1020-1119, and 1184-1258.
FIG. 9 shows the genomic DNA of SEQ ID NO:2, which encodes the ver-1 gene from Aspergillus parasiticus (GenBank accession M91369). The gene consists of nucleotides 396-1526 and the exons consist of nucleotides 496-822, 873-1196, and 1258-1395.
FIG. 10 shows the cDNA of SEQ ID NO:3, which encodes the omt-1 gene from Aspergillus parasiticus (GenBank accession L22091). The coding region consists of nucleotides 12-1268.
FIG. 11 shows the genomic DNA of SEQ ID NO:5, which encodes the aflR gene from Aspergillus parasiticus (GenBank accession L26220). The gene consists of nucleotides 224-2379 and the coding region consists of nucleotides 418-1551.
FIG. 12 shows the genomic DNA of SEQ ID NO:6, which encodes the verA gene from Emericella nidulans (GenBank accession L27825). The gene spans nucleotides 555-1449 and consists of three exons, which span 555-887, 939-1262, and 1312-1449.
FIG. 13 shows the genomic DNA of SEQ ID NO:7, which encodes the Tri5 gene from Gibberella pulicaris (GenBank accession M64348). The gene spans nucleotides 401-1612 and consists of two exons, which span 401-869 and 930-1612.
FIG. 14 shows the genomic DNA of SEQ ID NO:8, which encodes the bena gene from Aspergillus flavus (GenBank accession M38265). The gene spans nucleotides 207-2121 and consists of eight exons, which span 207-218, 347-370, 440-466, 578-619, 701-754, 817-1607, 1672-2031, and 2085-2121.

DESCRIPTION OF PREFERRED EMBODIMENTS

All patents., patent applications, and literature references cited in this specification are hereby incorporated herein by reference in their entirety. In case of conflict, the present description, including definitions, will control.
Provided herein is a method for determining whether a compound or an extract can inhibit biosynthesis of a secondary metabolite in a fungus. In particular, a method for identifying and purifying compounds that inhibit transcription of promoters that regulate transcription of genes encoding proteins, enzymes, or regulatory factors that are involved in biosynthesis of mycotoxins. As demonstrated herein the method is particularly useful for identifying and isolating compounds that inhibit biosynthesis of aflatoxin or deoxynivalenol. A novel aspect of the method is that it enables compounds that inhibit biosynthesis of mycotoxins to be distinguished and separated from compounds that inhibit growth of fungi.
The method disclosed herein enabled identification and isolation of an alkaloid from Piper nigrum (pepper), which is an alkenyl piperidine amide wherein the alkenyl group is a C18 alkenyl with two or more double bonds, that inhibits biosynthesis of aflatoxin and deoxynivalenol. In one embodiment, the alkaloid compound has the structure

wherein the compound inhibits biosynthesis of a mycotoxin produced by a fungus. The position of the double bonds in any one of the species of the present invention is determined by the method of Cahoon et al., Proc. Natl. Acad. Sci. USA 89: 11184-11188 (1992).
The alkaloid of the present invention, designated Cp2, inhibits transcription from the nor-1 promoter, the ver-1 promoter, the tri5 promoter, and the ipnA promoter. These are all promoters for fungus genes that encode enzymes involved in biosynthesis of secondary metabolism products. The nor-1 and ver-1 genes encode enzymes involved in biosynthesis of aflatoxin, the tri5 gene encodes an enzyme involved in biosynthesis of deoxynivalenol, and ipnA encodes an enzyme involved in biosynthesis of penicillin. While Cp2 inhibits biosynthesis of the above secondary metabolites, Cp2 does not inhibit growth of the fungus in vitro. Thus, the inhibitory activity of Cp2 is distinguishable from piperine, which is also an alkaloid that is isolatable from Piper nigrum, because piperine inhibits fungus growth and transcription from the tri5 promoter but not transcription from the ipnA promoter.
Cp2 is useful as an inhibitor of mycotoxin biosynthesis by fungi and, therefore, is useful for preventing the contamination of plant material with mycotoxins. Cp2 is particularly useful for preventing contamination of the plant material with aflatoxin or deoxynivalenol. Plant material includes, but is not limited to, grains, nut products such as peanuts, or animal feeds. Cp2 can be isolated from pepper extracts or it can be produced by chemical synthesis. To prevent mycotoxin biosynthesis in a plant material contaminated with a fungus, Cp2 is admixed with the plant material in an amount sufficient to inhibit the contaminating fungus from producing its mycotoxin, in particular, inhibit the fungus from producing aflatoxin or deoxynivalenol. A carrier or solution comprising Cp2 can be used in a spray solution for treating the plant material, in dry admixture with a carrier that is ingestible, or in a wash solution for washing the plant material. The Cp2 concentration in the solution can be between about 1 and 100 μg/ml. In culture, Cp2 at a concentration of about 52 μg/ml or more was shown to completely inhibit transcription from the nor-1 promoter. Under particular conditions, the concentration of Cp2 in the solution can be less than 1 μg/ml. Treating the plant material with Cp2 enables the plant material to be stored for an extended period of time with reduced risk that the plant material will become contaminated with a mycotoxin. Thus, Cp2, which inhibits Aspergillus spp. and Gibberella spp. from producing aflatoxin and deoxynivalenol, respectively, when applied to the plant material, reduces the risk that stored plant material will be contaminated with aflatoxin or deoxynivalenol. Furthermore, it has been reported that in vivo mycotoxins perform an essential role in the ability of the fungus in invading and colonizing plant tissues. Thus, the present invention not only inhibits mycotoxin biosynthesis but can further prevent fungal growth on the plant material. The Cp2 is particularly useful inhibitor of mycotoxin and fungal growth in vivo because it inhibits transcription of several of the genes involved in mycotoxin biosynthesis simultaneously. The simultaneous inhibition of transcription of several genes indicates that fungi may be less able to mutate around the inhibitory effect of Cp2 than they would be in the case of an inhibitor directed against a single gene target. Thus, Cp2 can be used to prevent mycotoxin contamination of plant material with a reduced risk that fungus mutants would arise that are resistant to Cp2 than with mycotoxin inhibitors that are directed against a single gene or gene product.
The genes encoding the enzymes involved in Cp2 biosynthesis can be isolated and used to produce transgenic plants wherein the Cp2 is produced in the seed, nut, or grain of the plant for control of mycotoxin biosynthesis by fungi growing on the seeds, nuts, or grain. These genes can also be used to transform commercial strains of fungus to control the synthesis of undesirable secondary metabolites in pharmaceutical or food fermentations. These genes can also be used to transform bacteria to enable biosynthesis of Cp2 by commercial fermentation methods.
As demonstrated by the identification and isolation of Cp2, the present invention provides a bioassay for identifying extracts and particular compounds within the extract that inhibit the biosynthesis of mycotoxins that include, but are limited to, aflatoxin, sterigmatocystin, or deoxynivalenol. In a preferred embodiment, chromatography is used to separate the compounds in the extract, which are then tested as disclosed herein for the ability to inhibit a promoter for a gene involved in the biosynthesis of the mycotoxin. In particular, a promoter selected from the group consisting of the nor-1 promoter, the ver-1 promoter, the verA promoter, the fas-1a promoter, the omt-1 promoter, the alfR promoter, the ipnA promoter, and the tri5 promoter.
To determine whether an extract contains at least one compound that inhibits mycotoxin biosynthesis in a fungus, or to identify and purify the inhibitory compound, a transgenic fungus is provided, which comprises a reporter gene operably linked to a promoter to a gene involved in mycotoxin biosynthesis. The transgenic fungus is grown with the extract in a culture under conditions that stimulate biosynthesis of the mycotoxin. Optionally, the method provides that several cultures are provided, each containing a transgenic fungus with one of the aforementioned promoters operably linked to a reporter gene. It is preferable that a control culture is also provided, which contains the same transgenic fungus grown in the absence of the extract, and control cultures that contain a transgenic fungus comprising a reporter gene operably linked to a promoter involved in primary metabolism, grown in cultures both with the extract and without the extract. When an extract contains a compound that inhibits mycotoxin biosynthesis, transcription of the reporter gene is inhibited when the transgenic fungus is grown with the extract but is not inhibited when grown in the absence of the extract. In contrast, the control transgenic fungus containing the reporter gene operably linked to a primary metabolism promoter is not inhibited by the extract. Determination of whether transcription of the reporter gene is inhibited or not is accomplished by adding an indicator substrate that enables detection of reporter activity. Preferably, the fungi are lysed prior to addition of the indicator substrate. A suitable method for lysing the fungi is freeze-thawing to break down the cell membranes to allow the reporter to leak out of the fungi. Alternatively, the fungi can be harvested and protein extracts made by methods well known in the art, and the protein extracts tested for reporter activity.
To further identify and isolate a compound in an extract that inhibits a promoter controlling transcription of a gene involved in biosynthesis of a mycotoxin, the compounds of the extract are separated into compounds, components, or fractions by a method such as column or thin-layer chromatography (TLC) chromatography. Preferably, the chromatography method is thin layer chromatography and separation of the extracts into compounds, fractions, or components is accomplished using various developing solvents, which are well known in the art. The separated compounds, fractions, or components are incubated as above with the transgenic fungus comprising a reporter gene operably linked to a promoter involved in mycotoxin biosynthesis under conditions that stimulate biosynthesis of the mycotoxin. A control transgenic fungus comprising a reporter gene operably linked to a promoter involved in primary metabolism is also provided in a control incubation. Inhibition of transcription of the reporter gene controlled by the mycotoxin promoter but not the primary metabolism promoter indicates that the compound, fraction, or component inhibits mycotoxin biosynthesis in the fungus. Inhibition of reporter transcription is determined by detecting reporter activity using an indicator substrate. By using the above method or combining the above method with other purification methods well known in the art, the inhibitory compound can be purified. Alternatively, the extract is resolved into compounds, fractions, or components by TLC. The TLC plates are then completely coated with a spore solution of a transgenic fungi containing the reporter gene operably linked to a promoter involved in mycotoxin biosynthesis in a molten agarose solution. The agarose immobilizes the spores on the plate and when the fungi germinates from the spores the agarose prevents cross-contamination between particular areas of the plate. After allowing the agarose to solidify, the plates are incubated under conditions that promote fungus growth. When fungus growth becomes manifest, the fungi coated plates are treated to lyse the fungi, preferably by freeze-thawing the fungi. The TLC plates are then treated with an indicator substrate that enables determination of whether transcription of the reporter has been inhibited. Preferably, the indicator substrate is applied as a solution that contains molten agarose. The agarose, after it solidifies, immobilizes the indicator substrate to prevent cross-contamination.
In a preferred embodiment, the method screens for compounds that inhibit promoters involved in mycotoxin biosynthesis, which include, but are not limited to, promoters for genes encoding the following enzymes involved in the biosynthesis of aflatoxin: nor-1 (FIG. 8; SEQ ID NO:1) from Aspergillus parasiticus (Chang et al., Curr. Genet. 21: 231-233 (1992); Trail et al., Appl. Environ. Microbiol. 60: 4078-4085 (1994)), ver-1 (FIG. 9; SEQ ID NO:2) from Aspergillus parasiticus (Skory et al., Appl. Environ. Microbiol. 58: 3527-3537 (1992)), omt-1 (FIG. 10; SEQ ID NO:3) from Aspergillus parasiticus (Yu et al., Appl. Environ. Microbiol. 59: 3564-3571 (1993)), fas-1A (SEQ ID NO:4) from Aspergillus parasiticus (Mahanti et al., Appl. Environ. Microbiol. 62: 191-195 (1996)), alfr (FIG. 11; SEQ ID NO:5) from Aspergillus parasiticus (Chang et al., Appl. Environ. Microbiol. 59: 3273-3279 (1993); Payne et al., Appl. Environ. Microbiol. 59: 156-162 (1993)), verA (FIG. 12; SEQ ID NO:6) from Emericella nidulans (Keller et al., Phytopathol. 84: 483-488 (1994); or deoxynivalenol such as tri5 (FIG. 13; SEQ ID NO:7) from Gibberella pulicaris (Holn et al., Molec. Plant-Microbe Interactions 5: 249-256 (1992)). In a preferred embodiment, the transgenic fungus comprises a reporter gene that is operably linked to the nor-1 promoter, which is involved in the conversion of norsolorinic acid to averantin in the aflatoxin biosynthesis pathway, or the reporter gene is operably linked to the promoter for the tri5 gene, which is involved in the biosynthesis of deoxynivalenol. In addition, it is preferable that the method further include a control transgenic fungus comprising a reporter gene that is operatively linked to a promoter for a gene known not to be involved in secondary metabolism. An example of such a promoter is the promoter for the benA gene (FIG. 14; SEQ ID NO:8), which encodes β-tubulin in Aspergillus flavus (Woloshuk et al., Appl. Environ. Microbiol. 60: 670-676 (1994)). For any one of the above promoters and gene sequences, the present invention includes mutants thereof.
As used herein, the phrase “operably linked to a promoter” refers to both a recombinant nucleic acid molecule wherein the reporter gene is directly linked to the promoter with little or no nucleotides between the promoter and the reporter gene and to a recombinant nucleic acid molecule wherein the reporter gene is inserted into a gene controlled by the promoter in the proper codon reading frame to produce a chimeric or fusion polypeptide comprising the promoter's gene product with the reporter polypeptide inserted therein. The advantage of the latter is that it facilitates generation of transgenic fungi with the reporter gene inserted into the promoter's gene in the fungi by double crossover or homologous recombination. In general, when making plasmid constructs containing a reporter gene for use as vectors for making transgenic fungi by double crossover, it is preferable that a deletion be made in the coding region of the promoter's gene and replacing the reporter gene with the deleted material.
In particular embodiments, the fungi used to make the transgenic fungi include, but are not limited to, Aspergillus spp. such as Aspergillus parasiticus, Aspergillus nidulans, Aspergillus versicolor, Aspergillus flavus, and Gibberella ssp. such as Gibberella zeae and Gibberella pulicaris. Methods for DNA transformation of fungi have been taught by Skory et al. Appl. Environ. Microbiol. 56: 3315-3320 (1990); Oakley et al., Gene 61: 385-399 (1987); and, a method is taught herein. It is also envisioned that the above assays can be performed using bacteria transformed with the any one of the aforementioned promoters operably linked to a gene encoding a reporter.
In particular embodiments, the reporter gene that is operably linked to the secondary or primary metabolite promoter, includes but is not limited to, the uidA gene, which encodes β-glucuronidase (GUS); the lacZ gene, which encodes β-galactosidase; the luc gene, which encodes firefly luciferase; the Rluc gene which encodes the Renilla luciferase; or the gene encoding the fluorescent green protein, which is disclosed in U.S. Pat. No. 5,958,713 to Thastrup et al. These reporter genes are commercially available and methods for their cloning and use are well known in the art. In the embodiment demonstrated herein, the reporter gene encodes the GUS enzyme. GUS was used as a reporter gene because GUS activity is easily monitored with a variety of indicator substrates including the histological stain, 5-bromo-4-chloro-3-indolyl-β-D-glucuronide (X-Gluc), which in the presence of the GUS enzyme is converted to a blue pigment, and 4-methylumbelliferyl-B-glucuronide (MUG), which in the presence of GUS can be converted to a fluorescent compound. These assays are well known in the art. The transgenic fungi herein comprise fungi wherein the GUS gene is operably linked to a promoter selected from the group consisting of the nor-1 promoter (isolatable from SEQ ID NO:1), the ver-1 promoter (isolatable from SEQ ID NO:2), the bena promoter (isolatable from SEQ ID NO:8), or the tri5 promoter (isolatable from SEQ ID NO:7) (Trail et al., Proc. Am. Phytopathol. Soc. Nat. Mtg., Albuquerque, N. Mex. 1994; Payne et al., Appl. Environ. Microbiol. 59: 156-162 (1993)). The above sequences are particularly suitable for making transgenic fungi by double crossover or homologous recombination that produce the gene product and reporter as a chimeric or fusion polypeptide.
Examples of the GUS reporter gene operably linked to a promoter are shown in FIGS. 4A-4D which show DNA plasmid vectors pAPGUSN, pPAGUSNN, pHD6-6 (Wu et al., Proc. Curr. Issues Food Safety, National Food Safety Toxicology Center, Michigan State University, East Lansing, Mich.), and pGAP2 (Woloshuk et al., Appl. Environ. Microbiol. 60: 670-676 (1994)), respectively. The above plasmid vectors have been used to transform Aspergillus parasiticus strain NR1 (niaD-mutant from ATCC 5862, afl+, disclosed in Chang et al., Curr. Genet. 21: 231-233 (1992)), strain NR2, or strain C2N (Trail et al., Appl. Environ. Microbiol. 60: 4078-4085 (1994)). While strains NR1 and C2N were used to demonstrate practice of the present invention, other fungal strains, transformed with the plasmid vectors disclosed herein or plasmid vectors containing other promoters involved in biosynthesis of secondary metabolites operably linked to a gene encoding a reporter such as the GUS gene, can be used to detect inhibitors of secondary metabolites according to the present invention.
Plasmid pAPGUSN (FIG. 4A) contains the GUS gene operably linked to the nor-1 promoter at its 5′ end and the nor-1 transcription terminator at its 3′ end. pAPGUSN was stablely integrated into the genome of C2N, a nor-1, niaD+ strain, by double crossover insertion. In the same manner as above, a second plasmid, pAPGUSNN, comprising the nor-1 promoter operably linked to GUS and a functional niaD gene (FIG. 4B) was stablely integrated into strain NR1, which restored the niaD+phenotype to strain NR1, and preserved aflatoxin biosynthesis. A third plasmid similar to pAPGUSNN was made with the ver-1 gene promoter operably linked to the GUS gene (plasmid pHD6-6) (FIG. 4C) and used to transform Aspergillus parasiticus. In addition to the above transgenic fungi, a transgenic Gibberella zeae expressing GUS controlled by the tris gene promoter, a transgenic Aspergillus nidulans expressing GUS controlled by the ipnA promoter, and a transgenic Aspergillus parasiticus expressing GUS controlled by the benA promoter (plasmid pGAP2 in FIG. 4D) were also made in the same manner. The method for making plasmid FLIRT comprising the ipnA promoter is disclosed by Bergh et al. in the J. Bacteriol. 178: 3908-3916 (1996). Methods for transforming fungi and recovering stable transformants are well known in the art. The aforementioned transgenic fungi can each be used in the present invention to identify and isolate compounds that inhibit biosynthesis of mycotoxins.
While particular plasmid vectors for making the transformed fungi are disclosed herein, transformed fungi equivalent to the transformed fungi disclosed herein can be made using any plasmid that contains the same or substantially the same sequences as disclosed by the particular plasmid vectors herein. Methods for isolating, cloning and manipulating DNA are well known in the are as are methods for producing transgenic fungi. Therefore, the DNA fragments comprising promoter and termination regions for any one of the mycotoxin biosynthesis genes can be isolated using in a polymerase chain reaction (PCR) appropriate primers to amplify the promoter and termination regions. Primers can be designed using methods well known in the arts. These amplified DNA fragments are then ligated to the 5′ and 3′ ends of a DNA encoding a reporter to form a cassette. DNA encoding reporters are commercially available. The cassette is inserted into any commercially available plasmid, which is linearized and used to transform any fungus, including those taught herein. Prior to transformation, additional genes such as the niaD gene can also be inserted into the plasmid. The novelty of the method resides in its ability to identify inhibitors of particular promoters controlling particular enzymes involved in mycotoxin biosynthesis and to distinguish the particular inhibitors from inhibitors that affect growth of the fungi, not in the particular transgenic fungi disclosed herein or the particular plasmids disclosed herein that were used to construct the transgenic fungi.
It is known that a wide variety of substances can inhibit aflatoxin biosynthesis; however, many of these substances also inhibit fungal growth, and often, the two processes have not been distinguished. However, unlike prior methods for identifying mycotoxin inhibitors, the present invention enables compounds that inhibit mycotoxin biosynthesis to be distinguished from compounds that inhibit both mycotoxin biosynthesis and primary metabolism. Hikoto et al. (Mycopathologia 66: 161-167 (1978)) reported that extracts of various condiments and herbal drugs inhibit mycotoxin biosynthesis. It is well known that pepper contains piperine, an alkaloid that inhibits fungus growth and therefor mycotoxin biosynthesis. However, using the method disclosed herein a novel compound in pepper extracts was identified that inhibits aflatoxin biosynthesis but not fungus growth in vitro. In particular, the method enabled the identification and isolation of compound Cp2, a novel alkaloid that inhibits the nor-1 promoter and thus, aflatoxin biosynthesis, but does not inhibit primary metabolism, since growth of the fungus was not inhibited in vitro. However, because mycotoxins have an important role in enabling fungi such as Aspergillus ssp. and Gibberella ssp. to invade and colonize plant tissues, providing Cp2 to a plant material can effectively prevent the fungi from growing in the plant material.
Identification of Cp2 was by the following culture or batch and chromatography methods of the present invention. Transgenic fungus strain G5 containing the nor-1 promoter operably linked to the GUS reporter gene was grown in PMS medium, which does not support aflatoxin biosynthesis. After 72 hours, the transgenic fungus was transferred to GMS medium, which is an aflatoxin induction medium (Buchanan et al., Appl. Environ. Microbiol. 48: 306-310(1984)). The above nutrient shift avoids differences in mycelial growth rates, which may occur between treatments. After the transfer to GMS medium, a chloroform extract of black pepper was added to medium. When GUS activity was measured, the pepper extract inhibited expression of GUS activity in transgenic strain G5 (FIG. 7) whereas a control culture of strain G5, to which no pepper extract was added, produced GUS activity (FIG. 7).
To show that the inhibition of GUS activity by the pepper extract was because of inhibition of the nor-1 promoter and not because some component of the pepper extract inhibited the GUS enzyme itself, the assay was repeated with transgenic fungus containing pGAP2, pAPGUSN (transformant G5), or pAPGUSNN. The pepper extract inhibited expression of GUS activity when GUS was under the control of the nor-1 promoter (pAPGUSN or pPAGUSNN), but not when GUS was under the control of the benA promoter (pGAP2). This indicated that the pepper extract did not inhibit the GUS enzyme by binding to the enzyme. Instead, inhibition was due to a component of the pepper extract, later identified as Cp2, interacting with the nor-1 promoter. The effect of black pepper extract on expression of GUS activity in Aspergillus transformed with the above vectors is shown in Table 1.

TABLE 1

GUS activity

plasmid (promoter) Pepper extract No extract

pGAP2 (benA) + +

pAPGUSN (nor-1) − +

pAPGUSNN (nor-1) − +
The method further enabled the novel Cp2 compound in the pepper extract that inhibited transcription of the nor-1 promoter, but not transcription of primary metabolism promoters, to be isolated. To isolate the Cp2 compound from the pepper extract, an aliquot of the pepper extract was resolved on silica plates by thin-layer chromatography (TLC). The TLC plates were then completely coated with a spore solution of transgenic fungus strain G5 containing about 0.3% molten agarose. After the agarose solidified, the plates were incubated under conditions that promoted growth of the transgenic fungus. Preferably, the plates were incubated at 30° C. for about two days in the dark. After fungus growth became manifest, the fungi covering the plates were lysed by freeze-thawing, preferably by freezing at −80° C. and thawing at room temperature. Next, the plates were covered with a solution of X-Gluc in about 0.6% molten agarose. The plates were then incubated 37° C. and monitored for β-glucuronidase activity. Blue areas signified that the hyphae in the area had β-glucuronidase activity whereas non-blue areas indicated the hyphae did not produce β-glucuronidase due to the presence of underlying inhibitory compounds. One area, designated Cp2, was subsequently shown to contain the novel Cp2 of the present invention. Control plates covered with a control fungus containing GUS under control of the benA promoter indicated that Cp2 did not inhibit fungus growth in vitro, only mycotoxin biosynthesis.
A preparative quantity of pepper extract was then resolved by flash column chromatography and column fractions were collected. The fractions were each analyzed by TLC as set forth above to identify those chromatography fractions that contained Cp2. The areas of the TLC plates corresponding to Cp2 were removed from the plate, pooled, concentrated, and re-chromatographed on TLC plates using a second solvent system to resolve the compounds. Optionally, the TLC purification method can be repeated until the compound is purified free of other compounds. After any purification step, an aliquot of the inhibitory material can be assayed on TLC plates or in the batch method to ensure that the material being purified retains the ability to inhibit mycotoxin biosynthesis. Purification of Cp2 required an additional TLC chromatography using a third solvent system which resulted in a Cp2 preparation of about 97% purity. As alternatives to TLC, column chromatography (e.g., ion exchange chromatography, size exclusion chromatography, HPLC, FPLC, etc.) or high voltage paper electrophoresis can be used to purify inhibitory compounds.
Cp2 was determined to inhibit transcription not only from the nor-1 promoter but also from the ver-1 promoter, the tri5 promoter, and the ipnA promoter. The inhibitory activity of Cp2 is distinguishable from piperine. Using the batch method of the present invention, piperine was unable to inhibit transcription from the ipnA promoter (FIG. 3A) even though it inhibited transcription from the tri5 promoter (FIG. 3B). Also, piperine inhibits fungus growth in vitro whereas Cp2 does not. Furthermore, Cp2 is distinguishable from other compounds that inhibit aflatoxin accumulation. For example, Huang et al. (Phytopathol. 87: 622-627 (1997)) identified from seeds of corn inbred for resistance to aflatoxin accumulation, a protein that inhibited aflatoxin accumulation by inhibiting growth of Aspergillus flavus, and another protein inhibited aflatoxin accumulation with little effect on fungal growth; Russin et al. (Phytopathol. 87: 529-533 (1997)) identified a component of unknown identity in the wax of kernels of corn bred to be resistant to Aspergillus flavus that is not present in the wax of corn kernels from corn not resistant to Aspergillus flavus; and Ghosh et al. (J. Stored Products Res. 32: 339-343 (1996)) identified three proteins in extracts of sorghum seeds that completely inhibited germination of Aspergillus flavus spores.
The following examples are intended to promote a further understanding of the present invention.

EXAMPLE 1

The method for identifying compounds that inhibit mycotoxin biosynthesis is illustrated using ground black pepper (Piper nigrum) extracts and assaying for compounds that inhibit aflatoxin biosynthesis.
Ground black pepper (500 mg) was extracted in 2 L cyclohexanol overnight under a fume hood with constant stirring. After 24 hours, the mixture was filtered through Whatman paper and the extracted pepper filtrate was removed and in a similar manner to the above, extracted with chloroform followed by absolute ethanol. The filtrates from each extraction were concentrated by rotary evaporation and analyzed by thin layer chromatography (TLC).
The extracts were separated on TLC plates using a solvent system consisting of chloroform:acetone:toluene (25:40:35 v:v:v). Afterwards, the TLC plates were dried in a fume hood to evaporate any solvents left on the plates, and then placed silica-side down on a UV transilluminator and UV absorbent spots were traced on an acetate sheet. To identify compounds with aflatoxin inhibitory properties, a bioassay was invented that modified the screening method of Homans et al. (J. Chromato., 51: 327-329 (1970)). The bioassay used two transgenic strains of Aspergillus parasiticus. The strain G5 had its nor-1 gene replaced by a DNA construct comprising the nor-1 promoter operably linked to GUS reporter gene (Xu et al. Physiol. Molec. Plant Pathol. 56: 185-196 (2000)). The strain GAPN2 had its niaD gene replaced by a DNA construct containing the β-tubulin benA promoter operably linked to the GUS reporter gene.
To perform the TLC bioassay, a spore solution of a transgenic Aspergillus parasiticus (strain G5) that contained the aflatoxin biosynthesis promoter nor-1 operably linked to the uidA gene, which encodes β-glucuronidase (GUS), was made at a final concentration of about 1×10⁶spores per ml in YES medium (a yeast extract and sucrose medium that induces aflatoxin biosynthesis in Aspergillus spp.) containing 0.3% agarose at 55 to 60° C. (20 ml will cover a plate that has a surface area of 200 cm²). The spore solution was evenly sprayed across the TLC plate inclined at a 600 angle in a sterile hood using a glass TLC spray apparatus with care taken to ensure an even coating. After the agarose solidified, the plate was balanced on two glass test tubes on dampened paper towels in a moist chamber assembled from a plastic storage container with a loose lid that was completely lined with plastic wrap. Plates were incubated for 2 days at 30° C. in the dark. Following two days of growth, the plates were frozen for one hour at −80° C. and then thawed at room temperature for at least 30 minutes to break down the cell membranes of the fungi and allow the GUS enzyme to leak out of the fungi. Areas of inhibition of fungal growth were traced on acetate sheets for comparison to the UV absorbent spots. Then the plates were inclined at a 600 angle and sprayed using a TLC plate sprayer with a mixture of 15 ml of 2× X-Gluc buffer (100 mM KPO₄, pH 7.0, 0.3% K ferricyanide, 0.1% X-Gluc (5-bromo-4-chloro-3-indolyl-β-D-glucuronide cyclohexylammonium salt (Gold Biotechnology, Inc., St. Louis, Mo.) added to 15 ml of 0.6% agarose at 55 to 60° C., immediately before spraying. The plates were wrapped in plastic film and incubated in a plastic storage container at 37° C. overnight. Areas that lacked β-glucuronidase activity were traced onto acetate sheets and compared to the locations for the UV absorbent spots and the areas that inhibited fungal growth.
Visual examination showed that the TLC plates were evenly covered with mycelia except for one area indicating that this area contained a compound that inhibited growth of the transgenic fungus (FIG. 1). GUS activity, which was indicated by a blue color, was not uniform over the entire surface of the TLC plate (FIG. 1, lane A). Additional area that lacked a blue color but had mycelial growth represented compounds that were inhibitory to the nor-1 promoter. Inhibition of GUS activity was not detected in a control consisting of a transgenic Aspergillus parasiticus containing plasmid pGAP2 comprising the GUS gene operatively linked to the promoter for the benA gene which encodes β-tubulin (FIG. 1, lane B). The control showed that the inhibitory effect of the compounds was not due to inhibition of GUS activity but was inhibition of GUS transcription via the nor-1 promoter. The area displaying inhibition of GUS transcription was chosen and the inhibitory compound was isolated.
This example shows that the present invention provides an easy and reliable TLC-based method to detect compounds inhibitory to transcription of secondary metabolism genes in Aspergillus parasiticus.

EXAMPLE 2

The following is a procedure for isolating Cp2 from ground pepper extracts.
Black ground pepper was suspended in cyclohexane (1:2 w:v) with constant stirring overnight. The mixture was filtered and then reextracted with cyclohexane for an additional four hours. The filtrates were combined and concentrated by rotary evaporation under vacuum at 40° C. After rotary evaporation, the cyclohexane extract was observed to have two phases. These phases were separated yielding a phase readily soluble in cyclohexane and a less dense phase readily soluble in ethanol. Both phases were tested to determine which phase contained the bioactive compound indicated in Example 1. An amount consisting of 10 μl of both phases were loaded onto separate TLC plates and the plates were developed using a solvent system consisting of chloroform:toluene:acetone (25-40:35 v:v:v). After development, the plates were dried overnight. Then the GUS bioassay was performed as shown in Example 1. The bioactive compound Cp2 was observed to be present in the ethanol soluble phase.
Flash chromatography was used to purify Cp2 from the pepper extract according to the method described by Still et al., J. Org. Chem. 43: 2923-2925 (1978). Flash chromatography uses an air pressure driven column which has been optimized for fast separations. The column used to purify Cp2 was dry packed with 6 inches of silica gel (grade 9385, 230-400 mesh, 60 angstrom available from Aldrich, Milwaukee, Wis.) between two layers of 50 mesh sand. Concentrated pepper extract filtrates made according to above were applied to the column and the column was then developed using a solvent system consisting of chloroform:toluene:acetone (25:40:6 v:v:v). Five ml aliquots were collected and analyzed on TLC plates to identify Cp2. The fractions containing Cp2 were pooled, concentrated by rotary evaporation, and resuspended in 100 proof ethanol and stored under refrigeration at 4° C. Approximately 10 mg of the concentrated fraction containing Cp2 was loaded onto a 20×20 cm preparatory TLC plate containing 1000 μm silica gel (60 angstrom, available from Whatman, Clinton, N.H.). The TLC plate was developed three times, each time using a solvent system consisting of hexane:acetone (2:1-v:v). After the third development, the TLC plate was dried and assayed for GUS inhibition as in Example 1. The bioactive band containing Cp2 was scraped out of the silica plate and eluted from the silica using chloroform:ethanol (4:1 v:v). The Cp2 product was then concentrated by gaseous hydrogen gas and loaded onto a TLC plate as above and developed three times, each time with a solvent system consisting of hexane:ethyl acetate (4:1 v:v) which provided sufficient separation to enable a single band containing Cp2 to be located. The Cp2 was recovered with a purity of approximately 97%. TLC bioassays as above were then used to confirm the inhibitory properties of the nearly homogenous preparation of Cp2. A preliminary structure of Cp2 was determined by mass spectrometry to comprise an unsaturated C18 fatty acid amide of piperidine.
Therefore, this example shows the isolation of the aflatoxin inhibitory compound of the present invention, Cp2, which had been identified by the method of the present invention demonstrated in Example 1.

EXAMPLE 3

Cp2 has a demonstrable effect on aflatoxin biosynthesis and nor-1 transcription. This example used Aspergillus parasiticus SU-1, a wild-type aflatoxin producing isolate, and consisted of analyzing aflatoxin biosynthesis by TLC and aflatoxin mRNA transcription by Northern analysis.
A nutritional shift protocol (Skory et al., Appl. Environ. Microbiol. 56: 3315-3320 (1990)) was used with the modifications below to determine the effect of Cp2 on expression of the nor-1 gene and aflatoxin biosynthesis. Six cultures of fungi were incubated in PMS (peptone mineral salts), a medium that does not induce biosynthesis of aflatoxin. After 48 hours, the mycelium from each culture were separately transferred to GMS (glucose mineral salts), a medium that induces biosynthesis of aflatoxin. At the same time Cp2 was added to five of the six cultures to provide cultures having a final concentration of 2.6 μg/ml, 26 μg/ml, 39 μg/ml, 52 μg/ml, and 78 μg/ml, respectively. Thirty-six hours later, a sample of each culture was removed for Northern analysis. Forty-eight hours later, the culture filtrate was analyzed by TLC for aflatoxin biosynthesis and RNA was extracted from the mycelium for Northern analysis. Direct competitive ELISA analyses were also performed to determine whether, for each sample, aflatoxin B1 (AFB1) was in the medium. The procedure was performed as described by Peska (J. Assoc. Off. Anal. Chem. 71: 1075-1081 (1988)) with anti-AFB1 antibodies and AFB1-horseradish peroxidase conjugate (both available by name from Michigan State University, East Lansing, Mich.).
FIG. 2A shows Cp2 caused a reduction in the amounts of aflatoxins B1 and G1. FIG. 2A further shows that Cp2 at a concentration greater than 26 μg/ml appeared to completely inhibit biosynthesis of aflatoxin. Interestingly, Cp2 also caused an increase in the amount of an unidentified pigment that correlated with the decrease in aflatoxin. The significance of the increase in this pigment is unknown. FIG. 2B shows that Cp2 had an effect on the accumulation of nor-1 transcripts in the wild-type isolate SU-1. Samples had been collected 36 hours and 48 hours after adding the Cp2. FIG. 2B shows that Cp2 at a final concentration of 52 μg/ml appeared to completely inhibit nor-1 transcription within 36 hours after addition to the medium (lane 3). Lower concentrations of Cp2 did not completely inhibit nor-1 transcription ( lanes 2, 5, 6 and 7).
This example demonstrates that Cp2 inhibits transcription of the nor-1 gene in Aspergillus parasiticus but does not inhibit growth of Aspergillus parasiticus itself.

EXAMPLE 4

The effect Cp2 and piperine on other secondary metabolite promoters is compared. For this comparison several transgenic fungi containing GUS operably linked to particular promoters involved in secondary metabolism pathways were used. A transgenic strain of Aspergillus nidulans (a penicillin producing fungus), FLIRT with the promoter for the ipnA gene (a gene involved in penicillin biosynthesis) operably linked to the GUS reporter gene, was provided by Dr. A. Brakhage, Technical University of Darmstädt, Germany and a transgenic strain of Gibberella zeae with the promoter for the tri5 gene (a gene involved in deoxynivalenol biosynthesis) operably linked to the GUS reporter gene, was provided by Dr. N. Alexander, U.S. Department of Agriculture, Peoria, Ill.
Each transgenic fungus was grown in mycotoxin inducing liquid medium containing Cp2 or piperine. Aliquots were harvested from the cultures 48, 72, and 96 hours after the addition of either the Cp2 or piperine. Protein extracts were made from the harvested mycelia, and 10 μg/ml aliquots were tested for GUS activity. The substrate used to measure GUS activity was the fluorescence compound 4-methylumbelliferyl (MUG), which fluoresces under UV light.
FIG. 3A shows that ipnA promoter in Aspergillus nidulans was inhibited by Cp2 but not by piperine whereas FIG. 3B shows that the tri5 promoter in Gibberella zeae was inhibited by both Cp2 and piperine.
The results in this example demonstrate that even though both Cp2 and piperine are obtainable from pepper, Cp2 and piperine are distinguishable compounds.

EXAMPLE 5

Construction of plasmid pAPGUSN having the GUS gene operably linked to the nor-1 promoter is illustrated. Standard molecular biology techniques were used to construct pAPGUSN.
A 3.1 kb HindIII-Bsu361 DNA fragment from the nor-1 gene (sequence available from Trail et al., Appl. Environ. Microbiol. 60: 4078-4085 (1994) that included the translational start site and 64 nucleotides 3′ of the translational start site was blunt-end ligated to a 5′ NcoI site of a DNA encoding uidA (GUS reporter gene) using a 10 bp HindIII DNA linker (available from New England Biolabs, Beverly, Mass.) to maintain integrity of the reading frame. To form the translational termination sequence, a 2.0 kb BstY1 DNA fragment from the nor-1 3′ untranslated region, including 18 bp upstream of the translational termination site, was ligated to the BamHI site at the 3′ end of the GUS gene. The ligated product inserted into an ampicillin resistant plasmid to generate plasmid vector pAPGUSN. The plasmid vector was sequenced at the 5′ junction of the nor-1-GUS fusion to confirm that the correct reading frame was preserved using primers for the promoter and for the GUS gene for double-stranded DNA sequencing. Sequencing was performed using a Sequenase chain-termination sequencing kit (available United States Biochemical Corp., Cleveland, Ohio) according to the manufacturer's instructions. FIG. 4A shows the plasmid map for pAPGUSN.

EXAMPLE 6

Aspergillus parasiticus C2N is transformed with pAPGUSN to illustrate construction of transgenic fungi that express GUS under the control of a secondary metabolism promoter.
Aspergillus parasiticus C2N is a nor-1 disrupted transformant that accumulates the aflatoxin precursor, norsolorinic acid (NA). C2N was derived from NR-1, which is a nitrate reductase-deficient aflatoxin producing isolate, derived from wild-type Aspergillus parasiticus SU-1 (available as NRRL 5862) by spontaneous mutation to chlorate resistance (Horng et al., Molec. Gen. Genet. 224: 294-296 (1990)), by inserting the nitrate reductase gene, niaD, into the nor-1 gene as described in Trail et al. (Appl. Environ. Microbiol. 60: 4078-4085 (1994)). C2N strains accumulate the orange-pigmented aflatoxin precursor norsolinic acid and makes reduced amounts of aflatoxin due to bifurcation in the aflatoxin biosynthesis pathway. Even though the aflatoxin produced by strain C2N is reduced in comparison to strain SU-1, the timing of aflatoxin biosynthesis is similar. Double recombination between the nor-1 flanking regions of pAPGUSN and the nor-1 flanking regions on the chromosome results in the replacement of the disrupted nor-1 in C2N with the nor-1-GUS fusion, resulting in a norsolorinic acid-accumulating, niaD−, GUS+ transformant. This causes the loss of a functional niaD gene present in the disrupted nor-1 gene of C2N, thus rendering the GUS expressing transformants to niaD−.
To make the transgenic Aspergillus parasiticus C2N, the plasmid pAPGUSN was linearized with Kpn1 before transformation. Polyethylene glycol-mediated transformation was carried out as described in Oakley et al., Gene 61: 385-399 (1987)) with modifications as disclosed in Skory et al. (Appl. Environ. Microbiol. 56: 3315-3320 (1990)).
Therefore, to perform the fungal transformations, 1×10⁸conidia were inoculated into a 250 ml Erlenmeyer flask containing 100 ml Czapek-Dox (CZ) medium (Difco) supplemented with 1% peptone or YES medium. Prior to transformation the flask was coated with a silanizing agent such as PROSIL, or dichlorodimethylsilane to prevent the mycelium from adhering to the glass and growing at the air interface. The culture was grown overnight at 29° C. in an orbital shaker (about 16-18 hours). Growth was visibly evident, but not excessive. Ideally, microscopic examination revealed that the majority of the conidia had formed germ tubes which were beginning to branch. The mycelial growth was harvested using a sterile Buchner funnel containing a MIRA-CLOTH filter and washed with water to remove spores. Then, the collected cells were transferred to a sterile 250 ml flask and resuspended in 40 CZ medium. To make protoplasts, 40 ml of digestion solution (filter sterilized 5 mg/ml Novozyme-234 in 1.1 M KCL, 0.1 M citrate, pH 5.8) was added. The cells were incubated for 3 hours at 30° C. with gentle shaking. Afterwards, the cells were filtered by gravity through a 29 μm nylon mesh weave filter and the protoplasts harvested at 5,000 rpm in a Sorval SS34 rotor (3,000×g) for 15 minutes at 4° C. The following operations were performed on ice. Next, the protoplasts were resuspended in 1 ml PEG buffer (0.6 M KCL, 0.05 M CaCl₂, 10 mM Tris-HCl, pH 7.5) and pelleted in a microcentrifuge at 4,500×g for 1 minute. This washing process was repeated three times. Afterwards, the washed protoplasts were resuspended in no more than 500 μl PEG-buffer and distributed in 100 μl aliquots for transformation. Five μl DNase inhibitor aurentricaboxylic acid (20 mM aurentricarboxylic acid, 5 mM Tris-HCl, pH 7.0) was added and mixed gently. Then, 1-10 μg of plasmid DNA was dissolved in a volume of less than 10 μl and added followed by addition of 50 μl of freshly prepared and filter sterilized PEG solution (25% polyethylene glycol 3350, 0.6 M KCL, 0.05 M CaCl₂, 10 mM Tris-HCl, pH 7.5). The transformation mixture was gently mixed and incubated on ice for 20 minutes. Then, 850 μl PEG solution was gently added and the mixture allowed to sit at room temperature for 30 minutes.
After the transformation reaction, nitrate non-utilizing transformants (niaD−, GUS+) of strain C2N were selected on CZ medium amended with 58 g/L potassium chlorate, 100 g/L glutamate, and 20% sucrose (CCGS medium). Coconut agar medium (CAM; made according to Arseculeratne et al. (Appl. Microbiol. 18: 88-94 (1969)), an aflatoxin inducing medium, was used to screen the transformants for changes in accumulation of aflatoxin or precursors according to Davis et al. (Microbiol. 53: 1593-1595 (1987)).
In two transformation experiments, six of the eight transformants recovered from the CCGS selection medium produced GUS activity as measured by a MUG assay. Southern analysis confirmed the presence of a single nor-1-GUS fusion genes at the site of nor-1 and the accompanying loss of the niaD gene from that region. Three of the GUS expressing transformants had unexpected rearrangements present in their DNA at the nor-1 region and were eliminated from further analysis. Unexpectedly, replacement of the disrupted nor-1 gene with the nor-1-GUS fusion resulted in the loss or NA accumulation in all cases, although aflatoxin B1 continued to be produced as expected. Further analysis by Southern hybridization did not reveal any explanation for the loss of pigment production, although small rearrangements of nucleotides in the region of insertion that might affect expression would not have been detected by Southern hybridizations. Among the three remaining transformants, transformant G5 produced the highest amount of GUS activity in culture and was chosen for further study.

EXAMPLE 7

Transformants made in Example 5 were analyzed for their growth, aflatoxin biosynthesis, and GUS activity in culture.
Flasks containing 100 ml of YES broth and 5 glass beads to keep the mycelia dispersed, were inoculated with 1.75×10⁷spores each. The cultures were grown in an orbital shaker at 175 rpm at 28° C. Mycelia was harvested 16, 24, 36, 48, and 72 hours after inoculation. Dry weights were determined by drying the mycelial mats overnight at 60° C. GUS activity assays were performed on 10 mg total protein from ground tissue extracts using the GUS substrate MUG as described in Liang et al., Appl. Environ. Microbiol. 63: 1058-1065 (1997). GUS activity was determined using a spectrofluorometer with an excitation wavelength of 365 nm and an emission wavelength of 455 nm. Protein content of the supernatant was determined by a BCA assay (available from Sigma Chemicals, St. Louis, Mo.). Direct competitive ELISA analyses were performed on samples of the culture medium to determine concentrations of aflatoxin B1. The procedures performed as described in Peska (J. Assoc. Off. Anal. Chem. 71: 1075-1081 (1988)) with aflatoxin B1 monoclonal antibodies and aflatoxin B1-horseradish peroxidase conjugate.
The results show that there was a similar temporal pattern of aflatoxin B1 biosynthesis between paternal isolate C2N and G5 with lower levels of biosynthesis by G5 (FIG. 8). Biosynthesis of aflatoxin B1 in NR1 (niaD parent of C2N) exhibited a similar temporal pattern but, as expected, reached a higher quantity, 42.5 μg/ml culture medium after 72 hours. Comparison of GUS expression by G5 and C2N showed that mycelial extracts of G5 had increased GUS activity for up to 72 hours and no GUS expression was detected in mycelial extracts of C2N (FIG. 9). Dry weights were not significantly different among all cultures at each time point.

EXAMPLE 8

The transgenic fungi were also evaluated for its ability to colonize peanut plants.
The transgenic fungus was introduced onto peanut plants and the peanut plants were cultivated under drought conditions. About 6-8 weeks after introduction, the peanuts were harvested and the harvested pods underwent treatment for GUS expression. The kernels were split and the halves were cut perpendicular to their longitudinal axis into pieces approximately 4 to 5 mm wide. To follow the path of infection of the fungus through the peanut, the correct orientation of the shell, integument, and kernel was maintained during fixation and embedding. This was accomplished by threading a nylon strand through the center of each peanut piece, penetrating the shell, integument, and kernel sequentially. The threaded pieces were prepared for cytological study by briefly fixing with 5% formaldehyde in 50 mM potassium phosphate, pH 7.0 for 5-30 minutes on ice under a vacuum. This preliminary fixation killed the fungus while not affecting GUS activity. After rinsing in sterile distilled water, the peanut parts were incubated overnight with X-Gluc in 50 mM potassium phosphate, pH 7.0, containing 0.5 mM potassium fericyanide and 10 mM EDTA for detection of GUS activity. Following a brief rinse in water, the stained tissues were transferred to a solution of 3.7% formaldehyde, 5% acetic acid, 47.5% ethanol for at least 24 hours. Then, the tissues were dehydrated through a tert-butanol series and embedded in paraffin (PARAPLAST PLUS available from Fisher Scientific, Inc., New Brunswick, N.J.). During the final embedding step, the nylon thread was removed and the peanut pieces were aligned to allow sectioning through all three tissues perpendicular to the original long axis of the peanut. Paraffin blocks were sectioned at 12-15 mm, and serial sections were placed on glass slides coated with Haupt's solution. Following dewaxing of the sections with xylene and rehydration through an ethanol series, some of the sections were stained with 0.2% Chlorazol Black. The sections were mounted in Permount. Microscopy was carried out on a Ziess Axioskop equipped with DIC optics or an Olympus Vanox-S microscope with phase contrast optics.
When the peanuts were stained for GUS using the substrate X-Gluc, the fungus was clearly visible in the infected kernels. However, the blue color associated with GUS activity was not observed in conidia, conidiophores, nor the external mycelia surrounding the pod. This was expected because aflatoxin precursors do not accumulate in conidia and conidiophores (Keller at al., Phytopathol. 84: 483-488 (1994)). FIG. 5 shows that the production of GUS activity in the transgenic fungus was similar in time course to aflatoxin biosynthesis. These results indicate that the nor-1 promoter was functioning in the transgenic fungus in the same manner as it functions in the wild-type fungus.
A transformant of Aspergillus parasiticus strain NR2, strain 664, was made, which contained the GUS reporter gene under the control of the ver-1 promoter integrated into the ver-1 gene without disrupting normal aflatoxin biosynthesis (Liang et al., ibid). Strains NR1 and NR2 are independent spore isolates of the same spontaneous mutant of chlorate resistance.
Transgenic strains G5 and 664 can be used as in the bioassay of the present invention to detect and identify compounds that inhibit the biosynthesis of aflatoxin.
While the present invention is described herein with reference to illustrated embodiments, it should be understood that the invention is not limited hereto. Those having ordinary skill in the art and access to the teachings herein will recognize additional modifications and embodiments within the scope thereof. Therefore, the present invention is limited only by the claims attached herein.

Claims

1-23. (canceled)

24. A method for determining whether a compound inhibits biosynthesis of a secondary metabolite comprising:

(a) providing a culture of a transgenic fungus comprising a reporter gene operably linked to a promoter for a gene involved in the biosynthesis of the secondary metabolite;

(b) providing to the culture the compound to be determined;

(c) incubating the culture containing the compound under conditions that cause biosynthesis of the secondary metabolite; and

(d) measuring expression of the reporter gene wherein absence of the expression of the reporter gene indicates that the compound inhibits biosynthesis of the secondary metabolite.

25. The method of claim 24 wherein the secondary metabolite is a mycotoxin.

26. The method of claim 25 wherein the mycotoxin is selected from the group consisting of aflatoxin, deoxynivalenol, or sterigmatocystin.

27. The method of claim 24 wherein the transgenic fungus is made from a fungus selected from the group consisting of Aspergillus parasiticus, Aspergillus nidulans, Aspergillus versicolor, Aspergillus flavus, Gibberella pulicaris, and Gibberella zeae.

28. The method of claim 24 wherein the promoter is selected from the group consisting of a nor-1 promoter, a ver-1 promoter, verA promoter, fas-1A promoter, omt-1 promoter, an alfR promoter, an ipnA promoter, a tri5 promoter, and mutant thereof.

29. The method of claim 24 wherein the reporter gene is selected from the group consisting of a gene encoding β-glucuronidase, a gene encoding β-galactosidase, a gene encoding luciferase, and a gene encoding fluorescence green protein.

30. The method of claim 24 wherein a transgenic fungus is provided that contains a reporter gene operatively linked to a constitutive promoter which provides a control for the method.

31. The method of claim 30 wherein the constitutive promoter is benA or mutant thereof.

32. A method for identifying a compound in a material that inhibits the biosynthesis of a secondary metabolite of a fungus comprising:

(a) providing an extract of the material;

(b) separating the material by a chromatography method;

(c) providing a spore suspension of a transgenic fungus comprises a reporter gene operatively linked to a promoter that is the same as the promoter that controls transcription of a gene involved in biosynthesis of the secondary metabolite;

(d) adding the spore suspension to the separated material;

(e) allowing the spores to germinate and grow fungi; and

(f) detecting expression of the reporter gene wherein absence of the expression of the reporter gene identifies the separated material that inhibits the biosynthesis of the secondary metabolite.

33. The method of claim 32 wherein the secondary metabolite is a mycotoxin.

34. The method of claim 33 wherein the mycotoxin is selected from the group consisting of aflatoxin, deoxynivalenol, and sterigmatocystin.

35. The method of claim 32 wherein the chromatography is thin layer chromatography (TLC) using TLC plates.

36. The method of claim 32 wherein the transgenic fungus is a fungus selected from the group consisting of Aspergillus parasiticus, Aspergillus nidulans, Aspergillus versicolor, Aspergillus flavus, Gibberella pulicaris and Gibberella zeae.

37. The method of claim 32 wherein the promoter is selected from the group consisting of a nor-1 promoter, a ver-1 promoter, verA promoter, fas-1a promoter, omt-1 promoter, an alfR promoter, an ipnA promoter, a tri5 promoter, and mutant thereof.

38. The method of claim 32 wherein the reporter gene is selected from the group consisting of a gene encoding β-glucuronidase, a gene encoding β-galactosidase, a gene encoding luciferase, and a gene encoding fluorescent green protein.

39. The method of claim 32 wherein a transgenic fungus is provided that contains a reporter gene operatively linked to a constitutive promoter which provides a control for the method.

40. The method of claim 39 wherein the constitutive promoter is benA or mutant thereof.

41. The method of claim 35 wherein the spores are applied to the TLC plates in an agarose solution.

42. The method of claim 32 or 35 wherein the fungi are incubated in a dark and moist atmosphere at 30° C. for a time sufficient for the fungi to cover the plate.

43. The method of claim 32 or 35 wherein the fungi are lysed to release the reporter gene expression product.

44. The method of claim 43 wherein the fungi are lysed by freeze-thawing.