WO2004090125A2

WO2004090125A2 - Method for producing l-amino acid using bacteria having enhanced expression of the gene pcka

Info

Publication number: WO2004090125A2
Application number: PCT/JP2004/004968
Authority: WO
Inventors: Andrey Yurievich Gulevich; Irina Vladimirovna Biryukova; Danila Vadimovich Zimenkov; Aleksandra Yurievna Skorokhodova; Aleksandr Dmitrievich Kivero; Alla Valentinovna Belareva; Sergei Vladimirovich Mashko
Original assignee: Ajinomoto Co., Inc.
Priority date: 2003-04-07
Filing date: 2004-04-06
Publication date: 2004-10-21
Also published as: BRPI0409196A; RU2268300C2; EP1611241B1; WO2004090125A3; US20060035348A1; WO2004090125A8; BRPI0409196B1; JP4500980B2; JP2006521816A; EP1611241A2

Abstract

The present invention relates to a method for producing L-amino acids, such as L-tryptophan, L-phenylalanine and L-tyrosine, using a bacterium belonging to the genus Escherichia, and wherein the L-amino acid productivity of said bacterium is enhanced by enhancing an activity of PEP carboxykinase, which is coded by the pckA gene.

Description

DESCRIPTION

METHOD FOR PRODUCING L-AMINO ACID USING BACTERIUM HAVING

ENHANCED EXPRESSION OF pclcA GENE

Technical field

The present invention relates to biotechnology, specifically to a method for producing L-amino acid by fermentation, and more specifically to a gene derived from bacterium Escherichia coli. The gene is useful for improving production of L-amino acids, namely aromatic L-amino acids, such as L-tryptophan, L-phenylalanine and L-tyrosine.

Background art

Conventionally, L-amino acids have been industrially produced by fermentation methods utilizing microorganism strains obtained from natural sources, or mutants thereof which have been especially modified to enhance L-amino acid production.

Many techniques have been disclosed which describe methods for enhancing L-amino acid production. For example, such techniques include transforming a microorganism with recombinant DNA (see, for example, US patent No. 4,278,765). Many of these techniques are based on increasing the activities of enzymes involved in amino acid biosynthesis, and/or desensitizing enzymes which are involved in feedback inhibition of the target L-amino acid (see, for example, Japanese Laid-open application No56-18596 (1981), WO 95/16042 or US patent Nos. 5,661,012 and 6,040,160).

Phosphoenolpyruvate (PEP) and erythrose 4-phosphate (E4p) are the essential precursors of the common biosynthetic pathway for aromatic L-amino acids. Also, phosphoenolpyruvate is a key intermediate involved in several other cellular processes. In wild-type E. coli, the major PEP consumer is the phosphotransferase system (PTS). Other PEP-consuming enzymes are phosphoenolpyruvate carboxylase, which is encoded by ppc gene, and pyruvate kinases, which are encoded by th&pykA andpykF genes. The reactions which result in PEP formation in E. coli are catalyzed by the glycolytic enzyme enolase and gluconeogenesis enzymes phosphoenolpyruvate synthase and phosphoenolpyruvate carboxykinase (PEP carboxykinase), which are encoded by the eno,ppsA and pckA genes, respectively (Escherichia coli and Salmonella, Second Edition, Editor in Chief: F.C.Neidhardt, ASM Press, Washington D.C., 1996). Therefore, optimization of the specific pathways of PEP and E4P biosynthesis can improve production of aromatic L-amino acids. In particular, increasing of the PEP supply is a standard method for increasing aromatic L-amino acid production. One way of increasing the PEP supply in a cell is to avoid PEP consumption in the phosphotransferase system by use of a non-PTS carbon source. Another way is to make glucose uptake and phosphorylation PEP-independent. Yet another way to increase the PEP supply is to recycle PEP from its derivatives, such as pyruvate and oxaloacetate. The inactivation of PEP carboxylase or PEP kinases to prevent the PEP utilization in glycolysis can also be utilized. Simultaneous inactivation of both pykA and pykF genes significantly increases carbon flow from glucose to L-phenylalanine (Grinter, N.J., ChemTech, 1998, 33-37 (July)). On the other hand, significantly increased formation of L-phenylalanine in the L-phenylalanine-producing E. coli strain containing an inactivated ppc gene is accompanied by increased formation of unwanted by-products, such as acetate and pyruvate (Miller J.E. et al., J. Ind. Microbiol., 1987, 2, 143-149).

Conversion of pyruvate back into PEP can be performed by the overexpression of phosphoenolpyruvate synthase, which is encoded by t eppsA gene. In this case, the carbon flux will be successfully directed toward DAHP (3-deoxy-D-arabino-heptulosonate 7-phosphate) production (Patnaik R. and Liao J.C., Appl. Environ. Microbiol, 1994, 60, 3903-3908, US patents 5906925, US5985617, US6489100; PCT application WO9608567A1). It was shown that increasing the phosphoenolpyruvate synthase activity in coryneform bacterium cells leads to increased production of L-amino acids, such as L-lysine, L-glutamic acid, L-threonine, L-isoleucine and L-serine (PCT application WO0056859A1). Also, it was shown that enhancing activity of phosphoenolpyruvate synthase in coryneform bacterium cells or in bacterium belonging to the genus Escherichia is useful for production of L-tryptophan, L-phenylalanine, L-tyrosine, L-threonine and L-isoleucine (European patent application EP0877090A1).

It is known that growth in the presence of dicarboxylic acids or acetate, formation of PEP and intermediates of the glycolytic pathway requires, in particular, decarboxylation of oxaloacetate by PEP carboxykinase (PckA), giving PEP in an ATP-dependent reaction. On the other hand, growth in the presence of glucose exhibits the highest expression for ppc gene and the lowest one for pclA gene (Teroaka H. et al., J. Biochem. 1970, 67, 567-575; Goldie H., J. Bacteriol. 1984, 59, 832-838 ). Using stoichiometric pathway analysis, a novel pathway to recycle pyruvate to PEP was proposed but not experimentally proven. This hypothetical cycle was considered to consist of PEP carboxykinase (PckA) that converted oxaloacetate to PEP and the glyoxylate shunt (Liao J.C. et al., Biotechnol. Bioeng., 1996, 52, 129-140).

Earlier it was shown that attenuation of, or switching off, i e pcl<A gene and/or yjfA and ytfP open reading frames in the cells of microorganisms of the Enterobacteriaceae family is useful for production of L-threonine (PCT application WO0229080A2). But at present there are no reports describing the fact that enhancement of PEP carboxykinase activity in the cell of L-amino acid-producing bacterium could lead to an increase in the L-amino acid production.

Disclosure of the invention

An object of the present invention is to enhance the productivity of aromatic L-amino acid-producing strains and to provide a method for producing the aromatic L-amino acid using the strain.

It is an object of the present invention to provide an L-amino acid- producing bacterium belonging to the genus Escherichia, wherein the bacterium has been modified to have an enhanced activity of PEP carboxykinase.

It is a further object of the present invention to provide the bacterium as described above, wherein the activity of PEP carboxykinase is enhanced by modifying an expression control sequence of the PEP carboxykinase gene on the chromosome of the bacterium so that the expression of the gene is enhanced, or by increasing copy number of the gene.

It is a further object of the present invention to provide the bacterium as described above, wherein the native promoter of said gene is replaced with more potent promoter.

It is a further object of the present invention to provide the bacterium as described above, wherein the native SD sequence of said gene is replaced with a more efficient SD sequence.

It is a further object of the present invention to provide the bacterium as described above, wherein the PEP carboxykinase gene originates from a bacterium belonging to the genus Escherichia.

It is a further object of the present invention to provide the bacterium as described above, wherein the PEP carboxykinase gene encodes the following protein (A) or (B):

(A) a protein which comprises the amino acid sequence shown in SEQ ID NO: 2 in Sequence listing; (B) a protein which comprises the amino acid sequence shown in SEQ ID NO: 2 in Sequence listing including deletion, substitution, insertion or addition of one or several amino acids, and which has an activity of PEP carboxykinase. (hereinafter, the proteins as defined in the above (A) or (B) are referred to as ^t,!protein(s) of the present invention")

It is a further object of the present invention to provide the bacterium as described above, wherein the PEP carboxykinase gene comprises the following DNA (a) or (b):

(a) a DNA which comprises the nucleotide sequence of the nucleotides 1 to 1623 in SEQ ID NO: 1; or

(b) a DNA which is hybridizable with the nucleotide sequence of the nucleotides 1-1623 in SEQ ID NO:l or a probe which can be prepared from the nucleotide sequence under stringent conditions and encodes a protein having an activity of PEP carboxykinase.

It is a further object of the present invention to provide the bacterium as described above, wherein the stringent conditions are conditions in which washing is performed at 60°C, and at a salt concentration corresponding to 1 x SSC and 0.1 % SDS.

It is a further object of the present invention to provide the bacterium as described above, wherein the bacterium is further modified to have enhanced expression of the yddG open reading frame.

It is a further object of the present invention to provide the bacterium as described above, wherein the L-amino acid is an aromatic L-amino acid selected from the group consisting of L-tryptophan, L-phenylalanine and L-tyrosine.

It is a further object of the present invention to provide a method for producing aromatic L-amino acids, which comprises cultivating the bacterium as described above in a culture medium to cause accumulation of an L-amino acid in the culture medium, and collecting the L-amino acid from the culture medium.

It is a further object of the present invention to provide the method as described above, wherein the L-amino acid is an aromatic amino acid selected from the group consisting of L-tryptophan, L-phenylalanine and L-tyrosine.

It is a further object of the present invention to provide the method as described above, wherein the bacterium has enhanced expression of genes for aromatic amino acid biosynthesis.

The method for producing an L-amino acid includes production of L-tryptophan using L-tryptophan-producing bacterium, wherein activity of the protein of the present invention is enhanced. Brief Description of Drawing

Figure 1 shows the structure of constructed chromosome region upstream of pckA gene.

Best Mode for Carrying Out the Invention

The present inventors have found that enhancement of transcription and/or translation of the pckA gene can enhance L-tryptophan production when expressed in an L-tryptophan-producing strain. The pckA gene encodes PEP carboxykinase, which catalyzes conversion of oxaloacetate to PEP. Thus the present invention has been completed.

The present invention will be explained in detail below.

A bacterium of the present invention is an L-amino acid-producing bacterium belonging to the genus Escherichia that has enhanced activity of a protein, and as a result, enhances production of a target L-amino acid. More specifically, the bacterium of present invention is an aromatic L-amino acid-producing bacterium belonging to the genus Escherichia that has enhanced activity of the protein of the present invention. Even more specifically, the bacterium of the present invention is an aromatic L-amino acid-producing bacterium, such as an L-tryptophan-producing bacterium, belonging to the genus Escherichia, wherein the bacterium has been modified to have enhanced activity of PEP carboxykinase. More specifically, the bacterium of present invention harbors the DNA comprising the pckA gene, and has a modified expression control sequence in the chromosome of the bacterium, and has enhanced ability to produce L- tryptophan.

The term "L-amino acid-producing bacterium" means a bacterium having an ability to cause accumulation of L-amino acid in a medium when the bacterium of the present invention is cultured in the medium. The L-amino acid producing ability may be imparted or enhanced by breeding. The term "L-amino acid-producing bacterium" used herein also means a bacterium, which is able to produce and cause accumulation of L-amino acid in a culture medium in an amount larger than a wild-type or parental strain, and preferably means that the microorganism is able to produce and cause accumulation in a medium of an amount not less than 0.5 g/L, more preferably not less than 1.0 g/L of a target L-amino acid. "L-amino acid" includes L-alanine, L-arginine, L-asparagine, L-aspartic acid, L-cysteine, L-glutamic acid, L-glutamine, L-glycine, L-histidine, L-isoleucine, L-leucine, L-lysine, L-methionine, L-phenylalanine, L-proline, L-serine, L-threonine, L-tryptophan, L-lyrosine and L-valine, and preferably includes aromatic L-amino acid, such as L-tryptophan, L-phenylalanine and L-tyrosine. The term "a bacterium belonging to the genus Escherichia" means that the bacterium is classified as the genus Escherichia according to the classification known to a person skilled in the art of microbiology. Examples of the microorganism belonging to the genus Escherichia used in the present invention include Escherichia coli (E. coli).

The term "activity of PEP carboxykinase" means an activity to catalyze the reaction of converting the oxaloacetate into phosphoenolpyruvate, while consuming ATP and releasing ADP and carbon dioxide. Activity of a PEP carboxykinase could be measured by the method described by, for example, Krebs A. and Bridger W.A. (Can. J. Biochem., 1980, 58, 309-318).

The term "modified to have enhanced activity of PEP carboxykinase" means that the activity of PEP carboxylase per cell has become higher than that of a non-modified strain, for example, a wild-type strain. For example, strains in which the number of PEP carboxykinase molecules per cell is increased, strains in which the specific activity per PEP carboxykinase molecule is increased, and so forth, are encompassed. Furthermore, a wild-type strain that serves as an object for comparison includes, for example, the Escherichia coli K-12. As a result of enhancement of intracellular activity of PEP carboxykinase, an effect is obtained whereby the amount of L-tryptophan which has accumulated in a medium increases.

Enhancement of PEP carboxykinase activity in a bacterial cell can be attained by enhancement of expression of a gene coding for PEP carboxykinase. As the PEP carboxykinase gene, any of the genes derived from bacteria belonging to the genus Escherichia, as well as genes derived from other bacteria such as coryneform bacteria, can be used. Of these, genes derived from bacteria belonging to the genus Escherichia are preferred.

As the gene encoding PEP carboxykinase of Escherichia coli (EC number 4.1.1.49), the pckA gene has been elucidated (nucleotide numbers 3530456 to 3532078 in the sequence of GenBank accession NC_000913.1, gi: 16131280). Therefore, thepckA gene can be obtained by PCR (polymerase chain reaction; refer to White, TJ. et al, Trends Genet., 5, 185 (1989)) utilizing primers prepared based on the nucleotide sequence of the gene. Genes encoding PEP carboxykinase of other microorganisms can be obtained in a similar manner. hepckA gene originated from the genus of Escherichia or its equivalents encompasses a DNA which encodes the following protein (A) or (B):

(A) a protein which comprises the amino acid sequence shown in SEQ ID NO:2 in Sequence listing; (B) a protein which comprises the amino acid sequence shown in SEQ ID NO:2 in Sequence listing including deletion, substitution, insertion or addition of one or several amino acids, and which has an activity of PEP carboxykinase.

The DNA coding for proteins of the present invention includes a DNA coding for the protein which include deletion, substitution, insertion or addition of one or several amino acids in one or more positions on the protein (A), as long as they do not lose the activity of the protein. Although the number of "several" amino acids referred to herein differs depending on the position or the type of amino acid residues in the three-dimensional structure of the protein, it may be 2 to 50, preferably 2 to 25, and more preferably 2 to 10 for the protein (A). The DNA coding for substantially the same protein as the protein defined in (A) may be obtained by, for example, modification of the nucleotide sequence coding for the protein defined in (A). For example, site-directed mutagenesis can be employed so that deletion, substitution, insertion or addition of an amino acid residue or residues occurs at a specific site. Such modified DNA can be obtained by conventional methods, such as by treating with reagents or other conditions generating mutations. Such methods includes treatment of the DNA coding for proteins of present invention with hydroxylamine, or treatment of the bacterium harboring the DNA with UV irradiation or a reagent such as N-methyl-N'-nitro-N-nitrosoguanidine or nitrous acid.

The DNA coding for the proteins of the present invention include variants which can be found in the different strains and variants of bacteria belonging to the genus Escherichia according to natural diversity. The DNA coding for such variants can be obtained by isolating the DNA, which hybridizes -withpckA gene or a part of the gene under stringent conditions, and which encodes the protein having the activity of PEP carboxykinase. The term "stringent conditions" referred to herein include conditions under which a so-called specific hybrid is formed, and a non-specific hybrid is not formed. For example, the stringent conditions includes a condition under which DNAs having high homology, for instance DNAs having homology of 70% or more, preferably 80% or more, more preferably 90% or more, and most preferably 95% or more, are hybridized with each other. Alternatively, stringent conditions can include conditions which comprise ordinary washing conditions for Southern hybridization, e.g., 60°C, 1 x SSC, 0.1% SDS, preferably 0.1 x SSC, 0.1% SDS. As a probe for the DNA that codes for variants and hybridizes withpckA gene, a partial sequence of the nucleotide sequence of SEQ ID NO: 1 can also be used. Such a probe may be prepared by PCR using oligonucleotides produced based on the nucleotide sequence of SEQ ID NO: 1 as primers, and a DNA fragment containing the nucleotide sequence of SEQ ID NO: 1 as a template. When a DNA fragment of about 300 bp in length is used as the probe, the washing conditions of the hybridization can be, for example, 50°C, 2 x SSC, and 0.1% SDS.

The activity of PEP carboxykinase can be enhanced by, for example, transforming the bacterium belonging to the genus Escherichia with a recombinant DNA, which has been prepared by ligating a gene fragment encoding PEP carboxykinase with a vector that functions in the bacterium belonging to the genus Escherichia, preferably a multiple copy type vector. The activity of PEP carboxylase is enhanced as a result of the increase in copy numbers of the genes encoding the enzymes in the transformant strain.

Transformation of bacterium with a DNA coding for a protein means introduction of the DNA into bacterium cell for example by conventional methods to increase expression of the gene coding for the protein of present invention and to enhance the activity of the protein in the bacterial cell. The activity of PEP carboxykinase can also be enhanced by enhancing transcription and/or translation of a gene encoding PEP carboxykinase. Thus the bacterium of the present invention includes one wherein the activity of the protein of the present invention is enhanced by alteration of an expression control sequence of the DNA coding for protein as defined in (A) or (B) on the chromosome of the bacterium. The enhancement of gene expression can be achieved by locating the DNA of the present invention under the control of a more potent promoter in place of the native promoter. The term "native promoter" means a DNA region present in a wild-type organism which is located upstream of the open reading frame (ORF) of the gene and has the function of promoting expression of the gene. Sequence of the native promoter of pckA gene was presented by Ramseier T.M. et al. (Mol. Microbiol., 1995, 16, 6, 1157-1169) and is available in EMBL/GenBank database under accession number U21325. Strength of a promoter is defined by frequency of RNA synthesis initiation acts. Method for evaluating the strength of a promoter is described by, for example, Deuschle U., Kammerer W., Gentz R., Bujard H. (Promoters in Escherichia coli: a hierarchy of in vivo strength indicates alternate structures. EMBO J. 1986, 5, 2987-2994).

The FruR protein (also known as Cra - Catabolite Repressor/ Activator) is a global regulatory protein that modulates the direction of carbon flow. Key enzymes of most central carbohydrate metabolic pathways are the targets of a cAMP-independent mechanism of catabolite repression in enteric bacteria mediated by the FruR protein. In the presence of appropriate exogenous sugar substrates, expression of genes encoding enzymes of glycolytic and Entner-Doudoroff pathways, including some genes encoding key proteins of the PTS, is activated. At the same time, expression of genes encoding key enzymes of gluconeogenic, glyoxalate shunt, and Krebs cycle pathways is inhibited. The FruR protein regulates expression of all of these genes. The FruR represses synthesis of key enzymes of glycolytic and Entner-Doudoroff pathways and activates synthesis of key enzymes of the three other pathways (Escherichia coli and Salmonella, Second Edition, Editor in Chief: F.C. Neidhardt, ASM Press, Washington D.C., 1996). PEP carboxykinase encoded by pclA gene is the gluconeogenic enzyme. Since it is known that P_PPSA promoter is up to 20 times more activated by the FruR protein while P_pckA promoter is only activated by FruR up to 4 times (Saier M.H., Jr., and Ramseier T. M., J. Bacteriol., 1996, 178, 12, 3411-3417), the P_PPSA promoter could be assumed to be a more potent promoter then P_{pC A} promoter in the strains constitutively expressing the fruR gene.

The enhancement of translation can be achieved by introducing into the DNA of the present invention a more efficient Shine-Dalgarno sequence (SD sequence) in the place of a native SD sequence. The SD sequence is a region upstream of the start codon of mRNA which interacts with the 16S RNA of ribosome (Shine J. and Dalgarno L., Proc. Natl. Acad. Sci. U.S.A., 1974, 71, 4, 1342-6). The term "native SD sequence" means SD sequence present in the wild-type organism. The nucleotide sequence of the native SD sequence of thepckA gene, which is a part of promoter region, was presented by Ramseier T.M. et al. (Mol. Microbiol., 1995, 16, 6, 1157-1169) and is available in EMBL/GenBank database under accession number U21325. SD sequence of the φlO gene from phage T7 can be exemplified as an efficient SD sequence (Olins P.O. et al., Gene, 1988, 73, 227-235).

Methods for preparation of chromosomal DNA, hybridization, PCR, preparation of plasmid DNA, digestion and ligation of DNA, transformation, selection of an oligonucleotide as a primer, and the like may be ordinary methods well known to one skilled in the art. These methods are described in, for example, Sambrook, J., and Russell D., "Molecular Cloning A Laboratory Manual, Third Edition", Cold Spring Harbor Laboratory Press (2001) and the like.

The bacterium of the present invention can be obtained by introduction of the aforementioned DNAs into bacterium inherently having ability to produce L-amino acid. Alternatively, the bacterium of present invention can be obtained by imparting to the bacterium already harboring the DNAs the ability to produce L-amino acid .

As a parent strain which is to be enhanced in activity of the protein of the present invention, an L-tryptophan-producing bacterium belonging to the genus Escherichia such as the E. coli strains JP4735/pMU3028 (DSM10122) and JP6015/pMU91 (DSM10123) deficient in the tryptophanyl-tRNA synthetase encoded by mutant trpS gene (US patent 5,756,345); E. coli strain SV164 (pGH5) having serA allele free from feedback inhibition by serine (US patent 6,180,373); E. coli strains AGX17 (pGX44) (NRRL B-12263) and AGX6(pGX50)aroP (NRRL B-12264) deficient in the enzyme tryptophanase (US patent 4,371,614); E. coli strain AGX17/pGX50,pACKG4-pps in which a phosphoenolpyruvate-producing ability is enhanced (WO9708333, US patent 6,319,696) and the like may be used.

Previously, authors of the present invention identified that when the wild-type allele of the yddG gene, which encodes a membrane protein is not involved in biosynthetic pathway of any L-amino acid, was amplified on a multi copy vector in a microorganism, resistance to L-phenylalanine and several other amino acid analogues was imparted to the microorganism. Besides, the yddG gene can enhance production of L-phenylalanine or L-tryptophan when additional copies of the gene are introduced into the cells of the respective producing strain (WO03044192). The L-tryptophan-producing bacterium can be further modified to have enhanced expression of yddG open reading frame or genes effective for L-tryptophan biosynthesis, which include genes of trpEDCBA operon, genes of common pathway for aromatic acids such as aroF, aroG, aroH, aroB, aroD, aroE, aroK, aroL, aroA and aroC genes, genes of L-serine biosynthesis such as serA, serB and serC genes, and the like.

The method of present invention includes a method for producing an L-amino acid comprising cultivating the bacterium of the present invention in a culture medium, allowing the L-amino acid to be produced and accumulated in the culture medium, and collecting the L-amino acid from the culture medium. Also the method of the present invention includes a method for producing L-tryptophan comprising cultivating the bacterium of the present invention in a culture medium, allowing L-tryptophan to be produced and accumulated in the culture medium, and collecting L-tryptophan from the culture medium.

In the present invention, cultivation, collection, and purification of L-amino acids such as L-tryptophan from the medium and the like may be performed in a manner similar to a conventional fermentation method wherein an amino acid is produced using a microorganism. A medium used for culture may be either a synthetic medium or a natural medium, so long as the medium includes a carbon source and a nitrogen source and minerals and, if necessary, appropriate amounts of nutrients which the microorganism requires for growth. The carbon source may include various carbohydrates such as glucose and sucrose, and various organic acids. Depending on the mode of assimilation of the chosen microorganism, alcohol, including ethanol, and glycerol may be used. As a nitrogen source, various ammonium salts such as ammonia and ammonium sulfate, other nitrogen compounds such as amines, a natural nitrogen source such as peptone, soybean-hydrolysate and digested fermentative microorganism can be used. As minerals, potassium monophosphate, magnesium sulfate, sodium chloride, ferrous sulfate, manganese sulfate, calcium chloride, and the like are used. Additional nutrients can be added to the medium, if necessary. For instance, if the microorganism requires tyrosine for growth (tyrosine auxotrophy) a sufficient amount of tyrosine can be added to the cultivation medium.

The cultivation is performed preferably under aerobic conditions such as by shaking and/or stirring with aeration, at a temperature between 20 to 42 °C, preferably between 37 to 40 °C. The pH of the culture is usually between 5 and 9, preferably between 6.5 and 7.2. The pH of the culture can be adjusted with ammonia, calcium carbonate, various acids, various bases, and buffers. Usually, a 1 to 5-day cultivation leads to the accumulation of the target L-amino acid in the liquid medium.

After cultivation, solids such as cells can be removed from the liquid medium by centrifugation or membrane filtration, and the target L-amino acid can be subsequently collected and purified by ion-exchange, concentration and/or crystallization methods.

Examples

The present invention will be more concretely explained below with reference to non-limiting Examples. In the Examples, an amino acid is of L-configuration unless otherwise noted.

Example 1: Construction of the plasmid carrying the fruR gene and the mutant serA gene encoding for a protein which is not subject to feedback inhibition by serine.

An L-tryptophan producing-strain SV164(pGH5) (US patent 6,180,373) contains the pGH5 plasmid which carries a mutant serA5 gene coding for a protein not subject to feedback inhibition by serine. Amplification of the serA5 gene is necessary for increasing the amount of serine, which is the precursor of L-tryptophan.

On the other hand, it is also known that P_PPSA promoter is activated by the FruR protein up to 20 times while P_pck_A promoter is activated by FruR only for 4 times (Saier M.H., Jr., and Ramseier T. M., J. Bacteriol., 1996, 178, 12, 3411-3417). Since the object is to enhance activity of PEP carboxykinase by substituting the P_pckA promoter with the P_PPSA promoter, it was necessary to express the fruR gene constitutively. For accomplishing the above two goals, the plasmid pGH5 in the L-tryptophan-producing strain was substituted with pMW-Pι_acuv5-serA5-fruR plasmid, which was constructed by the following scheme.

Based on the known nucleotide sequence of pGH5 plasmid (US patent 6,180,373) the primers depicted in SEQ ID No.3 (primer A) and No. 4 (primer B) were synthesized. Primer A is complementary to a sequence at the start codon region of the serA5 gene which has a restriction enzyme Xbal recognition site introduced at the 5'-end thereof. Primer B is complementary to a sequence of the termination codon region of serA5 gene which has a restriction enzyme Sail recognition site at the 5 '-end thereof.

The pGH5 plasmid was isolated by ordinary methods. PCR was carried out on "ThermoHybaid PCRExpress PCR system" under the following conditions: 30 sec at 95 °C, 30 sec at 55 °C, 30 sec at 72 °C, 25 cycles with a Taq polymerase (MBI Fermentas).

Concurrently, amplification of the fi-uR gene was performed. The chromosomal DNA of E. coli strain W 3350 was isolated by ordinary methods. Primers C (SEQ ID No. 5) and D (SEQ ID No. 6) were used. Primer C (complementary to a start codon region of fruR gene) contains a Sfl I-restriction site. Primer D (complementary to a stop codon region of fruR gene) contains a H lll-restriction site. PCR conditions were as follows: 30 sec at 95 °C, 30 sec at 53 °C, 30 sec at 72 °C, 25 cycles with a Taq polymerase(MBI Fermentas).

The PCR fragments containing serA5 and fruR genes obtained as described above were treated with Salϊ and then ligated. The ligated product was digested by Xbal and Hindϊlϊ, and inserted into the pMW-Pι_acuv₅-lacZ plasmid (Mashko S.V., et al., Biotekhnologiya (rus), 2001, 5, 3-20), which had been previously treated with the same restriction enzymes. Thus the pMW-Pi_acuv₅-serA5-fruR plasmid was obtained.

Example 2: Cloning the yddG gene from E. coli.

The yddG gene, which encodes a transmembrane protein useful for L-tryptophan production (WO03/044192), was cloned using the primers depicted in SΕQ ID No. 7 (primer yddgl) and No. 8 (primer yddg2). Primer yddgl is complementary to a sequence from 91 to 114 bp downstream of the termination codon of the yddG gene which has a restriction enzyme BamHl recognition site introduced at the 5 '-end thereof. Primer yddg2 is complementary to a sequence from 224 to 200 bp upstream of the start codon of the yddG gene, which has a restriction enzyme Sail recognition site introduced at the 5'-end thereof. The chromosomal DNA of E. coli strain TGI was prepared by ordinary methods. PCR was carried out using "Perkin Elmer GeneAmp PCR System 2400" under the following conditions: 40 sec. at 95 °C, 40 sec. at 47 °C, 40 sec. at 72 °C, 30 cycles with a Taq polymerase (Fermentas). The obtained PCR fragment containing yddG gene with its own promoter was treated with BamHl and Sail and inserted into the multicopy vector pAYCTER3, which had been previously treated with the same enzymes. Thus, the plasmid pYDDG2 was obtained.

The pAYCTER3 vector is a derivative of pAYC32. pAYC32 is a moderate copy- number vector and very stable, and is constructed on the basis of plasmid RSF1010 which contains a marker for streptomycin resistance (Christoserdov A. Y., Tsygankov Y. D, Broad-host range vectors derived from a RSF 1010 Tnl plasmid, Plasmid, 1986, v. 16, pp. 161-167). The pAYCTER3 vector was obtained by introduction of the polylinker from the pUC19 plasmid and the strong terminator rrnB into the pAYC32 plasmid instead of its promoter as follows. At first, the polylinker from pUC19 plasmid was obtained by PCR using the primers depicted in SEQ ID No. 9 and No. 10. The obtained PCR product was treated with Eco RI and Bglϊl restriction enzymes. The terminator rrnB was also obtained by PCR using the primers depicted in SEQ ID No. 11 and No. 12. The obtained PCR product was treated with Bglll and Bell restriction enzymes. Then, these two DNA fragments were ligated into pAYC32 plasmid, which had been previously treated with EcøRI and Bell restriction enzymes. Thus the pAYCTΕR3 plasmid was obtained.

Example 3. Substitution of the native upstream region of the pckA gene with the hybrid regulatory element carrying the P_ppSA promoter and SD_φlo in the E. coli chromosome.

To enhance thepckA gene expression, the promoter region of the ppsA gene (P_PPSA) derived from the E. coli chromosome was linked to the Shine-Dalgarno sequence (SD sequence) of the φlO gene from phage T7. This fragment was integrated upstream of thepckA coding region in the chromosome of the E. coli strain BW25113(pKD46) in place of the native region using a method described by Datsenko K.A. and Wanner B.L. (Proc.Natl.Acad.Sci.USA, 2000, 97, 6640-6645), which is also called "Red-mediated integration". In addition, the artificial DNA fragment integrated into the corresponding region of the bacterial chromosome carried chloramphenicol resistance gene (Cm^R) (Fig. 1). The nucleotide sequence of the substituted native region located upstream of pckA gene is presented in the Sequence listing (SEQ ID NO: 1). Escherichia coli strain BW25113 containing the recombination plasmid pKD46 can be obtained from the E. coli Genetic Stock Centre, Yale University, New Haven, USA, the accession number of which is CGSC7630 (WO02103010A1).

Construction of the above-mentioned artificial DNA fragment was accomplished by the following steps. First, a DNA fragment was obtained by PCR having the Bglll - restriction site in the "upstream" region, the P_ppSA promoter, and the SD sequence from the phage T7 φlO gene linked directly to the ATG-initiation codon of thepckA gene in the "downstream" region. The chromosomal DNA from E. coli W3350 strain was as the template in PCR. PCR was conducted using primers PI (SEQ ID NO: 13) and P2 (SEQ ID NO: 14). Primer PI contains a Bglll - restriction site. Primer P2 contains the SD sequence of the phage T7 φlO gene as well as 36 nucleotides from thepckA open reading frame. The sequence from thepckA gene was introduced into primer P2 for further Red-mediated integration of the fragment into the bacterial chromosome.

In all cases, PCR was conducted using the amplificatory "TermoHybaid PCR Express PCR System". The reaction mixture having a total volume of 50 μl contains 5 μl of lOx PCR-buffer with 15 mM MgCl₂ ("Fermentas", Lithuania), 200 μM each of dNTP, 20 nM each of the exploited primers and 1 u Taq-polymerase ("Fermentas", Lithuania). 0.5 μg of the chromosomal DNA was added to the reaction mixture as the template DNA for the further PCR-driven amplification. The PCR conditions were as follows: initial DNA denaturation for 5 min at 95 °C, followed by 25 cycles of denaturation at +95 °C for 30 sec, annealing at +55 °C for 30 sec, elongation at +72 °C for 30 sec, and the final polymerization for 7 min at +72 °C.

Concurrently, the second stage of construction of the DNA fragment of interest was conducted. The Cm^R gene was amplified by PCR using the commercially available plasmid pACYC184 (GenBank/EMBL accession number X06403, "Fermentas", Lithuania) as the template, and primers P3 (SEQ ID NO: 15) and P4 (SEQ ID NO: 16). Primer P3 contains the jBg/II-restriction site used for joining the previously obtained DNA fragment containing the P_PPSA promoter. Primer P4 contains 36 nucleotides which are located upstream of pckA gene derived from E. coli and are necessary for further Red-mediated integration of the fragment into the bacterial chromosome.

Amplified DNA fragments were then concentrated by agarose gel-electrophoresis and extracted from the gel by centrifugation through "GenElute Spin Columns" ("Sigma", USA), and followed by ethanol precipitation.

Two obtained DNA fragments were treated with Bglll restriction endonuclease followed by ligation using T4 DNA ligase (Maniatis T., Fritsch E.F., Sambrook, J.: Molecular Cloning: A Laboratory Manual. 2^nd edn. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press,1989).

The ligated product was amplified by PCR using primers P2 and P4. PCR was conducted in a reaction mixture having a total volume of 50 μl and containing 5 μl of lOx AccuTaq LA buffer ("Sigma",USA) , 200 μM each of dNTP, 20 nM each of the exploited primers and 1 μ AccuTaq LA polymerase ("Sigma", USA). Approximately 50 ng of the ligated DNA products was added to the reaction mixture as the template DNA. The PCR conditions were as follows: initial DNA denaturation for 5 min at 95 °C followed by 25 cycles of denaturation at 95 °C for 30 sec, annealing at 55 °C for 30 sec, elongation at 72 °C for 4 min and the final polymerization for 7 min at 72 ^QC.

The structure of constructed DNA region upstream of the pckA gene is shown in Fig.l. The nucleotide sequence of the constructed DNA region is presented in SEQ ID NO: 18.

The DNA fragment obtained and purified as described above was used for electroporation and Red-mediated integration into the bacterial chromosome of the E. coli strain BW25113(ρKD46). The recombinant plasmid pKD46 (Datsenko, K.A., Wanner, B.L., Proc.Natl.Acad.Sci.USA, 2000, 97, 6640-6645) having the thermosensitive replicon was used as the donor of the phage λ-derived genes responsible for the Red-mediated recombination system.

The cells of BW25113(pKD46) were grown overnight at 30 °C in a liquid LB-medium also containing ampicillin (100 μg/ml), then diluted 1:100 with the SOB-medium (Yeast extract, 5 g/1; NaCl, 0.5 g/1; Tryptone, 20 g/1; KC1, 2.5 mM; MgCl₂, 10 mM) containing ampicillin (100 μg/ml) and L-arabinose (10 mM) (arabinose is used for inducing the plasmid encoding genes of Red system) and grown at 30 °C until reaching the optical density OD₆oo=0.4-0.7 of the bacterial culture. The grown cells from 10 ml of the bacterial culture were washed 3 times by ice-cold de-ionized water followed by suspension in 100 μl of water. 10 μl of DNA fragment (100 ng) dissolved in de-ionized water was added to the cell suspension. The electroporation was performed by "Bio-Rad" electroporator (USA) (No. 165-2098, version 2-89) according to the manufacturer's instructions. Shocked cells were added to 1-ml of SOC medium (Sambrook et al., "Molecular Cloning A Laboratory Manual, Second Edition", Cold Spring Harbor Laboratory Press (1989)), incubated 2 hours at 37 °C, and then spread onto L-agar containing 25 μg/ml of chloramphenicol. Colonies which grew within 24 h were tested for the presence of Cm^R marker upstream of thepckA gene by PCR using primers P4 (SEQ ID NO: 16) and P5 (SEQ ID NO: 17). The same colonies were also tested for the presence of the P_ppSA promoter region upstream of pckA gene by PCR using primers PI (SEQ ID NO: 13) and P5 (SEQ ID NO: 17). For these tests, a freshly isolated colony was suspended in 20μl water, of which lμl was used in PCR. PCR conditions were as follows: initial DNA denaturation for 10 min at 95 °C; then 30 cycles of denaturation at 95 °C for 30 sec, annealing at 55 °C for 30 sec and elongation at 72 °C for 1 min; the final polymerization for 7 min at 72 °C. A few of the Cm^R colonies tested contained the desired 1749 nt and 697 nt DNA fragments, which confirmed the presence of the entire constructed DNA region upstream of thepckA gene and the hybrid regulatory element, carrying the P_PPSA promoter and SD_φι₀ in E. coli chromosome, respectively. One of the obtained strains was cured from the thermosensitive plasmid pKD46 by culturing at 37 °C. The resulting strain was named E. coli strain BW25113 (Cm-P_ppSA-SD_φιo-_.pc 4).

Example 4: Effect of enhanced pckA gene expression on tryptophan production.

The tryptophan-producing E. coli strain SV164 (pMW-P_/βcuv₅-.s'er45-/ruR, pYDDG2) was used as a parental strain for evaluating the effect of enhanced pckA gene expression on tryptophan production. The strain SV164 is described in detail in US patent 6,180,373.

To test the effect of enhancement of pckA gene expression under control of the P_ppSA promoter and SD of phage T7 φlO gene on tryptophan production, the above-mentioned DNA fragment from the chromosome of E. coli strain BW25113 (Cm-P_ppSA-SD_φιo-i?cfc4) was transferred to the tryptophan-producing E. coli strain SV164 (pMW-P_/acuv5-ser./45-/rwR) by PI transduction (Miller, J.H. (1972) Experiments in Molecular Genetics, Cold Spring Harbor Lab. Press, Plainview, NY). Then, plasmid pYDDG2 was introduced into both the SV164

strain and the resulting transductant.

Both the SV164 (pMW-Pι_acuv₅-ser,45-./ αR, pYDDG2) and the SV164 P_ppsA-SD_φlo-pckA (pMW-Pi_acuv₅-_'Ser.15-/r«R, pYDDG2) strains were cultivated overnight with shaking at 37°C in 3 ml of nutrient broth supplemented with 30 μg/ml of ampicillin and 25 μg/ml of streptomycin. 0.3 ml of the obtained cultures were inoculated into 3 ml of a fermentation medium containing tetracycline (20 μg/ml) in 20 x 200 mm test tubes, and cultivated at 37°C for 48 hours on a rotary shaker at 250 rpm.

The composition of the fermentation medium is presented in Table 1. Table 1

Section A had pH 7.1 adjusted by NH₄OH. Each section was sterilized separately.

After the cultivation, the amount of L-tryptophan which accumulated in the medium was determined by TLC. 10 x 15 cm TLC plates coated with 0.11 mm layers of Sorbfil silica gel without fluorescent indicator (Stock Company Sorbpolymer, Krasnodar, Russia) were used. Sorbfil plates were developed with a mobile phase: propan-2-ol : ethylacetate : 25% aqueous ammonia : water = 16 : 16 : 3 : 9 (v/v). A 2% solution of ninhydrin in acetone was used as a visualizing reagent.

The results are presented in the Table 2. Table 2

E. coli strain OD₆QO Amount of tryptophan, g/1

SV164 (pMΨ-?ι_acVy5-serA5-fruR, pYDDG2) 7.5 4.20

SV164 Ppps_A-SDφio-pc .4 (pMW-Pι_acυv₅-serA5-frιιR, 7.5 4.61 pYDDG2)

As it can be seen from the Table 2, the enhancement of pckA gene expression improved tryptophan productivity of the SV164 (pMW-?i_aCχjv₅-serA5-fi-uR) strain.

While the invention has been described with reference to preferred embodiments thereof, it will be apparent to one skilled in the art that various changes can be made, and equivalents employed, without departing from the scope of the invention. All the cited references herein, including the foreign priority document, RU 2003109477, are incorporated as a part of this application by reference in its entirety.

Industrial Applicability

According to the present invention, productivity of an aromatic L-amino acid-producing strain can be enhanced and a method for producing the aromatic L-amino acid using the strain is provided. The present invention is useful in the fermentation industry.

Claims

1. An L-amino acid-producing bacterium belonging to the genus Escherichia, wherein the bacterium has been modified to have enhanced activity of PEP carboxykinase.

2. The bacterium according to claim 1, wherein the activity of PEP carboxykinase is enhanced by modifying an expression control sequence of the PEP carboxykinase gene on the chromosome of the bacterium so that the expression of the gene is enhanced, or by increasing copy number of the gene.

3. The bacterium according to claim 2, wherein a native promoter of said gene is replaced with more, potent promoter.

4. The bacterium according to claim 2, wherein a native SD sequence of said gene is replaced with a more efficient SD sequence.

5. The bacterium according to any of claims 2 to 4, wherein the PEP carboxykinase gene is originated from a bacterium belonging to the genus Escherichia.

6. The bacterium according to claim 5, wherein the PEP carboxykinase gene encodes the following protein (A) or (B):

(A) a protein, which comprises the amino acid sequence shown in SEQ ID NO: 2 in Sequence listing;

(B) a protein which comprises the amino acid sequence shown in SEQ ID NO: 2 in Sequence listing including deletion, substitution, insertion or addition of one or several amino acids, and which has an activity of PEP carboxykinase.

7. The bacterium according to claim 5, wherein the PEP carboxykinase gene comprises the following DNA (a) or (b):

8. The bacterium according to claim 7, wherein the stringent conditions are conditions in which washing is performed at 60°C, and at a salt concentration corresponding to 1 x SSC and 0.1 % SDS.

9. The bacterium according to any of claims 1 to 8, wherein the bacterium is further modified to have enhanced expression of yddG open reading frame.

10. The bacterium according to any of claims 1 to 9, wherein the L-amino acid is an aromatic L-amino acid selected from the group consisting of L-tryptophan, L-phenylalanine and L-tyrosine.

11. A method for producing an aromatic L-amino acid comprising cultivating the bacterium according to any of claims 1 to 10 in a culture medium to cause accumulation of L-amino acid in the culture medium, and collecting the L-amino acid from the culture medium.

12. The method according to claim 11, wherein the L-amino acid is an aromatic amino acid selected from the group consisting of L-tryptophan, L-phenylalanine and L-tyrosine.

13. The method according to claim 12, wherein the bacterium has enhanced expression of genes for aromatic amino acid biosynthesis.