Abstract:
The folic acid concentration in amino acid producing organisms is reduced or completely removed in order to increase the production of L-serine. The enzymes and genes, which are involved in the biosynthesis of folic acid, can be completely excluded or at least reduced in activity. In a preferred embodiment, SEQ ID NOs: 1-7 are utilized.

Description:
[0001]    The invention relates to a method for producing L-serine, and to gene sequences, vectors, and microorganisms suitable therefor. 
       BACKGROUND OF THE INVENTION 
       [0002]    The amino acid L-serine is used in human medicine, the pharmaceutical industry, the food industry, and animal nutrition. 
         [0003]    It is known that amino acids are produced by fermenting strains of coryneform bacteria, in particular  Corynebacterium glutamicum . Due to its great importance there are continuous efforts to improve the production process. Such improvements can relate to fermentation measures such as e.g. stirring and supplying with oxygen or the composition of nutrient media such as e.g. the sugar concentration during fermentation or processing for product form using e.g. ion exchange chromatography or the intrinsic properties of the microorganism itself. 
         [0004]    Methods for mutagenesis, selection, and mutant selection are used for improving the properties of these microorganisms. In this manner strains are obtained that are resistant to antimetabolites or that are auxotrophic for metabolites that are meaningful in terms of regulator issues, and that produce L-amino acids and L-serine. Thus a coryneform bacterium is described that is resistant to azaserine or beta-(2-thienyl)-DL-alanine and produces L-serine (EP0943687). It is furthermore prior art that biosynthesis activities can be can be enhanced by means of genetic engineering methods and this leads to enhanced amino acid formation. Thus for example EP 0931833A2 describes that an increase in the biosynthesis enzyme phosphoserine-phosphatase and phosphoserine-transaminase is advantageous for L-serine formation. It is furthermore described that a gene that codes for D-3-phosphoglycerate-dehydrogenase can be used for L-serine formation (EP 0931833A2, PCT WO 93/12235). 
         [0005]    Moreover it is known that, in addition to an increase in the activity of enzymes of the L-amino acid synthesis pathway, the decrease or even the exclusion of enzymes that are involved in degradation reactions for the L-amino acid can lead to an improvement in L-amino acid accumulation. Thus it has been found that excluding the enzyme L-serine dehydratase leads to higher L-serine accumulations (PCT/DE 2004/000248). Furthermore, a reduction in the activity of the enzyme serine hydroxymethyltransferase leads to reduced L-serine degradation (U.S. Pat. No. 6,596,516; Simic et al. (2002) Appl. Environ Microbiol. 68:3321-3327). 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    It is the object of the invention to provide a method, a microorganism suitable therefor, and vectors with which the production of L-serine can be increased. Moreover, a method, microorganisms, and vectors should be provided with which it is possible to attain an increase in the production of cysteine, tryptophan, and methionine. 
         [0007]    Surprisingly, it was found that coryneform bacteria produce L-serine in an improved manner after modification or exclusion of the coding genes for folic acid synthesis. With the inventive method and the inventive bacteria and the inventive gene sequences for enzymes or regulators that catalyze folic acid synthesis or are involved in regulating folic acid synthesis it is also possible to produce L-serine with a yield that is substantially higher than that of strains not inventively modified. Table 2 portrays the extent of the increase in L-serine production. 
         [0008]    The invention is described in general in the following: 
         [0009]    In accordance with the invention, L-serine production and the production of cysteine, tryptophan, and methionine are increased in that the folic acid concentration is reduced in an organism that produces amino acid. The organism preferably already produces L-serine prior to the inventive modification. 
         [0010]    For instance, the reduction in the folic acid concentration can be attained by reducing the synthesis of folic acid or by its degradation. 
         [0011]    The reduction or prevention of synthesis of folic acid can be attained by directed or nondirected mutation of genes that are involved in the biosynthesis of folic acid. 
         [0012]    Examples of mutations that lead to a reduction or exclusion of folic acid production are deletion mutation, insertion mutation, substitution mutation, or point mutation of genes that are involved in the biosynthesis of folic acid. 
         [0013]    Furthermore, there can be a reduction or exclusion of folic acid production in genetically modified or unmodified genes of proteins that are involved in the biosynthesis of folic acid in that the expression of genes involved in the biosynthesis of folic acid is reduced or prevented. 
         [0014]    It is possible to attain a reduction or exclusion of the expression of genes that are involved in the biosynthesis pathway of folic acid using modification of promoters, preferably weakening of promoters, particularly preferred exclusion of promoters such as signal structures, repressor genes, activators, operators, attenuators, ribosome binding sites or start codons, terminators, or furthermore by modifying, preferably weakening, particularly preferred exclusion or attenuating of regulators or the stability of the transcripts. Furthermore, regulatable promoters can be used, in particular weakened promoters can be used. 
         [0015]    At the enzyme level, the activity of the enzymes involved in the biosynthesis of folic acid can be attained by reduction or exclusion of the catalytic activity and stability of the enzymes. The same effect can be attained by modifying the allosteric center or a feedback inhibition of the enzymes. One typical option for reducing or excluding the activity of the enzymes is protein modification, for instance by phosphorylation or adenylation. It is also possible to increase the proteolytic degradation of enzymes involved in the biosynthesis pathway of folic acid. 
         [0016]    Typical enzymes, the activity of which can be reduced or excluded in the manner described, are GTP cyclohydrolase, neopterine triphosphate pyrophosphatase, neopterin aldolase, 6-hydroxymethylpterinpyrophosphokinase, 4-amino-4-deoxy-chorismate synthase, 4-amino-4-deoxy-chorismate lyase, pteroate synthase, folate synthase, and dihydrofolate reductase. 
         [0017]    The subject-matter of the invention is also a vector that contains a tool that is suitable for causing inventive genetic modifications in a production organism. 
         [0018]    The inventive vectors contain tools that are suitable for deleting the gene for synthesizing 4-amino-4-deoxy-chorismate synthase and/or 4-amino-4-deoxy-chorismate lyase. 
         [0019]    The sequence for the deletion of the 4-amino-4-deoxy-chorismate synthase gene is depicted in SEQ ID NO: 1. SEQ ID NO: 2 illustrates the structure of a vector that bears SEQ ID NO: 1. 
         [0020]    The sequence for the deletion of the 4-amino-4-deoxy-chorismate lyase gene is depicted in SEQ ID NO: 3. SEQ ID NO: 4 illustrates the structure of a vector that bears SEQ ID NO: 3. 
         [0021]    The sequence for the deletion of the 4-amino-4-deoxy-chorismate synthase gene and the 4-amino-4-deoxy-chorismate lyase gene is depicted in SEQ ID NO: 5. 
         [0022]    SEQ ID NO: 6 illustrates the structure of a vector that bears the SEQ ID NO: 5 for the deletion of the 4-amino-4-deoxy-chorismate synthase gene and the 4-amino-4-deoxy-chorismate lyase gene. 
         [0023]    SEQ ID NO: 7 depicts a plasmid that is suitable for further increasing L-serine production. In  FIG. 4  it is the plasmid designated pEC-T18mob2-serA fbr CB-. 
         [0024]    The structures responsible for the deletion can also be added to other vector models that are suitable for the specific organism. In addition to the frequently used cyclical factors, linear vectors or phages are also suitable for vectors. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    The figures depict vectors that can be used for instance for the inventive modifications of the organisms producing L-serine. 
           [0026]      FIG. 1 : pK19mobsacB_pabAB corresponds to SEQ ID NO: 2; 
           [0027]      FIG. 2 : pK19mobsacB_pabC corresponds to SEQ ID NO: 4; 
           [0028]      FIG. 3 : pK19mobsacB_pabABC corresponds to SEQ ID NO: 6; 
           [0029]      FIG. 4 : pEC-T18mob2-serA fbr CB corresponds to SEQ ID NO: 7. 
       
    
    
       [0030]    The tools in accordance with SEQ ID NOs: 1, 3, and 5 are preferably added to vectors in L-serine production organisms that already produce L-serine prior to the modification. 
         [0031]    All of the sequences indicated should also include variants that have 90%, preferably 95% homology. 
         [0032]    Suitable organisms are for instance coryneform bacteria such as  Corynebacterium glutamicum  or  Brevibacterium . It is also possible to use  Enterobacteria, Bacillaceae , or yeasts types that exhibit a reduced folic acid concentration as production organisms. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0033]    The invention shall be described more precisely in the following. 
         [0034]    The subject-matter of the invention is a method for fermentative production of L-serine using coryneform bacteria in which the genes that code for folic acid synthesis are modified, excluded, or changed in their expression or a deficiency of folic acid is caused in bacteria that naturally require folic acid or regulation of folic acid synthesis is influenced such that this deficiency in the folic acid occurs. Since folic acid synthesis proceeds from an intermediate of nucleotide synthesis and from an intermediate of the synthesis of aromatic amino acids, in principle these reactions can also be modified or excluded provided that nucleotides and aromatic amino acids themselves are available in sufficient quantities, as is the case for example with external supplementation. Moreover, the deficiency in folic acid can also be caused by the deletion or modification of regulators that control the expression of genes of folic acid or the metabolism pathways connected thereto. 
         [0035]    These strains used preferably produce L-serine prior to the modification of the folic acid synthesis. 
         [0036]    The term “modification” includes weakening folic acid synthesis genes and complete deletion of folic acid synthesis genes. This includes nondirected mutageneses and directed recombinant DNA techniques. Using these methods it is possible for instance to delete genes for folic acid synthesis in the chromosome. Suitable methods for this are described in Schäfer et al. (Gene (1994) 145: 69-73) and also Link et al. (J. Bacteriology (1998) 179: 6228-6237). It is also possible to delete only portions of the gene or even to exchange mutated fragments of genes. Thus the loss of or a reduction in folic acid synthesis activity is attained by depletion or exchange. One advantageous embodiment of the inventive method is for instance the inventively modified  C. glutamicum  strain ATCC13032DpykdsdaADpabABpserABC, which among other things bears a deletion in the pabAB gene. 
         [0037]    Mutagenesis methods represent another option for weakening or excluding folic acid synthesis activity. Among these are nondirected methods that use chemical reagents such as e.g. N-methyl-N-nitro-N-nitrosoguanidine or even UV irradiation for mutagenesis, and a subsequent hunt in the desired microorganisms for a reduction in or loss of folic acid synthesis activity. Methods for initiating mutations and for mutant hunts are known in general and can be researched, inter alia, in Miller (A Short Course in Bacterial Genetics, A Laboratory Manual and Handbook for  Escherichia coli  and Related Bacteria (Cold Spring Harbor Laboratory Press, 1992)) or in the “Manual of Methods for General Bacteriology” from the American Society for Bacteriology (Washington D.C., USA, 1981). 
         [0038]    In addition, the expression of genes from folic acid synthesis can be reduced by modifying the signal structures for gene expression. Signal structures are for instance repressor genes, activator genes, operators, promoters, attenuators, ribosome binding sites, the start codon, and terminators. One skilled in the art can find information regarding these e.g. in patent application WO 96/15246, in Boyd and Murphy (J. Bacteriol. 1988. 170: 5949), in Voskuil and Chambliss (Nucleic Acids Res. 1998. 26; 3548), in Jensen and Hammer (Biotechnol. Bioeng. 1998 58: 191), in Patek et al. (Microbiology (1996) 142: 1297) and in known textbooks in the field of genetics and molecular biology such as e.g. the textbook by Knippers (“Molekulare Genetik” [Molecular Genetics], 8th edition, Georg Thieme Verlag, Stuttgart, Deutschland, 2001) or that of Winnacker (“Gene und Klone” [Genes and Clones], VCH Verlagsgesellschaft, Weinheim, Deutschland, 1990). The promoter and regulation region that is disposed upstream of the structure gene can be mutated. 
         [0039]    Using regulatable promoters it is also possible to reduce the expression during the course of the fermentative L-serine formation. In addition, however, it is possible to regulate the translation in that for instance the stability of the m-RNA is reduced. This can be attained in that the stability is weakened by additional and/or modified sequences at the 5′-end or 3′-end of the gene. Examples of this are described for genes from  Bacillus subtilis  (Microbiology (2001) 147; 1331-41) or yeast (Trends Biotechnol. 1994, 12:444-9). As described, it is furthermore possible to influence enzyme activity using intrinsic proteolytic activity (Mol. Microbiol. (2005) 57:576-91). 
         [0040]    Moreover, genes can be used that code with less activity for the corresponding enzyme of the folic acid synthesis. Mutations that lead to a modification or reduction in the catalytic activity of enzyme proteins are known. Examples of this can be found in the works of Qiu and Goodman (J Biological Chemistry (1997) 272: 8611-8617), Sugimoto et al. (Bioscience Biotechnology and Biochemistry (1997) 61: 1760-1762), and Möckel (“Die Threonindehydratase aus  Corynebacterium glutamicum : Auflebung der allosterischen Regulation und Struktur des Enzyms” [The Threonine Dehydratase from  Corynebacterium glutamicum : Removing the Allosteric Regulation and Structure of the Enzyme], Reports from the Jülich Research Centre, Jül-2906, ISSN09442952, Jülich, Germany, 1994). Summarizing descriptions can be found in textbooks for the fields of genetics and molecular biology, such as e.g. by Hagemann (“Allgemeine Genetik” [General Genetics], Gustav Fischer Verlag, Stuttgart, 1986). Alternatively, moreover, it is possible to attain reduced expression and activity of the folic acid synthesis enzyme by modifying media composition and culturing. One skilled in the art can find instructions, inter alia, in Martin et al. (Bio/Technology 5, 137-146 (1987)), Guerrero et al. (Gene 138, 35-41 (1994)), Tsuchiya and Morinaga (Bio/Technology 6, 428-430 (1988)), Eikmanns et al. (Gene 102, 93-98 (1991)), European patent EP 0 472 869, U.S. Pat. No. 4,601,893, Schwarzer and Pühler (Bio/Technology 9, 84-87 (1991)), Reinscheid et al. (Applied and Environmental Microbiology 60, 126-132 (1994)), LaBarre et al. (Journal of Bacteriology 175, 1001-1007 (1993)), and in patent application WO 96/15246. 
         [0041]    In this manner genes for folic acid synthesis of  C. glutamicum  can be expressed in a reduced manner or deleted or the enzyme activities can be reduced. 
         [0042]    Furthermore, in addition to the induced deficiency of folic acid, it can be advantageous for the production of L-serine to strengthen, in particular to overexpress, individually or in combinations, one or a plurality of the genes selected from the group:
       the serA gene that codes for the 3-phosphoglycerate-dehydrogenase;   the serC gene that codes for the phosphoserine-transaminase;   the serB gene that codes for the phosphoserine phosphatase;
 
or to strengthen or overexpress, individually or in combinations, alleles of these genes, in particular
   the serA genes that code for feedback-resistant 3-phosphoglycerate-dehydrogenase.       
 
         [0047]    Moreover, in addition to the induced deficiency of folic acid, it can be advantageous for the production of L-serine to reduce or delete, individually or in combination, one or a plurality of the genes selected from the group:
       the aecD gene that codes for the cystathionine lyase;   the metC gene that codes for the cystathionine lyase;   the sdaA gene that codes for the serine dehydratase;   the glyA gene that codes for the serine hydroxymethyltransferase;   the trpB gene that codes for the beta subunit of the tryptophan synthase;   the ilvA gene that codes for the threonine dehydratase;   the pyk gene that codes for the pyruvate kinase.       
 
         [0055]    In the framework of the present invention, the inventive microorganisms include bacteria from the  Corynebacterium  or  Brevibacterium  genera that are modified using classical and/or genetic-molecular methods such that their metabolism flow is strengthened toward biosynthesis of amino acids or derivatives thereof. The present invention includes all of the already known amino acid production strains. Furthermore included in accordance with the invention are those production strains that one skilled in the art can produce according to current methods in analogy with experiences from other microorganisms, for instance Enterobacteria, Bacillaceae, and types of yeast. Furthermore included in accordance with the invention are also those amino acid production strains in which the degradation of L-serine is modified or weakened. This can occur for instance using directed genetic engineering modifications of enzymes that degrade L-serine or corresponding genes. A few microorganisms that are suitable in accordance with the invention are cited as examples in the following. This list shall not be restricting, however: 
         [0000]      Corynebacterium glutamicum  ATCC13032;
   Corynebacterium acetoglutamicum  ATCC 15806;
   Corynebacterium acetoacidophilum  ATCC13870;
   Corynebacterium thermoaminogenes  FERM BP-1539;
   Brevibacterium flavum  ATCC14067;
   Brevibacterium lactofermentum  ATCC13869; and
   Brevibacterium divaricatum  ATCC 14020.
 
         [0056]    The subject-matter of the present invention is also a pabAB and pabC gene SEQ ID NOs: 1, 3, or 5, that is characterized in that a part of the sequence was cut out by means of defined deletion so that only inactivated or weakened amino deoxychorismate synthase or amino deoxychorismate lyase activity can result. The pabAB and pabC gene sequences are preferably isolated from microorganisms from the  Corynebacterium  or  Brevibacterium  genus. A few of these more specifically identified microorganisms are listed here: 
         [0000]      Corynebacterium glutamicum  ATCC13032;
   Corynebacterium acetoglutamicum  ATCC15806;
   Corynebacterium acetoacidophilum  ATCC13870;
   Corynebacterium thermoaminogenes  FERM BP-1539;
   Brevibacterium flavum  ATCC14067;
   Brevibacterium lactofermentum  ATCC13869; and
   Brevibacterium divaricatum  ATCC14020.
 
       Example 1 
     Structure of the Plasmid pK19mobsacBDpabAB 
       [0057]    Using known methods, the pabAB gene from  C. glutamicum  is replaced in the chromosome by a pabAB gene shortened by 1734 bp (J. Bacteriol. (1997) 179:6228-37; Gene (1994) 145: 69-73). For constructing the vector used for the gene exchange, the primers listed in following were synthesized, and they were derived from the publicly accessible genome sequence (NCBI accession number YP — 225287; NC — 006985): 
         [0000]    
       
         
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
           
               
                 pabAB-del-A 
                   
               
             
          
           
               
                 (SEQ ID NO:8) 
                   
               
             
          
           
               
                 5′-CGGGATCCTCAGGCTCGCACGTTGGAGGG-3′ 
                   
               
               
                   
               
               
                 pabAB-del-B: 
                   
               
             
          
           
               
                 (SEQ ID NO:9) 
                   
               
             
          
           
               
                 5′-CCCATCCACTAAACTTAAACAAAACGTGAAAGAATCATAATT-3′ 
                   
               
               
                   
               
               
                 pabAB-del-C: 
                   
               
             
          
           
               
                 (SEQ ID NO:10) 
                   
               
             
          
           
               
                 5′-TGTTTAAGTTTAGTGGATGGGGAGTGGGAGGAAATCCGCGTT-3′ 
                   
               
               
                   
               
               
                 pabAB-del-D: 
                   
               
             
          
           
               
                 (SEQ ID NO:11) 
                   
               
             
          
           
               
                 5′-GTGGATCCGCCCAAAACACCACGGTGGCGT-3′ 
                   
               
             
          
         
       
     
         [0058]    Primer pabAB-del-A begins 522 bp prior to the translation start and pabAB-del-D 436 bp behind the translation stop for the pabAB gene. The primer pabAB-del-B is 21 bp behind the translation start, the primer pabAB-del-C is 66 bp prior to the translation stop, and both have complementary linker regions, as indicated in Link et al. (J. Bacteriol. (1997) 179: 6228-37). PCR amplifications were performed in parallel with primer combination pabAB-del-A and pabAB-del-B and with primer combination pabAB-del-B and pabAB-del-C with chromosomal DNA from  C. glutamicum  ATCC13032. The PCR reaction was performed in 30 cycles in the presence of 200 μM deoxynucleotide triphosphates (dATP, dCTP, dGTP, dTTP), 600 nM each of the corresponding oligonucleotides, 100 ng chromosomal DNA of  Corynebacterium glutamicum  ATCC13032, 1/10 volume 10-fold reaction buffer, and 2.6 units of a heat-stable Taq/Pwo DNA polymerase mixture (Expand High Fidelity PCR System from Roche Diagnostics, Mannheim, Germany) in a thermocycler (PTC-100, MJ Research, Inc., Watertown, USA) under the following conditions: 
         [0000]    94° C. for 30 seconds, 50° C. for 30 seconds, and 72° C. for 40 seconds. The elongation step at 72° C. was extended after 10 cycles by five seconds per cycle. After the PCR reaction, the 563 bp fragment obtained from the 5′-flanking area and the 528 bp fragment from the 3′-flanking area were isolated from a 0.8% agarose gel using the QIAExII gel extraction kit (Qiagen) according to manufacturer instructions, and both fragments were used as templates in the second PCR with the primers pabAB-del-A and pabAB-del-D. Amplification occurred in 35 cycles in the presence of 200 μM deoxynucleotide triphosphates, 600 nM each of the corresponding oligonucleotides, 20 ng of the isolated template DNA from the first PCR, 1/10 volume 10-fold reaction buffer, and 2.6 units of the Taq/Pwo DNA polymerase mixture under the following conditions:
 
94° C. for 30 seconds, 50° C. for 30 seconds, and 72° C. for 80 seconds. The elongation step was extended after 10 cycles by five seconds each. After the PCR reaction, the 1122 bp DNA fragment obtained, which now contains the inactivated pabAB gene with a 1734 bp central deletion, was isolated from a 0.8% agarose gel and cloned in the vector pK19mobsacB (Gene 145: 69-73 (1994)). The resulting plasmid pK19mobsacBDpabAB ( FIG. 1 ) was confirmed in terms of correctness by sequencing.
 
       Example 2 
     Structure of the Plasmid pK19mobsacBDpabC 
       [0059]    Analogous to the procedure described in Example 1, the vector pK19mobsacBDpabC suitable for the gene exchange was constructed for deleting the pabC gene of  C. glutamicum . The primers required for this for PCR amplification were again derived from the publicly accessible genome sequence (NCBI accession number YP — 225288.1; NC — 006958). They are provided in the following: 
         [0000]    
       
         
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
           
               
                 pabC-del-A: 
                   
               
             
          
           
               
                 (SEQ ID NO:12) 
                   
               
             
          
           
               
                 5′-GAGGATCCAATCATTGCTGAGCTGCGCAG-3′ 
                   
               
               
                   
               
               
                 pabC-del-B: 
                   
               
             
          
           
               
                 (SEQ ID NO:13) 
                   
               
             
          
           
               
                 5′-CCCATCCACTAAACTTAAACAATCAACAACTGTGGGTGTTGA-3′ 
                   
               
               
                   
               
               
                 pabC-del-C: 
                   
               
             
          
           
               
                 (SEQ ID NO:14) 
                   
               
             
          
           
               
                 5′-TGTTTAAGTTTAGTGGATGGGTCGGTGAAGCCCTGGAATGAA-3′ 
                   
               
               
                   
               
               
                 pabC-del-D: 
                   
               
             
          
           
               
                 (SEQ ID NO:15) 
                   
               
             
          
           
               
                 5′-AGGGATCCGTGATGAGTCCGATCTCGGAA-3′ 
                   
               
             
          
         
       
     
         [0060]    Primer pabC-del-A begins 500 bp prior to the translation start and pabC-del-D 500 bp behind the translation stop of the pabC gene. The primer pabC-del-B is 51 bp behind the translation start and pabC-del-C 48 bp prior to the translation stop. The last two primers each have complementary linker regions. In parallel, a 602 bp 5′ flanking area was amplified with the primary combination pabC-del-A and pabC-del-B and a 597 bp 3′ flanking area of the fragment to be deleted was amplified with the primary combination pabC-del-C und pabC-del-D. The PCR reaction was performed using the standard procedure, such as for instance in Example 1 with chromosomal DNA of  C. glutamicum  ATCC13032. After the PCR reaction, the DNA fragments obtained were isolated with the QIAExII gel extraction kit (Qiagen) and both fragments were used as templates in a further PCR. pabC-del-A and pabC-del-D were used as primers. The PCR reaction was performed using the standard procedure such as for instance in Example 1 with chromosomal DNA from  C. glutamicum  ATCC13032. After the PCR reaction, the 1178 bp DNA fragment obtained, which now contained the inactivated pabC gene with a 585 bp central deletion, was isolated from a 0.8% agarose gel and ligated with the vector pK19mobsacB (Schäfer et al. Gene 145: 69-73 (1994). The  Escherichia coli  strain DH5αmcr (Grant et al., Proceedings of the National Academy of Sciences of the United States of America USA (1990) 87: 4645-4649) was transformed with the ligation preparation. The plasmid pK19mobsacBDpabC obtained ( FIG. 2 ) was examined for correctness using restriction digestion and sequencing. 
       Example 3 
     Structure of the Plasmids pK19mobsacBDpabABC 
       [0061]    Analogous to the procedure described in Example 1, the vector pK19mobsacBDpabABC suitable for the gene exchange was constructed for deleting the pabABC gene of  C. glutamicum . The primers required for this PCR amplification, derived from the publicly accessible genome sequence (NCBI accession number YP — 225287; YP — 225288.1; NC — 006958), are provided in the following: 
         [0000]    
       
         
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
           
               
                 pabABC-del-A: 
                   
               
             
          
           
               
                 (SEQ ID NO:8) 
                   
               
             
          
           
               
                 5′-CGGGATCCTCAGGCTCGCACGTTGGAGGG-3′ 
                   
               
               
                   
               
               
                 pabABC-del-13: 
                   
               
             
          
           
               
                 (SEQ ID NO:9) 
                   
               
             
          
           
               
                 5′-CCCATCCACTAAACTTAAACAAAACGTGAAAGAATCATAATT-3′ 
                   
               
               
                   
               
               
                 pabABC-del-C: 
                   
               
             
          
           
               
                 (SEQ ID NO:14) 
                   
               
             
          
           
               
                 5′-TGTTTAAGTTTAGTGGATGGGTCGGTGAAGCCCTGGAATGAA-3′ 
                   
               
               
                   
               
               
                 pabABC-del-D: 
                   
               
             
          
           
               
                 (SEQ ID NO:15) 
                   
               
             
          
           
               
                 5′-AGGGATCCGTGATGAGTCCGATCTCGGAA-3′ 
                   
               
             
          
         
       
     
         [0062]    Primer pabABC-del-A begins 500 bp prior to the translation start and pabABC-del-D begins 500 bp behind the translation stop of the pabABC gene. Primer pabABC-del-B is 51 bp behind the translation start of pabAB and pabABC-del-C is 48 bp prior to the translation stop of pabC. The latter two primers each have complementary linker regions. In parallel, a 593 bp 5′ flanking area was amplified with the primary combination pabABC-del-A and pabABC-del-B and a 597 bp 3′ flanking area of the fragment to be deleted was amplified with the primary combination pabABC-del-C and pabABC-del-D. The PCR reaction was performed using the standard procedure, such as for instance in Example 1 with chromosomal DNA of  C. glutamicum  ATCC13032. After the PCR reaction, the DNA fragments obtained were isolated with the QIAExII gel extraction kit (Qiagen) and both fragments were used as templates in a further PCR. Primers pabABC-del-A and pabABC-del-D were used as primers. The PCR reaction was performed using the standard procedure such as for instance in Example 1 with chromosomal DNA from  C. glutamicum  ATCC13032. After the PCR reaction, the 1169 bp DNA fragment obtained, which contained the inactivated pabABC gene with a 2475 bp central deletion, was isolated from a 0.8% agarose gel and ligated with the vector pK19mobsacB (Schäfer et al. Gene 145: 69-73 (1994)). The  Escherichia coli  strain DH5αmcr (Grant et al., Proceedings of the National Academy of Sciences of the United States of America USA (1990) 87: 4645-4649) was transformed with the ligation preparation. The plasmid pK19mobsacBDpabC obtained ( FIG. 3 ) was examined for correctness using restriction digestion and sequencing. 
       Example 4 
     Structure of the Folic Acid Auxotrophic Strains  C. glutamicum  DsdaADpabAB,  C. glutamicum  DsdaADpabC,  C. glutamicum  DsdaADpabABC 
       [0063]    The plasmid pK19mobsacBDpabAB, or pK19mobsacBDpabC and pK19mobsacBDpabABC, was added to  C. glutamicum  DsdaA, which is described in patent application PCT/DE2004/000248, by means of electroporation, and selected for kanamycin resistance. Only those clones in which the plasmid was integrated into the chromosome using homologous recombination were kanamycin-resistant. In each case, one clone was selected that after examination demonstrated the saccharose sensitivity imparted by the plasmid (Gene (1994) 145: 69-73). This clone was cultivated in 50 ml BHI medium (brain-heart infusion medium, Difco Laboratories, Detroit, USA) without kanamycin or saccharose. Then 100 μl from 10-2, 10-3, and 10-4 dilutions of the culture were each plated on BHIS plates (BHI medium with 0.5 M sorbitol) with 10% (w/v) saccharose (FEMS Microbiol Lett. (1989) 53:299-303). The saccharose-resistant clones obtained were then tested for kanamycin sensitivity and verified using PCR analysis. The following primer combinations were used: 
         [0000]    
       
         
               
               
               
               
             
           
               
                 Deletion of pabAB: 
                   
                   
                   
               
               
                 pabAB-del-pr-u2 
                 5′-TGGCTCACTTCGCTGGTCTTGTTG-3′ 
                 (SEQ ID NO:16) 
               
               
                   
               
               
                 pabAB-del-pr-12 
                 5′-GAATGGTTGCGGCGAGTGTCA-3′ 
                 (SEQ ID NO:17) 
               
               
                   
               
               
                 Deletion of pabC: 
               
               
                 pabC-del-pr-u2 
                 5′-GTTGGGGGAGCAGGACGAGTGGT-3′ 
                 (SEQ ID NO:18) 
               
               
                   
               
               
                 pabC-del-pr-12 
                 5′-TACGCGCATCTGGAAGCCTGGTTA-3′ 
                 (SEQ ID NO:19) 
               
               
                   
               
               
                 Deletion of pabABC: 
               
               
                 pabABC-del-pr-u2 
                 5′-CGTTCCGGCATCATTCTGGCTAAG-3′ 
                 (SEQ ID NO:20) 
               
               
                   
               
               
                 pabABC-del-pr-12 
                 5′-GCGACTCCGGGTTGTTCCTGATAA-3′ 
                 (SEQ ID NO:21) 
               
             
          
         
       
     
         [0064]    The successful deletion resulted in a band of 1426 bp for the pabAB gene site, a band of 1663 bp for the pabC gene site, and a band of 1447 bp for the pabABC gene site. In this manner it was possible to obtain starting strain clones that have specific deletions in genes for folic acid synthesis. 
         [0065]    These strains were designated  C. glutamicum  DsdaADpabAB,  C. glutamicum  DsdaADpabC, and  C. glutamicum  DsdaADpabABC. 
         [0066]    These three strains were plated on the minimal medium CGXII (J Bacteriol. (1993) 175:5595-603) and on identical medium that contained the 1 mM folic acid. Table 1 provides the results. 
         [0000]    
       
         
               
             
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Folic acid requirement for the  Corynebacterium glutamicum   
               
               
                 mutants produced. 
               
             
          
           
               
                   
                 Growth 
               
             
          
           
               
                   
                 
                   C. glu. 
                 
                 
                   C. glu. 
                 
                 
                   C. glu. 
                 
                 
                   C. glu. 
                 
               
               
                 Folic acid 
                 DsdaA 
                 DsdaADpabAB 
                 DsdaADpabC 
                 DsdaADpabABC 
               
               
                   
               
               
                 0 mM 
                 + 
                 − 
                 − 
                 − 
               
               
                 1 mM 
                 + 
                 + 
                 + 
                 + 
               
               
                   
               
             
          
         
       
     
       Example 5 
     Structure and L-Serine Formation of  C. glutamicum  DsdaADpabAB pserABC Strain 
       [0067]    The plasmid pEC-T18mob2-serAfbrserCserB was added to the  C. glutamicum  DsdaADpabAB strain using electroporation. The plasmid ( FIG. 4 ) comprises the vector pEC-T18mob2 (Curr. Microbiol. (2002) 45, 362-367), the corynebacterial genes serAfbr (Appl Microbiol Biotechnol. (2002) 60:437-41) and serC and serB (German patent application 10044 831.3). Tetracycline-resistant clones were tested using known standard methods for the presence and integrity of the plasmid pEC-T18mob2-serAfbrserCserB (Sambrook et al. (1989) Molecular Cloning, A Laboratory Manual, Cold Spring Harbor Laboratory Press) and a clone was designated  C. glutamicum  DsdaADpabAB pserABC. 
         [0068]    For forming L-serine, the strain  C. glutamicum  DsdaADpabAB pserABC in complex medium (CgIII with 2% glucose, 5 μg/l tetracycline) was used and the fermentation medium CGXII (J Bacteriol (1993) 175: 5595-5603) was inoculated from it. The fermentation medium CGXII contains 0.1 or 1 mM folic acid. The strain  C. glutamicum  DsdaA pserABC was cultivated as a control in the same manner. At least two independent fermentations were performed for each. After cultivation for 30 hours at 30° C. on the rotation shaker at 120 rpm, the quantity of L-serine accumulated in the medium was measured. The amino acid concentration was analyzed by means of high pressure liquid chromatography (J Chromat (1983) 266: 471-482). The results of the fermentation are presented in Table 2. It is apparent that the use of the constructed and described mutants in folic acid synthesis represents a method that significantly improves L-serine formation. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Accumulation of L-serine in the culture supernatant 
               
               
                 of  C. glutamicum  DsdaADpabAB pserABC and  C. glutamicum   
               
               
                 DsdaA pserABC. 
               
             
          
           
               
                 Folic acid 
                   
                 L-serin3 
               
               
                 (mM) 
                 Strain 
                 (mM) 
               
               
                   
               
             
          
           
               
                 0.01 
                 
                   C. glutamicum 
                 
                 49 
               
               
                   
                 DsdaADpabAB pserABC 
               
               
                 0.1 
                 
                   C. glutamicum 
                 
                 89 
               
               
                   
                 DsdaADpabAB pserABC 
               
               
                 1.0 
                 
                   C. glutamicum 
                 
                 2 
               
               
                   
                 DsdaADpabAB pserABC 
               
               
                 0.01 
                 
                   C. glutamicum 
                 
                 2 
               
               
                   
                 DsdaA pserABC 
               
               
                 0.1 
                 
                   C. glutamicum 
                 
                 1 
               
               
                   
                 DsdaA pserABC 
               
               
                 1.0 
                 
                   C. glutamicum 
                 
                 1 
               
               
                   
                 DsdaA pserABC 
               
               
                   
               
             
          
         
       
     
       Example 6 
     Structure and L-Serine Formation of the  C. glutamicum  DpykDsdaADpabAB pserABC Strain 
       [0069]    The plasmid pK19mobsacBDpyk (Arch Microbiol. (2004) 182:354-63) was added to the strain  C. glutamicum  13032DsdaADpabABC by means of electroporation and selected for kanamycin-resistance. Only those clones in which the plasmid was integrated into the chromosomal pyk gene locus using homologous recombination were kanamycin-resistant. One clone was selected after examination demonstrated the saccharose sensitivity imparted by the plasmid and it was cultivated in 50 ml BHI medium (brain heart infusion medium, Difco Laboratories, Detroit, USA) without kanamycin and saccharose. Then 100 μl from 10-2, 10-3 and 10-4dilutions of the culture were each plated on BHIS plates (BHI medium with 0.5 M sorbitol) with 10% (w/v) saccharose. The saccharose-resistant clones obtained were then tested for kanamycin sensitivity and the successful deletion was verified using PCR analysis. The  C. glutamicum  DpykDsdaADpabAB strain could be obtained from the starting strain in this manner. As described in Example 5, this strain was transformed with pEC-T18mob2-serAfbrserCserB and the  C. glutamicum  DpykDsdaADpabAB pserABC strain was obtained. As described in Example 5, this strain was used for L-serine formation, in comparison to  C. glutamicum  DsdaADpabAB pserABC. Table 3 provides the results. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Accumulation of L-serine in culture supernatant 
               
               
                 of  C. glutamicum  DpykDsdaADpabAB pserABC and  C. glutamicum   
               
               
                 DsdaADpabAB pserABC. 
               
             
          
           
               
                 Folic acid 
                   
                 L-serine 
               
               
                 (mM) 
                 Strain 
                 (mM) 
               
               
                   
               
             
          
           
               
                 0.1 
                 
                   C. glutamicum 
                 
                 89 
               
               
                   
                 DsdaADpabAB pserABC 
               
               
                 0.1 
                 
                   C. glutamicum 
                 
                 98 
               
               
                   
                 DpykDsdaADpabAB pserABC 
               
               
                 1.0 
                 
                   C. glutamicum 
                 
                 2 
               
               
                   
                 DpykDsdaADpabAB pserABC