WO2005108561A2 - Recombinant enzyme with altered feedback sensitivity - Google Patents

Abstract

The present invention relates to the use of recombinant homoserine transsuccinylase enzymes with altered feedback sensitivity (MetA*) and possibly recombinant S-adenosyl methionine synthetase enzymes with reduced activity (MetK*) for the production of methionine, its precursors or derivatives thereof.

Description

RECOMBINANT ENZYME WITH ALTERED FEEDBACK SENSITIVITY

DESCRIPTION

Field of the invention The present invention relates to the use of recombinant homoserine transsuccinylase enzymes with altered feedback sensitivity (MetA*) and eventually, recombinant S-adenosyl methionine synthetase enzymes with reduced activity (MetK*) for the production of methionine, its precursors or derivatives thereof.

Prior art Sulfur-containing compounds such as cysteine, homocysteine, methionine or S- adenosylmethionine are critical to cellular metabolism and are produced industrially to be used as food or feed additives and pharmaceuticals. In particular methionine, an essential amino acid, which cannot be synthesized by animals, plays an important role in many body functions. Aside from its role in protein biosynthesis, methionine is involved in transmethylation and in the bioavailability of selenium and zinc. Methionine is also directly used as a treatment for disorders like allergy and rheumatic fever. Nevertheless most of the methionine which is produced is added to animal feed. With the decreased use of animal-derived proteins as a result of BSE and chicken flu, the demand for pure methionine has increased. Chemically D,L-methionine is commonly produced from acrolein, methyl mercaptan and hydrogen cyanide. Nevertheless the racemic mixture does not perform as well as pure L-methionine, as for example in chicken feed additives (Saunderson, C.L., (1985) British Journal of Nutrition 54, 621-633). Pure L-methionine can be produced from racemic methionine e.g. through the acylase treatment of N-acctyl-D,L-mcthioninc which increases production costs dramatically. The increasing demand for pure L-methionine coupled to environmental concerns render microbial production of methionine attractive. Microorganisms have developed highly complex regulatory mechanisms that fine-tune the biosynthesis of cell components thus permitting maximum growth rates. Consequently only the required amounts of metabolites, such as amino acids, arc synthesized and can usually not be detected in the culture supernatant of wild-type strains. Bacteria control amino acid biosynthesis mainly by feedback inhibition of enzymes, and repression or activation of gene transcription. Effectors for these regulatory pathways are in most cases the end products of the relevant pathways. Consequently, strategies for overproducing amino acids in microorganisms require the deregulation of these control mechanisms. The pathway for L-methionine synthesis is well known in many microorganisms (Fig. 1A/B). Methionine is derived from the amino acid aspartate, but its synthesis requires the convergence of two additional pathways, cysteine biosynthesis and Cl metabolism (N- methyltetrahydrofolale). Aspartate is converted into homoserine by a sequence of three reactions. Homoserine can subsequently enter the threonine/isoleucine or methionine biosynthetic pathway. In F.. coli entry into the methionine pathway requires the acylation of homoserine to succinyl- homoserine. This activation step allows subsequent condensation with cysteine, leading to the thioether-containing cystadiionine, which is hydrolyzed to give homocysteine. The final methyl transfer leading to methionine is carried out by either a Bi₂-dcpcndent or a B_]2-indcpcndcnt methyltransferase.

Methionine biosynthesis in E. coli is regulated by repression and activation of methionine biosynthetic genes via the MeU and MetR proteins, respectively (reviewed in Neidhardt, F. C. (Ed. in Chief), R. Curliss III, J. L. Ingraham, E. C. C. Lin, K. B. Low, B. Magasanik, W. S. Reznikoff, M. Rilcy, M. Schaechtcr, and H. E. Umbargcr (eds). 1996. Escherichia coli and Salmonella: Cellular and Molecular Biology. American Society for Microbiology; Weissbach et al., 1991 Mol. Microbiol., 5, 1593-1597). MetJ uses S-adenosylmethionine as a corepressor, that is made from methionine by the enzyme S-adenosylmethionine synthetase (EC 2.5.1.6) encoded by the essential gene metK. metK encoding homoserine transsuccinylase (EC 2.3.1.46), the first enzyme unique to the synthesis of methionine is another major control point for methionine production. Besides the transcriptional regulation of met A by MeU and MetR the enzyme is also feedback regulated by the end-products methionine and S-adenosylmethionine (Lee, L.-W et al. (1966) Multimetabolite control of a biosynthetic pathway by sequential metabolites, JBC 241 (22), 5479-5780). Feedback inhibition by these two products is synergistic meaning that low concentrations of each metabolite alone are only slightly inhibitory, while in combination a strong inhibition is exerted. Amino acid analogues inhibit bacterial growth through the synthesis of abnormal polypeptides and by interfering with feedback inhibition, which lead to detrimental processes inside the cell. Analogue-resistant mutants have been obtained which show altered and deregulated enzymes leading to excess synthesis of the corresponding metabolites that can outcompele the analogue. Several groups have used the methionine analogues norlcucinc, cthioninc, and α- methylmethionine to generate microbial strains that overproduce methionine. It was shown that α- mcthylmethionine is a potent inhibitor of the homoserine transsuccinylase enzyme MetA (Rowbury RJ, (1968) The inhibitory effect of α-methylmethionine on Escherichia coli, J. gen. Microbiol., 52, 223-230). Feedback resistant mutants in Salmonella typhimurium that mapped to the metK locus were isolated (Lawrence D. A., (1972) Regulation of the methionine feedback-sensitive enzyme in mutants of Salmonella typhimurium; Lawrence, D.A., Smith, D.A., Rowbury, R.J. (1967) Regulation of methionine synthesis in Salmonella typhimurium: Mutants resistant to inhibition by analogues of methiomne, Genetics 58, 473-492). Norleucine resistant mutants were shown to map to the metK locus. (Chattopadhyay, M.K., Ghosh, A. K. and Sengupta, S. (1991), Control of methionine biosynthesis in Escherichia coli K12: a closer study with analogue resistant mutants, Journal of General Microbiology, 137, 685-691). The same authors have reported the isolation of feedback resistant metK mutants and norleucine resistant mutants in E. coli, but the actual mutations in met A and metK. have not been described. The critical role of homoserine transsuccinylase for bacterial methionine production by fermentation has been demonstrated (Kumar, D. Garg, S. Bisaria V S., Sreekrishnan, T.R. and Gomes, J. (2003) Production of methionine by a multi-analogue resistant mutant of Corynebacterium lilium, Process Biochemistry 38, 1165-1171). The patent application JP2000139471 describes a process for the production of L-methionine using mutants in the genes metK and metK. In this case the precise location of the mutations has been determined. The MetA mutant enzymes have partially lost the sensitivity to feed-back inhibition by methionine and S- adenosyl-methionine. Nevertheless their initial activities are decreased down to about 25% when compared to the wildtype enzyme, and at concentrations of 1 mM methionine for some mutants or 1 mM SAM for others another 25-90 % of the enzyme activity is lost. The metK mutants were not characterized enzymatically but used in fermentations to increase the amount of methionine produced.

General disclosure of the invention This invention relates to a method for the preparation of methionine, its precursors or products derived thereof in a fermentative process with a microorganism where L-homoserine is converted into O-succinyl-homoserine with a homoserine transsuccinylase, comprising the step of culturing the said microorganism on an appropriate medium and recovering methionine, its precursors or products derived thereof once produced, wherein the homoserine transsuccinylase is a mutated homoserine transsuccinylase with reduced sensitivity for the feedback inhibitors S- adenosylmethionine and methionine. The invention also relates to the same method with microorganisms where the S-adenosylmethionine synthetase enzyme activity is reduced. ? The present invention also concerns the mutated enzymes, nucleotide sequences coding for the said enzymes with reduced sensitivity or activity and microorganisms comprising the said nucleotide sequences as disclosed above and below. The recombinant enzymes can be used together and can also be combined with several other changes in the corresponding microorganisms such as overexpression of genes or their deletion. Preferentially the gene encoding aspartokinase/homoserine dehydrogenase (meth or thrK) is over expressed and the gene encoding the methionine rcprcssor me is deleted. In the description of the present invention, genes and proteins are identified using the denominations of the corresponding genes in E. coli. However, and unless specified otherwise, use of these denominations has a more general meaning according to the invention and covers all the corresponding genes and proteins in other organisms, more particularly microorganisms. PFAM (protein families database of alignments and hidden Markov models; http://www.sanger.ac.uk/Software/PfamΛ represents a large collection of protein sequence alignments. Each PFAM makes il possible to visualize multiple alignments, sec protein domains, evaluate distribution among organisms, gain access to other databases, and visualize known protein structures. COGs (clusters of orthologous groups of proteins; http://www.ncbi.nlm nih. gov/COG/1 are obtained by comparing protein sequences from 43 fully sequcnccd genomes representing 30 major phylogenic lines. Each COG is defined from at least three lines, which permits the identification of former conserved domains. The means of identifying homologous sequences and their percentage homologics arc well known to those skilled in the art, and include in particular the BLAST programs, which can be used from die website http://www. ncbi.nlm.nih.gov BLAST/ wi th the default parameters indicated on that website. The sequences obtained can then be exploited (e.g., aligned) using, for example, the programs CLUSTALW (http://www.ebi.ac.uk/clustal\vΛ or MULTALIN (http://prodcs.toulouse.inra.fr/multalin/cgi-bin/mul talin.pl). with the default parameters indicated on those websites. Using the references given on GenBank for known genes, those skilled in the art are able to determine the equivalent genes in other organisms, bacterial strains, yeasts, fungi, mammals, plants, etc. This routine work is advantageously done using consensus sequences that can be determined by carrying out sequence alignments with genes derived from other microorganisms, and designing degenerate probes to clone the corresponding gene in another organism. These routine methods of molecular biology are well known to those skilled in the art. and are described, for example, in Sambrook et al. (1989 Molecular Cloning: a Laboratory Manual. 2^nd ed. Cold Spring Harbor Lab., Cold Spruig Harbor, New York.).

Detailed description of the invention The modified homoserine succinyltransferases showing a decreased feed-back sensitivity

«? to methionine and S-adcnosylmcthionine in comparison to the wild-type enzyme, according to the present invention, comprises at least one amino acid mutation when compared with the wild-type sequence. The mutation is preferentially selected in the conserved regions coding for amino acids 24 to 30 or in the region coding for amino acids 58 to 65 or in the region coding for amino acids 292 to 298 with the first amino acid proline after the formylmethionine counting as number 1. All references to amino acid positions are made based on the homoserine succinyltransf erase encoded by the me i gene of E. coli represented in figure 2. The relative positions of corresponding conserved regions in other homoserine succinyltransferases from different organisms can be found by a person skilled in the art by simple sequence alignment as represented on figure 3 with the enzymes listed below: gi|20138683|sp|Q97I29|META Clostridnim acetobutylicum Homoserine O-succinyltransferasc gi|12230304|spjQ9PLV2|META Campylobacter Jejuni Homoserine O-succinyllransferase - gi|12230277|sp|Q9KAK7|META Bacillus halodura s Homoserine O-succinyltransferase gi|20138686|sp|Q97PM9|META Streptococcus pneumoniae Homoserine O-succinyltransferase gi|20138715 |sp|Q9CEC5|META Lactococcus lactts Homoserine O-succinyltransferase gi|20138656|sp|Q92L99|META Sinorhizobium meliloti Homoserine O-succinyltransfcrase gi|20138618|sp|Q8YB V5|META Brucella melitensis Homoserine O-succinyltransferase gi|20141549|sp|P37413|META Salmonella typhimurium Homoserine O-succinyltransferase - gi|20138601 |sp|Q8X610|META &cΛeπcΛ/α co// O157:H7 Homoserine O-succinyltransferase gi|12231004|sp|P07623|META Escherichia coli Homoserine O-succinyltransferase gi|12230285|sp|Q9KRM5|META Vibrio cholerae Homoserine O-succinyltransferase - gi|38258142|sp|Q8FWG9|META Brucella melitensis biovar suis Homoserine O- succinyltransferase gi|20138631 |sp|Q8ZAR4|META Yersinia pestis Homoserine O-succinyltransferase gi|12231010|sp|P54167|META Bacillus subtilis Homoserine O-succinyltransferase gi|12230320|sp|Q9WZY3|META Thermotoga maritima Homoserine O-succinyltransferase - gi|20138625|sp|Q8ZlWl|META Salmonella typhi Homoserine O-succinyltransferase >gi|31340217|sp|Q8D937|META Vibrio vulniβcus Homoserine O-succinyltransferase gi|31340213 |sp|Q87NW7|META Vibrio parahaemolyticus Homoserine O-succinyltransferase The modified homoserine succinyltransferases preferentially exhibit a specific activity which is at least ten times that of the wild-type enzyme in the presence of 10 mM methionine and 1 mM S-adenosylmethionine and at least 80 times that of the wild-type enzyme in the presence of 10 mM methionine and 0,1 mM S-adenosylmethionine. Preferred enzymes retain an activity of at least two percent of their initial activity in the presence of 100 mM methionine and 1 mM S- adenosy lmethionine . Preferentially, the protein sequence of the modified homoserine succinyltransferase according to the invention contains the amino acid mutation of at least one of the conserved regions sequences specified below. Preferentially at least one mutation is present in the conserved region 1 comprising the amino acid sequence defined below, in the N-terminal part of the wild type homoserine transsuccinylases, corresponding to amino acid 24 to 30 in the amino acid sequence of E. coli MetA shown in SEQ ID NO 1. This non mutated conserved region 1 has the following sequence: X1-X2-X3-A-X4-X5-Q In which

XI represents E, D, T, S, L, preferentially T X2 represents D, S, K, Q, E, A, R, preferentially S X3 represents R, E, D, preferentially R

X4 represents Y, I, F, A, K, S, V, preferentially S X5 represents H, S, N, G, T, R, preferentially G. In a preferred embodiment the unmodified homoserine succinyltransferase conserved region 1 has the following amino acid sequence: T-S-R-A-S-G-Q. In another preferred embodiment of the invention, at least one mutation is present in the conserved region 2, also in the N-terminal part of the wild type homoserine transsuccinylase, corresponding to amino acid 58 to 65 in the amino acid sequence of E. coli MetA shown in SEQ ID NO 1. The non mutated conserved region 2 has the following formula: XI -X2-X3-P-L-Q-X4-X5 hi which

X 1 represents G, A, S, preferentially S X2 represents N, A, preferentially N X3 represents S, T, preferentially S X4 represents V, L, I, preferentially V

X5 represents N, E, H, D, preferentially D. In a preferred embodiment the unmodified homoserine succinyltransferase conserved region 2 has the following amino acid sequence: S-N-S-P-L-Q-V-D. In a third embodiment of the invention the homoserine succinyltransferase comprises at least one mutation in a conserved region in its C-terminal part, corresponding to amino acid 292 to 298 in the amino acid sequence of E. coli MetA shown in SEQ ID NO 1. The non mutated conserved region 3 has the following formula: X1-Y-Q-X2-T-P-X3 In which XI represents V, I, M, preferentially V

X2 represents E, K, G, I, Q, T, S, preferentially I X3 represents F, Y, preferentially Y. In a preferred embodiment, the unmodified homoserine succinyltransferase conserved region 3 has the following amino acid sequence: V-Y-Q-I-T-P-Y. In a preferred embodiment, the conserved alanine in conserved region 1 is replaced with another amino acid, more preferentially with a valine. The modified conserved region has most preferentially the following amino acid sequence: T-S-R-V-S-G-Q.

In another preferred embodiment, the conserved amino acids L and/or Q in conserved region 2 are replaced with other amino acids. Preferentially, leucine is replaced by phenylalanine and/or glutaminc is replaced with a glutamatc or aspartate. Most preferentially, the modified conserved region 2 has the following amino acid sequence: S-N-S-P-L-E-V-D. In a further preferred embodiment, the conserved amino acids L and or Q in conserved region 3 are replaced with another amino acid. In a preferred application the metK gene is the homoserine transsuccinylase enzyme of E. coli K12 represented by the SEQ ID NO 1 and sequences homologous to that sequence that have homoserine transsuccinylase activity and that share at least 80% homology, preferentially 90% homology with the amino acid sequence of SEQ ID NO 1. Modified homoserine succinyltransferases may be obtained, for example, by selecting strains growing in the presence of methionine analogues such as α-methylmethionine, norleucine or ethionine. Preferentially these strains will be selected while growing in the presence of α- methylmethionine. The present invention furthermore relates to nucleotide sequences, DNA or RNA sequences, which encode a mutated homoserine succinyltransferase according to the invention as defined above. In a preferred embodiment, the DNA sequence is characterized by the fact that it comprises at least one mutation in the coding regions of the conserved regions 1 to 3 of the wild type met A gene, represented in SEQ ID NO 1 , the said mutation being not a silent mutation. These DNA sequences can be prepared, for example, from the strains growing in the presence of methionine analogues. The starting DNA fragment encompassing the modified met A gene, is cloned in a vector using standard known techniques for preparing recombinant DNA. These DNA sequences can also be prepared, for example, by non-specific or by targeted mutagenesis methods from strains harboring the DNA sequence encoding wild-type homoserine succinyltransferase. Non-specific mutations within the said DNA region may be produced, for example, by chemical agents (e.g. nitrosoguanidine, cthylmethancsulfonic acid and the like) and/or by physical methods and/or by PCR reactions, which are carried out under particular conditions. Methods for introducing mutations at specific positions within a DNA fragment are known and are described in Molecular Cloning: a Laboratory Manual, 1989,. 2^nd ed. Cold Spring Harbor Lab., Cold Spring Harbor, New York. Using the afore mentioned methods, one or more mutations which cause the modified homoserine succinyltransferase to possess an amino acid sequence which leads to methionine and S-adenosylmethionine insensitivity can be introduced in the said DNA region of any met A gene. Preferentially, one or more nucleotides in the DNA sequence encoding homoserine succinyltransferase are changed such that the amino acid sequence that is encoded by the gene exhibits at least one mutation. Another method of producing feedback-resistant metK alleles consists in combining different point mutations which lead to feedback resistance, thereby giving rise to multiple mutants possessing new properties. The modified S-adenosylmethioninc synthetase according to the invention has decreased activity in comparison to the wild-type enzyme, and has at least one mutation in its protein sequence when compared to the wild-type sequence. The mutation is preferentially in one of the conserved regions defined below. All references to amino acid positions are made based on the S-adenosylmetiiionine synthetase encoded by the metK gene of E. coli. The relative positions of corresponding conserved regions in other S-adenosylmethionine synthetases from different organisms can be found by a person skilled in the art by simple sequence alignment as represented in figure 4 with enzymes listed below:

>gi|39574954|emb|CAE78795.1| methionine adenosyltransferase Bdellovibrio bacteriovorus HD100]

>gi|45657232|ref|YP_001318.1| s-adenosylmethionine synthetase protein Leptospira interrogans serovar Copenhageni str. Fiocruz LI -130

>gi|28378057|ref|NP_784949.1| methionine adenosyltransferase Lactobacillus plantarum WCFS1

>gi|26453553|dbj|BAC43885.1| S-adenosylmethionine synthetase Mycoplasma penetrans >gι|24212014|sp|Q9K5E4| S-adenosylmethionme synthetase Corynebacterium glutamicum >gι| 18145842|dbj|BAB81883 1 | S-adenosylmethionine synthetase Clostridium perfnngens str 13 >gι|13363290|dbj|BAB37241 1| methionme adenosyltransferase 1 Escherichia coli 0157 H7 >gι|45443250|ref|NP_994789 1| S-adenosylmethionme synthetase Yersima pestis biovar Mcdiacvails str 91001

>gι|44888151|sp|Q7WQX8|METK S-adenosylmethionine synthetase Borreha burgdorfen >gι|44888141|sp|Q7U4S6| S-adenosylmethionine synthetase Synechococcus sp WH8102 >gι|44888135|sp|Q7MHK6| S-adenosylmethionme synthetase Vibrio vulnificus YJ016 >gι|23466330|ref|NP_696933 1| S-adenosylmethionine synthetase Bifidobacterium longum NCC2705]

>gι|21219978|ref|NP_625757 1| S-adenosylmethiomne synthetase Streptomyces coelicolor A3(2)]

>gι|39937076|ref|NP_949352 1| methiomne S-adenosyltransferase Rhodopseudomonas palustns

CGA009

>gι|l 676639 l|ref|NP_462006 1| methionine adenosyltransferase 1 Salmonella typhimurium LT2 >gι|33594910|ref|NP_882553 1| S-adenosylmethionme synthetase Bordetella parapertussis 12822 >gι|44888148|sp|Q7VRG5| S-adenosylmethionine synthetase Candidatus Blochmanma βondanus >gι|44888147|sp|Q7VNG7|METK_HAEDU S-adenosylmethionme synthetase Haemophilm ducreyi >gι|44888146|sp|Q7VFY5| S-adenosylmethionme synthetase Hehcobacter hepaticus >gι|44888145|sp|Q7VDM7| S-adcnosylmethionme synthetase Prochlorococcus marinus >gι|44888142|sp|Q7URU7| S-adenosylmethionme synthetase Pirellula spec >gι|44888138|sp|Q7NHG0| S-adenosylmethionine synthetase Gloeobacter violaceus >gι|44888137|sp|Q7N119| S-adenosylmethiomne synthetase Photorhabdus lummescens subsp laumondii >gι|44888136|sp|Q7MTQ0| S-adcnosylmcthionine synthetase Porphyromonas gingivalis

>gι|39650934|emb|CAE29457 1| methiomne S-adenosyltransferase Rhodopseudomonas palustns

CGA009

>gι|l 5792421 |ref|NP_282244 1| S-adenosylmethiomne synthetase Campylobacter jejuni subsp jejum NCTC 11168 >gι|39574954|cmb|CAE78795 1| methionine adenosyltransferase Bdellovibno bacteriovorus HD100

>gι|45657232|ref|YP_001318 1 | s-adenosylmethiomne synthetase protein Leptospira interrogans serovar Copenhagem str Fιocruz Ll-130 >gι|28378057|ref|NP_784949 1| methionme adenosyltransferase Lactobacillus plantarum WCFSl >gι|45600470|gb|AAS69955 1| s-adenosylmethionine synthetase protein Leptospira interrogans serovar Copenhagem str Fiocruz LI -130

>gι|26453553|dbj|BAC43885 1| S-adenosylmethiomne synthetase Mycoplasma penetrans >gι|18145842|dbj|BAB81883 1| S-adenosylmethionme synthetase Clostridium perfnngens str 13 >gι|13363290|dbj|BAB37241 1| methionine adenosyltransferase 1 Escherichia coli 0157 H7 >gi|45443250|ref|NP_994789.1| S-adenosylmethionine synthetase Yersinia pestis biovar.

Mediaevails str. 91001

>gi|44888153|sp|Q8CXS7| S-adenosylmethionine synthetase Leptospira interrogans

>gi|44888151|sp|Q7WQX8| S-adenosylmethionine synthetase Bordetella bronchiseptica >gi|44888150|sp|Q7W200| S-adcnosylmcthioninc synthetase Bordetella parapertussis

>gi|44888149|sp|Q7VUL5| S-adenosylmethionine synthetase Bordetella pertussis The modified S-adenosylmethionine synthetase preferentially exhibits a specific activity which is at least five times less than that of the wild-type enzyme. Preferentially, the protein sequence of a modified S-adenosylmethionine synthetase contains the amino acid mutation of at least one of the sequences specified below. In one embodiment of the invention the amino acid changes concern the cystein at position

89 or the cystein at position 239 that both reduce the activity of MetK (Reczkowski and Markham,

1995, JBC 270, 31, 18484-18490). In a preferred embodiment at least one mutation is present in the conserved region 1, corresponding to amino acid 54 to 59 in the amino acid sequence off. coli MetK shown in SEQ ID

NO 2, the non mutated conserved region 1 comprising the amino acid sequence defined below:

G-E-X1 -X2-X3-X4 wherein

XI represents I, V, L, T preferentially I X2 represents T, K, S, R, preferentially T

X3 represents T, S, G, preferentially T

X4 represents S, N, T, K, E, R, P, N, A preferentially S. Preferentially the unmodified S-adenosylmethionine synthetase conserved region 1 has the following amino acid sequence: G-E-I-T-T-S. In another preferred embodiment at least one mutation is present in the conserved region

2, corresponding to amino acid 98 to 107 in the amino acid sequence of E. coli MetK shown in

SEQ ID NO 2, the non mutated conserved region 2 comprising the amino acid sequence defined below:

Q-S-Xl -D-I-X2-X3-G-V-X4 wherein

XI represents P, Q, A, S, preferentially P

X2 represents A, N, Q, S, F, preferentially N

X3 represents V, Q, Y, R, M, N preferentially Q

X4 represents K, D,N, T, S, A, E preferentially D. Preferentially the unmodified S-adenosylmethionine synthetase conserved region 2 has the following amino acid sequence: Q-S-P-D-I-N-Q-G-V-D. In another preferred embodiment at least one mutation is present in the conserved region 3, corresponding to amino acid 114 to 121 in the amino acid sequence of £ coli MetK shown in SEQ

ID NO 2, the non mutated conserved region 3 comprising the amino acid sequence defined below: X1-G-A-G-D-Q-G-X2 wherein

XI represents Q, A, I, V, E, T, preferentially Q

X2 represents L, I, S, V, M, preferentially L. Preferentially the unmodified S-adcnosylmcthioninc synthetase conserved region 3 has the following amino acid sequence: Q-G-A-G-D-Q-G-L. In another preferred embodiment at least one mutation is present in the conserved region 4, corresponding to amino acid 137 to 144 in the amino acid sequence of E. coli MetK shown in SEQ

ID NO 2, the non mutated conserved region 4 comprising the amino acid sequence defined below: X1-I-X2-X3-X4-H-X5-X6 wherein

XI represents S, P, T, A, preferentially P

X2 represents A, T, W, Y, F, S, N preferentially T

X3 represents M, Y, L, V preferentially Y X4 represents S, A, preferentially A

X5 represents K, R, E, D, preferentially R

X6 represents L, I, preferentially L. Preferentially the unmodified S-adenosylmethionine synthetase conserved region 4 has the following amino acid sequence: P-I-T-Y-A-H-R-L. In another preferred embodiment at least one mutation is present in the conserved region 5, corresponding to amino acid 159 to 163 in the amino acid sequence of E. coli MetK shown in SEQ

ID NO 2, the non mutated conserved region 5 comprising the amino acid sequence defined below:

X1-L-X2-X3-D wherein XI represents W, F, Y, V, E preferentially W

X2 represents R, G, L, K, preferentially R

X3 represents P, L, H, V, preferentially P. Preferentially the unmodified S-adenosylmethionine synthetase conserved region 5 has the following amino acid sequence: W-L-R-P-D. Preferentially at least one mutation is present in the conserved region 6, corresponding to to amino acid 183 to 189 in the amino acid sequence of E. coli MetK shown in SEQ ID NO 2, the non mutated conserved region 6 comprising the amino acid sequence defined below.

X1-X2-X3-S-X4-Q-H

In which XI represents V, I, preferentially V

X2 represents V, L, I, preferentially V

X3 represents V, L, I, M, preferentially L

X4 represents T, V, A, S, H, preferentially T. hi a preferred embodiment the unmodified S-adenosylmethionine synthetase conserved region 6 has the following amino acid sequence: V-V-L-S-T-Q-H. Preferentially the mutations are introduced in the conserved region 7, corresponding to amino acid 224 to 230 in the amino acid sequence off. coli MetK shown in SEQ ID NO 2, the non mutated conserved region 7 comprising the amino acid sequence defined below: X1-N-P-X2-G-X3-F In which

XI represents I, V preferentially I X2 represents T, G, S, preferentially T X3 represents R, T, Q, K, S, preferentially R. In a preferred embodiment the unmodified S-adenosylmethionine synthetase conserved region 7 has the following amino acid sequence: I-N-P-T-G-R-F. Preferentially the mutations are introduced into the conserved region 8, corresponding to amino acid 231 to 237 in tire amino acid sequence ofE. coli MetK shown in SEQ ID NO 2, the non mutated conserved region 8 comprising the amino acid sequence defined below: X1-X2-G-X3-P-X4-X5 In which

XI represents T, V, 1, Y, E, preferentially V X2 represents V, I, L, N, preferentially I X3 represents G, S, preferentially G

X4 represents M, I, Q, A, H, D preferentially M X5 represents G, S, A, H preferentially G. In a preferred embodiment the unmodified S-adenosylmethionine synthetase conserved region 8 has the following amino acid sequence: V-l-G-G-P-M-G. Preferentially the mutations arc introduced in the conserved region 9, corresponding to amino acid 246 to 253 in the amino acid sequence of E. coli MetK shown in SEQ ID NO 2, the non mutated conserved region 9 comprising the amino acid sequence defined below: X1-X2-D-T-Y-G-G In which XI represents M, I, preferentially I X2 represents V, I preferentially I. In a preferred embodiment the unmodified S-adenosylmethionine synthetase conserved region 9 has the following amino acid sequence: I-V-D-T-Y-G-G. Preferentially the mutations are introduced into the conserved region 10, corresponding to amino acid 269 to 275 in the amino acid sequence off. coli MetK shown in SEQ ID NO 2, the non mutated conserved region 10 comprising the amino acid sequence defined below: K-V-D-R-S-X1-X2 In which XI represents A, G, preferentially A X2 represents A, S, L, preferentially A. In a preferred embodiment the unmodified S-adenosylmethionine synthetase conserved region 10 has the following amino acid sequence: K-V-D-R-S-A-A In another preferred application of the invention at least one mutation is present in the conserved region 11. The unmodified S-adcnosylmethioninc synthetase harbors the conserved region in its C-terminal part with the following amino acid sequence: XI -X2-Q-X3-X4-Y-A-I-G-X5-X6 In which

XI represents L, E, I, Q, T preferentially E X2 represents V, I, L preferentially I X3 represents V, L, 1 preferentially V X4 represents A, S preferentially S X5 represents V, I, R, K, A preferentially V X6 represents A, V, T, S, preferentially A. This region corresponds to amino acid 295 to 305 in the amino acid sequence of E. coli

MetK shown in SEQ ID NO 2. In a preferred embodiment the unmodified S-adenosylmethionine synthetase conserved region 11 has the following amino acid sequence: E-I-Q-V-S-Y-A-I-G-V-A. In another preferred application of the invention the mutations are introduced into the N- terminus of the protein leading to a frameshift and changes in the last 6 amino acids. In the S-adenosylmethionine synthetase with decreased activity preferentially the serine in conserved region 1 is replaced with another amino acid. In a preferred application of the invention the serine is replaced with a asparagine. In a preferred embodiment the modified S-adenosylmethionine synthetase has the following amino acid sequence in conserved region 1: G-E-I-T-T-N. In the S-adenosylmethionine synthetase with decreased activity preferentially the conserved glycine in conserved region 2 is replaced with another amino acid. In a preferred application of the invention the conserved glycine is replaced with a serine. In a preferred embodiment the modified S-adenosylmethionine synthetase has the following amino acid sequence in conserved region 2: Q-S-P-D-I-N-Q-S-V-D. In the S-adenosylmethionine synthetase with decreased activity preferentially the conserved glycine in conserved region 3 is replaced with another amino acid. In a preferred application of the invention the conserved glycine is replaced with a serine. In a preferred embodiment the modified S-adenosylmethionine synthetase has the following amino acid sequence in conserved region 3: Q-S-A-G-D-Q-G-L. In the S-adenosyhnethionine synthetase with decreased activity preferentially die conserved histidine and/or the semi-conserved proline in conserved region 4 is/are replaced with other amino acids. In a preferred application of the invention the conserved histidine is replaced with a lyrosine and/or the semi -conserved proline is replaced with a leucine. In a preferred embodiment the modified S-adenosylmethionine synthetase has one of the following amino acid sequences in conserved region 4: P-I-T-Y-A-Y-R-L, L-I-T-Y-A-H-R-L, L-I-T-Y-A-Y-R-L. In the S-adenosylmethionine synthetase with decreased activity preferentially the semi- conserved arginine in conserved region 5 is replaced with another amino acid. In a preferred application of the invention the arginine is replaced with a cysteine. In a preferred embodiment the modified S-adenosylmethionine synthetase has the following amino acid sequence in conserved region 5: W-L-C-P-D. In the S-adenosylmethionine synthetase with decreased activity preferentially the conserved histidine or semi-conserved valine in conserved region 6 is replaced with another amino acid. In a preferred application of the invention the conserved histidine is replaced with a tyrosine and/or the valine with aspartate. In a preferred embodiment the modified S-adenosylmethionine synthetase has one of the three following amino acid sequences in conserved region 6: V-V-L-S-T-Q-Y V-D-L-S-T-Q-H V-D-L-S-T-Q-Y. In the S-adenosylmethioninc synthetase wid decreased activity preferentially the semi- conserved threonine in conserved region 7 is replaced with another amino acid. In a preferred application of the invention the threonine is replaced with an isoleucine. In a preferred embodiment the modified S-adenosylmethionine synthetase has the following amino acid sequence in conserved region 7: 1-N-P-I-G-R-F. In the S-adenosylmethionine synthetase with decreased activity preferentially the conserved proline in conserved region 8 is replaced with another amino acid. In a preferred application of the invention the proline is replaced with a serine. In a preferred embodiment the modified S-adenosylmethionine synthetase has the following amino acid sequence in conserved region 8: V-I-G-G-S-M-G. In the S-adenosylmethioninc synthetase with decreased activity preferentially the second conserved glycine in conserved region 9 is replaced with another amino acid. In a preferred application of the invention the glycine is replaced with an aspartate. In a preferred embodiment the modified S-adenosylmethionine synthetase has the following amino acid sequence in conserved region 9: 1-V-D-T-Y-G-D. In the S-adenosylmethionine synthetase with decreased activity preferentially the conserved serine in conserved region 10 is replaced with another amino acid. In a preferred application of the invention the serine is replaced with a phenylalanine. In a preferred embodiment the modified S-adenosylmethionine synthetase has the following amino acid sequence in conserved region 10: K-V-D-R-F-A-A. In the S-adenosylmethionine synthetase with decreased activity preferentially the conserved isoleucine in conserved region 1 1 is replaced with other amino acids. In a preferred application of the invention isoleucine is replaced by leucine. In a preferred embodiment the modified S-adenosylmethionine synthetase has the following amino acid sequence in conserved region 11: E-I-Q-V-S-Y-A-L-G-V-A. The present invention furthermore relates to nucleotide sequences, DNA or RNA sequences, which encode a mutated S-adenosylmethionine synthetase according to the invention as defined above. In a preferred embodiment, these DNA sequences are characterized by the fact that they comprise al least one mutation in the coding DNA sequence regions for the conserved regions 1 to 11 of the wild type metK gene, represented as wild type in SEQ ID NO 2, the said mutation being not a silent mutation. In a preferred application the metK gene is the S-adenosylmethionine synthetase off. coli K12 represented by the SEQ ID NO 2 and sequences homologous to that sequence that have S- adenosylmethionine synthetase activity and that share at least 80% homology, preferentially 90% homology with the amino acid sequence of SEQ ID NO 2. The mutated S-adenosylmethionine synthetase genes described above may be obtained by conventional techniques known to the person skilled in the art disclosed above and below, including random or targeted mutagencsis or synthetic DNA construction. The metK gene encoding modified homoserine succinyltransferase and/or the metK gene encoding modified S-adenosylmethionie synthetase may be encoded chromosomally or extrachromosomally. Chromosomally there may be one or several copies on the genome that can be introduced by methods of recombination known to the expert in the field. Extrachromosomally both genes may be carried by different types of plasmids that differ with respect to their origin of replication and thus their copy number in the cell. They may be present as 1-5 copies, ca 20 or up to 500 copies corresponding to low copy number plasmids with tight replication (pSClOl, RK2), low copy number plasmids (pACYC, pRSFlOlO) or high copy number plasmids (pSK bluescript II). The metK and/or metK gene may be expressed using promoters with different strength that need or need not to be induced by inducer molecules. Examples are the promoter Ptrc, Ptac, Plac, the lambda promoter cl or other promoters known to the expert in the field. MetA and/or MetK expression may be boosted or reduced by elements stabilizing or destabilizing the corresponding messenger RNA (Carrier and Keasling (1998) Biotechnol. Prog. 15, 58-64) or the protein (e.g. GST tags, Amersham Biosciences). The present invention also relates to microorganisms which contain a feedback-resistant metK allele according to the invention and eventually a metK allele with reduced activity according to the invention. Such strains are characterized by the fact that they possess a methionine metabolism which is deregulated by at least one feedback-resistant metK allele and/or by a reduced production of SAM caused by a MetK enzyme with reduced activity. Novel strains may be prepared from any microorganism in which methionine metabolism proceeds by the same metabolic pathway. Gram-negative bacteria, in particular E. coli, are especially suitable. For the purpose of expressing the modified homoserine succinyltransferase enzyme and S- adcnosylmcthioninc synthetase, the feedback-resistant metK allclcs and metK allclcs with reduced activity are transformed in a host strain using customary methods. The screening for strains possessing modified homoserine succinyltransferase and S-adenosylmethionine synthetase properties is, for example, realized using enzymatic tests. For example, the homoserine succinyltransferase activity can be determined in an enzymatic test with homoserine and succinyl-CoA as substrates. The reaction is started by adding the protein extract containing the homoserine succinyltransferase enzyme, and the formation of O- succinylhomoserine is monitored by GC-MS after protein precipitation and derivatization with a silylating reagent. Feedback inhibition is tested in the presence of methionine and S- adenosylmethionine in the reaction mixture. S-adenosylmethionine synthetase activity can be determined in an enzymatic test with methionine and ATP as substrates. The reaction is started by adding the protein extract containing the S-adenosylmethionine synthetase enzyme, and the formation of S-adenosylmethionine is monitored by FIA-MS/MS. Preferentially, use is made of E. coli strains in which the endogenous met A and metK genes are inactivated and complemented by novel recombinant genes. The feedback-resistant metK alleles and the metK alleles with reduced activity render it possible to abolish the control at important biosynthetic control points, thereby amplifying the production of a large number of compounds which are situated downstream of aspartate. These include, in particular, homoserine, O-succinylhomoserine, cystathionine, homocysleine, methionine and S-adenosylmethionine. In particular the invention relates to the preparation of L-methionine, its precursors or compounds derived thereof, by means of cultivating novel microorganisms. The above-described products are classified below as compound (I). An increase in the production of compound (I) can be achieved by changing the expression levels or deleting the following genes implicated in the production of aspartate, a precursor of compound (I).

Gene genbank entry name ackK g 1788633 acetate kinase pta g 1788635 phosphotransacetylase acs gl790505 acetate synthase aceK g 1790445 isocitrate lyase aceB g 1790444 malate synthase aceE g 1786304 pyruvate deydrogenase El αce¥ g 1786305 pyruvate deydrogenase E2

Ipd g 1786307 pyruvate deydrogenase E3 αceK g 1790446 lsocitrate dehydrogenase kinase/phosphatase sucC g 1786948 succinyl-CoA synthetase, beta subunit sucD g 1786949 succinyl-CoA synthetase, alpha subunit ppc g 1790393 phosphoenolpyruvate carboxylase pck g 1789807 phosphoenolpyruvate carboxykinase pykA gl788160 pyruvate kmase II pykF g 1787965 pyruvate kmase I p pooxxBB S g11778877009966 pyruvate oxidase pps g 1787994 phosphoenolpyruvate synthase ilvB gl790104 acetohydroxy acid synthase I, large subunit

</vN gl790103 acetohydroxy acid synthase I. small subunit

;/vG g 1790202 acetohydroxy acid synthase II, large subunit g 1790203

;/vM g 1790204 acetohydroxy acid synthase II, small subunit i/vl g 1786265 acetohydroxy acid synthase III, large subunit ilvU g 1786266 acetohydroxy acid synthase III, small subunit αroF g 1788953 DAHP synthetase aαrrooGG g g 11778866996699 DAHP synthetase αrυW g 1787996 DAHP synthetase αspC g!787159 aspartate aminotransferase In addition pyruvate carboxylase. from Rhizobium etli (accession nombct r* 1 39"> tra> be introduced

ge etic engineering wi I cob and An additional JKC tease m tac psodnet

ωn of compound (!) can be achieved oics vpressm" <>eπes of the h sme/mcthionine paihvtsv. such as the homosvrine SΛ iithcsutng π s nies encoded b t e genes t t <\ (homoseππe delrvdr genase' aspartυkmasc }>178 83) or WK'/L (homoserine dchsdrogcnase'asparlokiuαse. g 1790 * 76) or h; (aρaιlok«>ase. g 179045*) or tTAj/ ias atlaliϊ semialckhs de dchvdrogenasc. gi 8 841) or a commnaliou i cteof A further met case m the pioducUσn of (I ) is possible

osercxprcssmg genes >molvcd in sulfaic assimilation and production of cv«lemc This can be achieved bv overc- xcssmg the following genes (see below) or bv dereι>.ulalmj> the μatliwov through the mliodυetion of a constitutive (

aliclc as described

CoK er and Ktcdtcli (1994 Viol Microb'ol 13 797-805) and by introducing a n >fc alkie encoding a serine acel> ! tsansR,rase w ilh decreased sensttiv J^'or tt« inhibitor l.-e stcine Λ patent application I 'S 6,218.168, Denk & Bock 1987.1 Gen Mieiobtol 133 515-25} 7 he following genes need to be overevpsessed CysK gl 788761 sulfalc pet me se

stem l g 178876? nicmbians bound sulphate lianspott sj stem CysZ g 1788753 ORF upstream of cysK cysH gl 789108 ATP sulfurylase cysD gl789109 sulfate adenylyltransferase cysC gl789107 adenylylsulfate kinase cysH gl789121 adenylylsulfatc rcductasc cysl gl789122 sulfite reductase, alpha subunit cy.si gl789123 sulfite reductase, beta subunit cysE g 1790035 serine acetyltransferase cysK g 1788754 cysteine syntliase cysM g2367138 O-acctyl-sulfhydrolase In addition genes involved in the production of Cl (methyl) groups may be enhanced by overexpressing the following genes: serK g 1789279 phosphoglycerate dehydrogenase serB g 1790849 phosphoserine phosphatase serC gl787136 phosphoserine aminotransferase glyK g 1788902 serine hydroxymethyltransferase metE gl790377 5, 10-Methylenetetrahydrofolate reductase In addition genes directly involved in the production of methionine may be overexpressed: metB gl790375 Cyslathionine gamma -synthase metC g 1789383 Cystathionine beta-lyase metH g 1790450 B12-dependent homocysteine-N5-methyltetrahydrofolate transmethylase metE •>g2367304 Tetrahydroptcroyltriglutamate methyltransferase metE g 1790377 5, 10-Methy lenetetrahy drofolate reductase metR g 1790262 Positive regulatory gene for metE and metH and autogenous regulation. Furthermore expression of genes in pathways degrading methionine or deviating from the methionine production pathway may be reduced or the genes may be deleted. speD gl 786311 S-Adenosylmethionine decarboxylase speC gl789337 Ornithine decarboxylase thrB gl786184 Homoserine kinase astK g 1788043 Arginine succinyltransferase dapK g 1788823 Dihydrodipicolinate synthase. A further increase in the production of (I) is possible by means of deleting the gene for the repressor protein MetJ, responsible for the down-regulation of the methionine regulon as was suggested in JP 2000157267-A 3 (see also GenBank gl790373). Production of (I) may be further increased by using an altered metB allele that uses preferentially or exclusively H₂S and thus produces homocysteine from O succinylhomoserine as has been described in the patent application PCT N° PCT/FR04/00354, which content is incorporated herein by reference. In another preferred embodiment, the gene encoding aspartokinase/homoserine dehydrogenase is over expressed and/or die gene encoding methionine repressor metJ is deleted in the microorganism of the present invention and in the microorganism used in the method according to the invention. Preferably, the aspartokinase/homoserine dehydrogenase is encoded by a feed-back deregulated ThrA allele. Such feed-back deregulation may be obtained by introducing the mutation Phe318Ser in the ThrA enzyme. Enzyme position is given here by reference to the ThrA sequence disclosed in gencbank accession number V00361.1 (gl786183). The metK and metK alleles described above may be used in eukaryotes or prokaryotes. Preferentially the organism used is a prokaryote. In a preferred application the organism is either E. coli or C. glutamicum. The invention also concerns the process for the production of compound (I). Compound (I) is usually prepared by fermentation of the designed bacterial strain. According to the invention, the terms 'culture' and 'fermentation' are used indifferently to denote the growth of a microorganism on an appropriate culture medium containing a simple carbon source. According to the invention a simple carbon source is a source of carbon that can be used by those skilled in the art to obtain normal growth of a microorganism, in particular of a bacterium. In particular it can be an assimilable sugar such as glucose, galactose, sucrose, lactose or molasses, or by-products of these sugars. An especially preferred simple carbon source is glucose. Another preferred simple carbon source is sucrose. ^■■^•' Those skilled in the art are able to define the culture conditions for the microorganisms according to the invention. In particular the bacteria are fermented at a temperature between 20°C and 55°C, preferentially between 25°C and 40°C, and more specifically about 30°C for C. glutamicum and about 37°C for E. coli. The fermentation is generally conducted in fermenters with an inorganic culture medium of known defined composition adapted to the bacteria used, containing at least one simple carbon source, and if necessary a co-substrate necessary for the production of the metabolite. In particular, the inorganic culture medium for E. coli can be of identical or similar composition to an M9 medium (Anderson, 1946, Proc. Natl. Acad. Sci. USA 32:120-128), an M63 medium (Miller, 1992; A Short Course in Bacterial Genetics: A Laboratory Manual and Handbook for Escherichia coli and Related Bacteria, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York) or a medium such as defined by Schaefer et al. (1999, Anal. Biochem. 270: 88- 96). Analogously, the inorganic culture medium for C. glutamicum can be of identical or similar composition to BMCG medium (Liebl et al., 1989, Appl. Microbiol. Biotechnol. 32: 205- 210) or to a medium such as that described by Riedel et al. (2001, J. Mol. Microbiol. Biotechnol. 3: 573-583). The media can be supplemented to compensate for auxotrophies introduced by mutations. After fermentation compound (I) is recovered and purified if necessary. The methods for the recovery and purification of compound (I) such as methionine in the culture media are well known to those skilled in the art. Figure 1A/B : Methionine metabolism in Escherichia coli. Figure 2 : Alignment of wildtype and recombinant metK genes obtained upon selection on α- methyl-methionine. Conserved residues are represented by light grey boxes and mutated residues are indicated by white boxes. - Figure 3 : Alignment of MetA sequences from different microorganisms. Figure 4 : Alignment of MetK sequences from different microorganisms. Figure 5 : Amino acid replacements in the E. coli MetK protein. Amino acids above the coherent lines indicate the position and the replacing amino acid.

EXAMPLES Example 1

Isolation of E. coli mutants containing homoserine transsuccinylase enzymes which show decreased feedback-sensitivity towards methionine and S-adenosylmethionine Isolation ofE. coli strains growing on a-methylmethionine α-mcthylmethionine is a growth-inhibitory analogue of methionine producing an immediate effect on the growth rate of E. coli at very low concentrations (minimal inhibitory concentration of lμg/ml and minimal concentration for a maximal inhibition 5μg/ml, Rowbury et al., 1968). Analogues can mimic methionine in the feedback inhibition of homoserine transsuccinylase and interfere with protein synthesis without being metabolized. Only mutant strains are able to grow in a medium containing the analogue. E. coli was routinely grown aerobically at 37°C in LB supplemented when needed with die appropriate antibiotic. The medium used for the α-methyl-methionine analogue resistant was a minimal medium containing (per liter): K₂HP0₄ 8g, Na₂HP0₄ 2g, (NH₄)₂S0₄ 0.75g, (NH₄)₂HP0₄ 8g, NH₄C1 0.13g, citric acid 6.55g, MgS0₄ 2.05g, CaCl₂ 40mg, FeS0₄ 40mg, MnS0 20mg, CoCl₂ 8 mg, ZnS0₄ 4mg, (NH₄)₂Mo₂0₇ 2.8mg, CuCl₂ 2mg, H₃BO₃ lmg. The pH was adjusted to 6.7 and the medium sterilized. Before use, glucose lOg l and thiamine lOmg/1 were added. The α-methylmethionine powder commercialized by Sigma contains methionine traces. An overnight liquid culture of an E. coli strain (MG1655 AmetE) unable to grow without methionine addition was carried out to eliminate the methionine (coming from α-methylmethionine) from the minimal medium. The culture was centrifuged 10 minutes at 7000rpm and the supernatant was filtered (Sartorius 0.2μm). Agar 15g l was added to this minimal medium.

1*10⁸ cells / ml from an overnight culture in minimal medium of the wild type strain (MG1655) were spread onto plates with minimal medium and α-methylmethionine after 4 washing steps in sterile water. The plates were incubated at 37°C until colonies appeared. Evidence of mutations in the coding sequence of the metA gene coding for the homoserine transsuccinylase enzyme. Genomic DNA was prepared from 4 clones that grew on α-methyl-methionine at 4mg/ml after cultivation in LB medium. The cell pellet was washed once with sterile water and resuspended in 30μl of sterile water. Cells were broken by heating 5 minutes at 95°C and the debris were pelleted. The met A gene was PCR-amplified using Taq polymerase and the following primers: MetAF (SEQ ID NO 3): tcaccttcaacatgcaggctcgacattggc (421 1759-421 1788) MetAR (SEQ ID NO 4): ataaaaaaggcacccgaaggtgcctgaggt (4212857-4212828). In three clones point mutations were detected which led to amino acid substitutions. In clone metKl 1 CAG was exchanged for a GAG leading to the replacement of Q by E. In metK* 13 TTG was exchanged for a TTT leading to the replacement of L by F. In metK* 14 GCG was exchanged for GTG leading to the replacement of A by V (see fig.2).

As can be seen from the alignment shown in fig. 3 all three amino acid that are replaced are highly conserved in MetA proteins from various species. The mutated homoserine transsuccinylase enzymes show decreased feedback-sensitivity towards methionine and S-adenosylmethionine. The activity of homoserine transsuccinylase was determined in vitro. E. coli strains carrying either wild-type or mutant enzymes were cultured in rich medium with 2.5 g/1 glucose and harvested at late log phase. Cells were resuspended in cold potassium phosphate buffer and sonicated on ice (Branson sonificr, 70W). After centrifugation, proteins contained in the supernatants were quantified (Bradford, 1976). Ten μl of the extracts were incubated for 30 minutes at 25°C with 30 M homoserine and

4 mM succinyl-CoA. Methionine and/or S-adenosylmethionine were added as indicated. The succinylhomoserine produced by homoserine transsuccinylase enzymes was quantified by GC-MS after dcrivatization with tcrt-butyldimcthylsilyltrifluoroacetamide (TBDMSTFA). L-Serine[l-13C] was included as an internal standard.

Results of homoserine transsuccinylase activities are reported in table 1 below:

Strain Methionine mM S-adenosylmethionine mM ",„ . . ^J mUI mg proteins metA 0 0 36.7 (100%) 0 0.1 27.9 (76%) 0 1 4.7 (12.8%) 10 0 0.4 (1.1%) 10 0.1 0.2 (0.5%) 10 1 0.1 (0.3%) metA*l l 0 0 19.2 (100%) 0 0.1 21.7 (113%) 0 1 18.5 (96%) 10 0 18.7 (97%) 10 0.1 16.9 (88%) 10 1 2.7 (14.1%) metA* 13 0 0 10.9 (100%) 0 1 12.7 (117%) 10 0 13.7 (126%) 10 1 1.0 (9.2%) metA*14 0 0 18.0 (100%) 0 1 15.1 (83.9%) 10 0 20.8 (116%) 10 1 2.0 (11.1%)

Table 1. Specific activities of wild-type and mutant homoserine transsuccinylase enzymes upon the addition of inhibitors L-methionine and S-adenosyl methionine.

The mutated homoserine transsuccinylase enzymes thus show decreased feedback- sensitivity towards methionine and S-adenosylmethionine.

Example 2

Isolation of E. coli mutants containing S-adcnosylmethionine synthetase enzymes with reduced activity Isolation ofE. coli strains growing on norleucine Norleucine is a growth-inhibitory analogue of methionine. At higher concentrations (50 mg/1) only mutant strains are able to grow in a medium containing the analogue. Most of these mutations map to the metK and met] loci. E. coli was routinely grown aerobically at 37°C in LB supplemented when needed with the appropriate antibiotic. The medium used for the norleucine analogue resistant was a minimal medium containing (per liter): K₂HP0₄8g, Na₂HP0₄2g, (NH₄)₂S0 0.75g, (NH₄)₂HP0₄8g, NH₄C1 0.13g, citric acid 6.55g, MgSQ, 2.05g, CaCl₂40mg_;sFeSO₄ 40mg, MnS0₄ 20mg, CoCl₂ 8 mg, ZnS0₄ 4mg, (NH₄)₂Mo₂0₇ 2.8mg, CuCl₂ 2mg, H₃B0₃ lmg. The pH was adjusted to 6.7 and the medium sterilized. Before use, glucose 1 g/l and thiamine 1 Omg/1 were added. 1*10⁸ cells / ml from an overnight culture in minimal medium of the wild type strain

(MG1655) were spread onto plates with minimal medium and norleucine (50 to 200 g/1) after 4 washing steps in sterile water. The plates were incubated at 37°C until colonies appeared. Evidence of mutations in the coding sequence of the metK gene coding for the S- adenosylmethionine synthetase enzyme Gcnomic DNA was prepared from cultures grown in LB liquid medium. The cell pellet was washed once with sterile water and resuspended in 30μl of sterile water. Cells were broken by heating 5 minutes at 95°C and the debris was extracted. DNA was PCR-amplified with Taq polymerase using the following primers: MetKpF : cccggctggaagtggcaacacg (3084372-3084393) (SEQ ID NO 05) MetKR : gccggatgcggcgtgaacgcctatcc (3085956-3085931) (SEQ ID NO 06). The metK gene was sequenced. In 10 clones point mutations were detected which led to amino acid substitutions. In clone efK*59/105 AGC was exchanged for AAC leading to the replacement of S by N (position 59) and GGC was exchanged for AGC leading to the replacement of G by S (position 105). In clone merK*105 GGC was exchanged for AGC leading to the replacement of G by S. In metK* 115 GGC was exchanged for AGC leading to the replacement of G by S. In metK* l37 CCT was replaced by CTT leading to the replacement of P by L. In e/K* 142 CAC was replaced by TAC leading to the replacement of H by Y. In metK* 161/235 CGC was replaced by TGC leading to the replacement of R by C and CCA was replaced by TCA leading to the replacement of P by S. In metK*lS9 CAC was replaced by TAC leading to the replacement of H by Y. In metK*227 ACC was replaced by ATC leading to the replacement of T by I. In mefK*253 GGC was replaced by GAC leading to the replacement of G by D. In metK*273 TCC was replaced by TTC leading to the replacement of S by F (see fig.5). As can be seen from the alignment shown in fig. 4 all amino acids that are replaced are conserved in MetK proteins from various species. Recombinant S-adenosylmethionine synthetase enzymes with decreased activity The activity of S -adenosylmethionine synthetase was determined in vitro. E. coli strains carrying either wild-type or mutant enzymes were cultured in minimal medium with 5 g/1 glucose and harvested at late log phase. Cells were resuspended in cold potassium phosphate buffer and sonicated on ice (Branson sonifier, 70W). After centrifugation, proteins contained in the supernatants were quantified (Bradford, 1976). One hundred μL of the extracts were incubated for 30 minutes at 37°C with 10 mM methionine and 10 mM ATP. Potassium chloride was included to activate the enzymes. The adenosylmethionine produced by methionine adenosyltransferase enzymes was quantified by FIA- MS/MS. Results of S-adenosylmethionine synthetase activities are reported in Table 2 below:

Table 2: S-adenosylmethionine synthetase activities (MAT) of WT and MetK* mutants. Example 3

Construction of E. coli strains for the production of O-succinylhomoserine or methionine by overexpressing altered homoserine transsuccinylase and other enzymes of the methionine biosynthesis pathway For the construction of E. coli strains that allow the production of O-succinylhomoserinc, a plasmid overexpressing the metL gene, coding for homoserine dehydrogenase and aspartokinase was introduced into strains harboring different alleles of met A. The plasmid overexpressing metL was constructed as follows: The following two oligonucleotides were used: - MctLF with 32 bases (SEQ ID NO 7): TATTCatgagtgtgattgcgcaggcaggggcg with

- a region (lower case) homologous to the sequence (4127415 to 4127441) of the gene metL (sequence 4127415 to 4129847, reference sequence on the website http.//genolist.pasteιu- fr/ColibriΛ, - a region (upper case) that together with the sequence pertaining to metL forms a restriction site for the enzyme BspHI (underlined). MetBLAR with 38 bases (SEQ ID NO 8): TATAAGCTTccataaacccgaaaacatgagtaccgggc with

- a region (lower case) homologous to die sequence (4129894 to 4129866) of the gene metL - a region (upper case) tiiat harbors die restriction site Hmd l. The gene metL was amplified by PCR using die oligonucleotides MetLF and MetBLAR and the restriction sites BspH] and Hindllϊ were introduced at the N- and C-terminus of the metL gene, ■respectively. The resulting PCR fragment was restricted by BspH\ and HmdWi and cloned into the vector pTRC99A (Slralagene) previously cut by Ncol and HindlU. Plasmid preparations were examined for the presence of inserts of the correct size. The DNA sequence of metL was verified and the resulting plasmid pTRCme/L transformed into the strains harboring the alleles e/A*l 1 and met A* 13. The activity of Homoserine dehydrogenase II (HDH) was determined in vitro. E. coli strains were cultured in minimal medium with 10 g.l^"1 glucose and harvested at late log phase. Cells were resuspended in cold potassium phosphate buffer and sonicated on ice (Branson sonificr,

70W). After centrifugation. proteins contained in die supernatants were quantified (Bradford.

1976). Thirty μl of the extracts were incubated at 30°C in a spectrophotomeler, wiUi 25 mM homoserine and 1 mM N ADP ' . Potassium chloride was included to activate die enzyme. Since E. colt harbors a second gene encoding a homoserine dehydrogenase activity (thrK), threonine was added which inhibits this activity. The appearance of NADPH was monitored for 30 minutes at

340 nm. Expression of metL from the Ptrc promoter increases drastically the HDH activity when compared to a similar strain with the plasmid.

Tab. 3 Homoserine dehydrogenase activities of the MetL protein in the MG1655 E. coli strain harbouring the metK* 1 1 mutation with or without overexpressing metL from the Ptrc promoter. In another application the methionine regulatory gene met] was deleted in the strains harboring the metA*l l, metA*13 and metA* 14 alleles. To inactivate the met] gene the homologous recombination strategy described by Datsenko & Wanner (2000) was used. This strategy allows the insertion of a chloramphenicol resistance cassette, while deleting most of the gene concerned. For this purpose 2 oligonucleotides were used: DmetJF with 100 bases (SEQ ID NO 9):

CaggcaccagagtaaacattgtgttaatggacgtcaatacatctggacatctaaacttctttgcgtatagattgagcaaaCATATGAATA TCCTCCTTAG with

- a region (lower case) homologous to the sequence (4126216 to 4126137) of the gene met] (sequence 4125658 to 4125975, reference sequence on the website http://genolist.pasteur.fr/Colibri ).

- a region (upper case) for the amplification of the chloramphenicol resistance cassette (reference sequence in Datsenko, K.A. & Wanner, B.L., 2000, PNAS, 97: 6640-6645),

- DmetJ R with 1 0 bases (SEQ ID NO 10): tgacgtaggcctgataagcglagcgcatcaggcgattccactccgcgccgctcttttttgctttagtattcccacgtctcTGTAGGCTGG AGCTGCTTCG with

- a region (lower case) homologous to die sequence (4125596 to 4125675) of the gene met]

- a region (upper case) for the amplification of the chloramphenicol resistance cassette. The oligonucleotides DmctFR and DmcUF were used to amplify the chloramphenicol resistance cassette from the plasmid pKD3. The PCR product obtained was then introduced by electroporation into the strain MG1655 (pKD46) in which the Red recombinase enzyme expressed permitted the homologous recombination. The chloramphenicol resistant transformants were then selected and the insertion of the resistance cassette was verified by a PCR analysis with the oligonucleotides McUR and MetTF defined below. The strain retained is designated MG1655 (Δmetf ::Cm) metK*. MetJR (SEQ ID NO 11): ggtacagaaaccagcaggctgaggatcagc (homologous to the sequence from 4125431 to 4125460). MetBR (SEQ ID NO 12): ttcgtcgtcatttaacccgctacgcactgc (homologous to the sequence from 4126305 to 4126276). The chloramphenicol resistance cassette was then eliminated. The plasmid pCP20 carrying recombinase FLP acting at the FRT sites of the chloramphenicol resistance cassette was introduced into the recombinant strains by electroporation. After a series of cultures at 42°C, the loss of the chloramphenicol resistance cassette was verified by a PCR analysis with the same oligonucleotides as those used previously. The strains retained were designated MG1655 (AmetJ) metK*. Subsequently the plasmid pTRC/nefL harbouring the metL gene was introduced into these strains giving rise to Kmet] metK*\ 1 pTRCwe/L, Amet] metK* \3 pTRCmetL and AmetJ metK*\4 pTRCmefL.

Example 4

Construction of homoserine succinyltransferase alleles expressing enzymes with further reduced feed-back sensitivity To further reduce the sensitivity of homoserine transsuccinylase to its feed-back inhibitors methionine and SAM other metK mutants were constructed by site directed mutagenesis. Initially the metK and met A*l 1 allele were cloned into the vector pTRC99A (Stratagene).

The following two oligonucleotides were used:

- MetA-Ncol with 49 bases (SEQ ID NO 13): TATTAAATTACCatggcaccgattcgtgtgccggacgagctacccgccg with - a region (lower case) homologous to the sequence (4211862 to 4211892) of the gene metK (sequence 4211859 to 4212788, reference sequence on the website http://genolist.nasteur.fr/Colibri/).

- a region (upper case) that together with the sequence pertaining to metK forms a restriction site for the enzyme Ncol (underlined), - MelA-EcoRI with 47 bases (SEQ ID NO 14):

TATTAAATTAGaattccgactatcacagaagattaatccagcgttgg with

- a region (lower case) homologous to the sequence (4212804 to 42127774) of the gene metK

- a region (upper case) tiiat harbors the restriction site £cøRI. The allclcs metK and melK*\ 1 were amplified by PCR using oligonucleotides MctAF and

MetAR and die restriction sites Ncol and £<røRI were introduced at the N- and C -terminus of die metK gene, respectively. The resulting PCR fragments were restricted by Ncol and EcoRl and cloned into the vector pTRC99A (Stratagene) previously cut by Ncol and EcoRl. Plasmid preparations were examined for the presence of inserts of die correct size and the DNA sequences of metK and met A*l I were verified by sequencing.

Enzymatic analysis of the clones expressing metK and metK* 11 gave very low MetA activity. We presumed that introducing an alanine between amino acid IM and 2P. which was required for cloning of metK into the vector pTRC99A, caused tiiis loss of activity. Therefore using site directed mutagenesis (Stratagene) Uie alanine was eliminated using the oligonucleotides: metA-alaF (Afllli) (SEQ ID NO 15): cacacaggaaacagaccatgccgatacgtgtgccggacgagctaccc metA-alaR (Afllli) (SEQ ID NO 16): gggtagctcgtccggcacacgtatcggcatggtctgtttcctgtgtg Subsequently the mutants metK* 15 (A27V + Q64E), metK* 16 (Q64D) and metK* 17 (A27V + Q64D) were constructed by site directed mutagenesis (Stratagene) using the corrected metA sequence as matrix. For the construction of pTRC/wefA*15 (A27V + Q64E) the following oligonucleotides were used:

MetAA28VF (Xbal) (SEQ ID NO 17): Gtgatgacaacttctagagtgtctggtcaggaaattcgtcc MetAA28VR (Xbal) (SEQ ID NO 18): Ggacgaatttcctgaccagacactctagaagttgtcatcac and pTRCmetK* 11 as matrix. For the construction of pTRCme/A*16 (Q64D) the following oligonucleotides were used:

MetAQ64DF (EcoRV) (SEQ ID NO 19): Gctttcaaactcacctttggatgtcgatatccagctgttgc MetAQ64DR (EcoRV) (SEQ ID NO 20) gcaacagctggatatcgacatccaaaggtgagtttgaaagc and pTRC/nef A as matrix. For the construction of pTRCmetA*17 (A27V + Q64D) the following oligonucleotides were used: MetAA28VF (Xbal) (SEQ ID NO 17) and MetAA28VR (Xbal) (SEQ ID NO 18); and pTRC efA*16 as matrix. Plasmids were verified by sequencing. To transfer the alleles metK*15, metK*l6 and metK*17 onto the metK locus on the chromosome, a metK deletion was constructed in the AmetJ background applying the same strategy used for the met] deletion. The following oligonucleotides were used for deleting metK: DmetAF (4211866-4211945) (SEQ ID NO 21) ttcgtgtgccggacgagctacccgccgtcaatttcttgcgtgaagaaaacgtctttgtgatgacaacttctcgtgcgtctTGTAGGCTGG AGCTGCTTCG DmetAR (4212785-4212706) (SEQ ID NO 22) AtccagcgttggattcatgtgccgtagatcgtatggcgtgatctggtagacgtaatagttgagccagttggtaaacagtaCATATGAAT ATCCTCCTTAG The region indicated in lower case corresponds to the sequence between metK and yjaB. The region in upper case is used to amplify the kanamycin resistance cassette (Datsenko & Wanner, 2000). (Numbers in parentheses correspond to the reference sequence on the website http://gcnolist.pastcur.fr/Colibri . The resulting deletion was verified using the following oligonucleotides. (Numbers in parentheses correspond to Uic reference sequence on the website http://κcnolist.pasteur fr/Colibri . MetAF (SEQ ID NO 3) and MetAR (SEQ ID NO 4). The resulting strain DmetJ DmelK was used to introduce die alleles werA*15, metK* 16 and metK* 17 onto the chromosome. For this piupose the plasmid pKD46 was introduced into the strain DmetJ OmelK (Datsenko and Wanner, 2000). The alleles were amplified from the vectors pTROwe/A*15, pTRCmetK*16 and pTRC/wefA* 17 using the following oligonucleotides: MetArcF (4211786-4211884) (SEQ ID NO 23) Ggcaaattttctggttatcttcagctatctggatgtctaaacgtataagcgtatgtagtgaggtaatcaggttatgccgattcgtgtgccggacga gc MetArcR (4212862-4212764) (SEQ ID NO 24) Cggaaataaaaaaggcacccgaaggtgcctgaggtaaggtgctgaatcgcttaacgatcgactatcacagaagattaatccagcgttggatt catgtgc The sequence in bold is homologous to the sequence of the gene metK; the rest of the sequence is adjacent to met A For verification by PCR the following oligonucleotides were used : MetAF (SEQ ID NO 3) and MetAR (SEQ ID NO 4). As described above, the resulting strains were cultivated in minimal medium, crude extracts were prepared and the activity of MetA was determined. As can be seen from Tab. 4 the newly constructed alleles are less sensitive to the inhibition by methionine and SAM than the alleles described above. Especially interesting is the allele metA* 15 with high intrinsic activity uiat cannot be inhibited by SAM and methionine.

Tab. 4 Homoserinetranssuccinylase activities of MetA mutants in absence and presence of 1 0 mM metiiionine and 1 mM SAM. Percentages values in parentheses indicate the amount of remaining activity upon inhibition.

Example 5

Construction of E. coli strains for the production of O-succinyl homoserine or methionine by combining feed-back resistant MetA alleles with MetK alleles with decreased activity In order to transfer the recombinant metK allclcs into strains harboring feedback resistant metK alleles, a kanamycin resistance cassette was introduced between die metK and gal? gene. To introduce the cassette the homologous recombination strategy described by Datsenko & Wanner (2000) is used. This strategy allows the insertion of a kanamycin resistance cassette, while deleting most of the gene concerned. For this purpose 2 oligonucleotides are used: - DMetKFscr with 100 bases (SEQ ID NO 25): ccgcccgcacaataacatcattcttcctgatcacgtttcaccgcagattatcatcacaactgaaaccgattacaccaaccTGTAGGCTGG AGCTGCTTCG with - a region (lower case) homologous to the sequence of the region between metK and gal? (sequence 3085964 to 3086043, reference sequence on the website http://genolist.pasteur.fr/Colibri/).

- a region (upper case) for the amplification of the kanamycin resistance cassette (reference sequence in Datsenko, K.A. & Wanner, B.L., 2000, PNAS, 97: 6640-6645), - DmetKRscr with 100 bases (SEQ ID NO 26): gagttatatcatcatagattaaacgctgttatctgcaattaagactttactgaaaagaaatgtaacaactgtgaaaaccgCATATGAATAT CCTCCTTAG with

- a region (lower case) homologous to the region between the gene metK and gal? (3086162 to 3086083)

- a region (upper case) for die amplification of the kanamycin resistance cassette. The oligonucleotides DMetKFscr and DmetKRscr are used to amplify the kanamycin resistance cassette from the plasmid pKD4. The PCR product obtained is then introduced by electroporation into the strain MG1655 (pKD46) in which the expressed Red recombinase enzyme allows the homologous recombination. The kanamycin resistant transfomiants arc then selected and the insertion of the resistance cassette is verified by a PCR analysis with the oligonucleotides MetKFscr and MetKRscr defined below. The strain retained is designated MG1655 (metK, Km). MetKFscr (SEQ ID NO 27): gcgcccatacggtctgatlcagatgctgg (homologous to the sequence, from 3085732 to 3085760). MetKRscr (SEQ ID NO 28): gcgccagcaatlacaccgatatccaggcc (homologous to the sequence from 3086418 to 3086390). To transfer the metK alleles. the method of phage PI transduction is used. The protocol followed is implemented in 2 steps with the preparation of the phage l sale of the strain MG1655 (melK, Km) and then transduction into strain MG1655 AmetJ metK*l l pTRCmelL. The construction of the strain {metK, Km) is described above. Preparation of phage lvsate PI : Inoculation with 100 μl of an overnight culture of the strain MG1 55 (melK, Km) of 10 ml of LB + Km 50 μg/ml + glucose 0.2 % + CaCl₂ 5 mM Incubation for 30 min at 37°C wiUi shaking - Addition of 100 μl of phage lysale PI prepared on d e wild strain MG1655 (about 1.10⁹ phage/ml) Shaking at 37°C for 3 hours until all the cells were lysed Addition of 200 μl chloroform and vortexing Centrifugation for 10 min at 4500 g to eliminate cell debris Transfer of supernatant to a sterile tube and addition of 200 μl chloroform Storage of lysate at 4°C. Transduction Centrifuging for 10 min at 1500 g of 5 l of an overnight culture of the strain MG1655 Δ efJ metK* 11 pTRCmetL in LB medium Suspension of the cell pellet in 2.5 ml of 10 mM MgS0₄, 5 mM CaCl₂ Control tubes: 100 μl cells 100 μl phages PI of strain MG1655 (AmetK, Km) - Test lube: 100 μl of cells + 100 μl of phages PI of the strain MG1655 (AmetK, Km) - Incubation for 30 min at 30°C without shaking Addition of 100 μl of 1 M sodium citrate in each tube and vortexing Addition of 1 ml of LB Incubation for 1 hour at 37°C with shaking Spreading on dishes LB + Km 50 μg/ml after centrifuging of tubes for 3 min at 7000 rpm - Incubation at 37°C overnight. Verification of the strain The kanamycin resistant transfomiants are then selected and the insertion of the region containing (metK, Km) is verified by a PCR analysis with the oligonucleotides MetKFscr and MetKRscr. The strain retained is designated MG1655 AmetJ metK*l l pTRCmefL metK*. The kanamycin resistance cassette can then be eliminated. The plasmid pCP20 carrying

FLP recombinase acting at the FRT sites of the kanamycin resistance cassette is then introduced into the recombinant sites by electroporation. After a series of cultures at 42°C, the loss of the kanamycin resistance cassette is verified by a PCR analysis with the same oligonucleotides as used • previously (MetKFscr and MetKRscr).

Example 6

Fermentation of E. coli production strains and analysis of yield Production strains were initially analyzed in small Erlenmeyer flask cultures using modified M9 medium (Anderson, 1946, Proc. Natl. Acad. Sci. USA 32:120-128) that was supplemented with 5 g/1 MOPS and 5 g/1 glucose. Carbcnicillin was added if necessary at a concentration of 100 mg/1. An overnight culture was used to inoculate a 30 ml culture to an OD600 of 0.2. After the culture had reached an OD600 of 4.5 to 5, 1 ,25 ml of a 50% glucose solution and 0.75 ml of a 2M MOPS (pH 6.9) were added and culture was agitated for 1 hour. Subsequently 1PTG was added if necessary. Extracellular metabolites were analyzed during the batch phase. Amino acids were quantified by HPLC after OPA/Fmoc derivatization and other relevant metabolites were analysed using GC-MS after silylation. The following results were obtained for the strains MG1655, MG1 55 pTRC e/L MG1655 metK* l l pΥRCmetL, MG1655 we/A* 13 pTRCmetL, MG1655 e/A*l l AmetJ pTRCmefL, MG1655 metK* 11 Δme J pTRCmefL wefK*H 142Y.

Tab. 5 Specific concentrations of extracellular metabolites (mmol g dry weight) after batch (Erlenmeyer flask) fermentation. ND not detected.

To further boost the production of homoserine the aspartokinase homoserine a thrA* allele with reduced feed-back resistance to threonine is expressed from the plasmid pCL1920 (Lerner & 10 Inouye, 1990, NAR 18, 15 p 4631) using the promoter Ptrc. For the construction of plasmid pME107 thrK was PCR amplified from genomic DNA using the following oligonucleotides: Bsp lt rA (SEQ ID NO 29): ttaTCATGAgagtgttgaagttcggcggtacatcagtggc SfotαlthrA (SEQ ID NO 30): ttaCCCGGGccgccgccccgagcacatcaaacccgacgc The PCR amplified fragment is cut with the restriction enzymes BspHl and Smal and 15 cloned into the Ncol / Smal sites of the vector pTRC99A (Stratagene). For the expression from a low copy vector the plasmid pMElOl is constructed as follows. The plasmid pCL1920 is PCR amplified using the oligonucleotides PME101F and PME101R and the BstZlT -Xmn fragment from the vector pTRC99A harboring the lacl gene and the Ptrc promoter is inserted into the amplified vector. The resulting vector and the vector harboring the thrK gene are restricted by Apal 0 and Smal and the thrK containing fragment is cloned into the vector pMElOl. To relieve ThrA from feed-back inhibition die mutation F318S is introduced by site-directed mutagenesis (Stratagene) using the oligonucleotides ThrAF F318S for and ThrAR F318S, resulting in the vector pME 107. The vector pME 107 was introduced into the AmetJ metK* 11 strains with differing metK* alleles. 5 PME101F (SEQ ID NO 31): Ccgacagtaagacgggtaagcctg PME101R (SEQ ID NO 32): Agcttagtaaagccctcgctag ThrAF F318S (Smal) (SEQ ID NO 33): Ccaatctgaataacatggcaatgtccagcgtttctggcccggg ThrAR F318S (Smal) (SEQ ID NO 34): Cccgggccagaaacgctggacattgccatgttattcagattgg Strains that produced substantial amounts of metabolites of interest were subsequently tested under production conditions in 300 ml fermentors (DASG1P) using a fed batch protocol. For this purpose the fermentor was filled with 145 ml of modified minimal medium and inoculated with 5 ml of preculture to an optical density (OD600nm) between 0.5 and 1.2. The temperature of the culture was maintained constant at 37 °C and the pH was permanently adjusted to values between 6.5 and 8 using an NH OH solution. The agitation rate was maintained between 200 and 300 rpm during the batch phase and was increased to up to 1000 rpm at the end of the fed-batch phase. The concentration of dissolved oxygen was maintained at values between 30 and 40% saturation by using a gas controller. When the optical density reached a value between three and five the fcd-batch was started with an initial flow rate between 0.3 and 0.5 ml/h and a progressive increase up to flow rate values between 2.5 and 3.5. ml/h. Ai this, point she How rate, was maintained constant for 24 to 48 hours. The medi of the fed was based on minimal media containing glucose al concentrations between 300 and 500 g/I Tab. 6 shows methionine concentrations for the strains AmetJ we/A*l l pME107 wefK*H142Y and AmetJ metK*ll pME107 metK*T227l compared to the reference strain AmetJ wefA*ll pME107 after about 75h of operation. Both strains harboring the metK* alleles attain an increased methionine concentration when compared to the reference strain. Thus the metK mutations confer an industrial advantage on methionine production by increasing the productivity of the strains.

Tab. 6 Metiiionine production of the reference strain AmetJ metA*l l pME107 and two strains harboring the metK* mutations after indicated time of operation including batch and fed- batch.

Claims

What is claimed is: 1. A method for the preparation of methionine, its precursors or products derived thereof, in a fermentative process with a microorganism where L-homoserine is converted into O- succinylhomoserine with a homoserine transsuccinylase, comprising the step of culturing the said microorganism on an appropriate medium and recovering methionine, its precursors or products derived thereof once produced, wherein the homoserine transsuccinylase is a mutated homoserine transsuccinylase with reduced sensitivity for the feedback inhibitors S-adenosylmethionine and methionine.

2. The method as claimed in claim 1, wherein the modified homoserine succinyltransferases exhibit a specific activity which is at least ten times that of the wild-type enzyme in the presence of 10 mM methionine and 1 mM S-adenosylmethionine and al least 80 times that of the wild-type enzyme in the presence of 10 mM methionine and 0.1 mM S- adenosy lmethionine .

3. The method as claimed in one of claims 1 or 2, wherein the modified homoserine succinyltransferases comprise at least one amino acid mutation when compared with the wild-type sequence in one of the following non mutated conserved regions and combinations diereof: conserved region 1 comprising the following sequence:

X1-X2-X3-A-X4-X5-Q

In which XI represents E, D, T, S, L, preferentially T

X2 represents D, S, K, Q, E, A, R, preferentially S

X3 represents R, E, D, preferentially R

X4 represents Y, I, F, A, K, S, V, preferentially S

X5 represents H, S, N, G, T, R, preferentially G conserved region 2 comprising the following sequence:

X 1 -X2-X3 -P-L-Q-X4-X5

In which

XI represents G, A, S, preferentially S

X2 represents N, A, preferentially N X3 represents S, T, preferentially S

X4 represents V, L, I, preferentially V

X5 represents N, E, H, D, preferentially D conserved region 3 comprising the following sequence:

X1-Y-Q-X2-T-P-X3 In which

XI represents V, I, M, preferentially V

X2 represents E, K, G, I, Q, T, S, preferentially I

X3 represents F, Y, preferentially Y.

4. The method as claimed in claim 3, wherein the modified homoserine succinyltransferases comprise at least one of the following mutations and combinations thereof: conserved alanine in conserved region 1 is replaced with a valine, conserved amino acids L and/or Q in conserved region 2 are replaced with other amino acids, preferentially Icucine is replaced by phcnylalanine and/or glutamine is replaced with a glutamatc or aspartate, and conserved amino acids L and/or Q in conserved region 3 are replaced with another amino acid.

5. The method as claimed in claim 4, wherein the modified conserved region 1 comprises the amino acid sequence T-S-R-V-S-G-Q, and/or the modified conserved region 2 comprises the amino acid sequence selected among S-N-S-P-F-Q-V-D, S-N-S-P-F-E-V-D, S-N-S- P-F-D-V-D D, S-N-S-P-L-E-V-D and S-N-S-P-L-D-V-D.

6. The method as claimed in any one of claims 1 to 5, wherein the microorganisms comprise a S-adenosylmethionine synthetase enzyme with reduced S-adenosylmethionine synthetase enzymatic activity.

7. The method as claimed in claim 6, wherein the modified S-adenosylmethionine synthetase contains at least one amino acid mutation of at least one of the sequences specified below: cysteine at position 89 and/or cysteine at position 239 conserved region 1, the non mutated region 1 comprising the following sequence: G-E-X1-X2-X3-X4 wherein

XI represents I, V, L, T preferentially 1

X2 represents T, K, S, R, preferentially T

X3 represents T, S, G, preferentially T X4 represents S, N, T, K, E, R, P, N, A preferentially S conserved region 2, the non mutated conserved region 2 comprising the following sequence:

Q-S-Xl -D-I-X2-X3-G-V-X4 wherein

XI represents P, Q, A, S, preferentially P X2 represents A, N, Q, S, F, preferentially N

X3 represents V, Q, Y, R, M, N, preferentially Q

X4 represents K, D, A, N, T, S, E, preferentially D conserved region 3, the non mutated conserved region 3 comprising the following sequence:

X1-G-A-G-D-Q-G-X2 wherein

XI represents Q, A, I, V, E, T, preferentially Q

X2 represents L, I, S, V, M, preferentially L conserved region 4, the non mutated conserved region 4 comprising the following sequence:

X1-I-X2-X3-X4-H-X5-X6 wherein

XI represents S, P, T, A, preferentially P

X2 represents A, T, W, Y, F, S, N preferentially T

X3 represents M, Y, L, V preferentially Y X4 represents S, A, preferentially A

X5 represents K, R, E, D, preferentially R

X6 represents L, 1, preferentially L conserved region 5, the non mutated conserved region 5 comprising the following sequence:

X1-L-X2-X3-D wherein

XI represents W, F, Y, V, E preferentially W

X2 represents R, G, L, K, preferentially R

X3 represents P, L, H, V, preferentially P conserved region 6, the non mutated conserved region 6 comprising the following sequence: X1-X2-X3-S-X4-Q-H

In which

X 1 represents V, I, preferentially V

X2 represents V, L, I, preferentially V

X3 represents V, L, I, M, preferentially L X4 represents T, V, A, S, H, preferentially T conserved region 7, the non mutated conserved region 7 comprising the following sequence:

X1-N-P-X2-G-X3-F

In which

XI represents V, I, preferentially 1 X2 represents T, G, S, preferentially T

X3 represents R, T, Q, K, S, preferentially R conserved region 8, the non mutated conserved region 8 comprising the following sequence:

X1-X2-G-X3-P-X4-X5

In which XI represents T, V, I, Y, E, preferentially V

X2 represents V, I, L, N, preferentially I

X3 represents G, S, preferentially G

X4 represents M, I, Q, A, H, D preferentially M

X5 represents G, S, A, H preferentially G conserved region 9, the non mutated conserved region 9 comprising the following sequence:

X1-X2-D-T-Y-G-G

In which

XI represents M, I, preferentially I

X2 represents V, I, preferentially I conserved region 10, the non mutated conserved region 10 comprising the following sequence:

K-V-D-R-S-X1-X2

In which

XI represents A, G, preferentially A X2 represents A, S, L, preferentially A conserved region 11, the non modified conserved region 11 comprising the following sequence:

X1-X2-Q-X3-X4-Y-A-I-G-X5-X6

In which

XI represents L, E, I, Q, T preferentially E X2 represents V, I, L preferentially I

X3 represents V, L, I preferentially V

X4 represents A, S preferentially S

X5 represents V, I, R, K, A preferentially V

X6 represents A, V, T, S, preferentially A.

8. The method as claimed in claim 7, wherein the modified S-adenosylmethionine synthetase with decreased activity comprises at least one of the following mutations and combinations thereof: the conserved serine in conserved region 1 is replaced with another amino acid, preferentially with a asparagine, the conserved glycine in conserved region 2 is replaced with another amino acid, preferentially with a serine, the conserved glycine in conserved region 3 is replaced with another amino acid, preferentially with a serine, the conserved histidine and/or semiconserved proline in conserved region 4 is replaced with another amino acid, preferentially the conserved histidine with a tyrosinc and/ or the conserved proline with a leucine, the semi-conserved arginine in conserved region 5 is replaced with another amino acid, preferentially with a cysteine, the conserved histidine and/or semi-conserved valine in conserved region 6 is replaced with another amino acid, preferentially the conserved histidine is replaced with a tyrosine and/or the valine with aspartate, the semi -conserved threonine in conserved region 7 is replaced with another amino acid, preferentially with an isoleucine, the conserved proline in conserved region 8 is replaced with another amino acid, preferentially with a serine, the second conserved glycine in conserved region 9 is replaced with another amino acid, preferentially with a aspartate, the conserved serine in conserved region 10 is replaced with another amino acid, preferentially with a phenylalanine, and the conserved isoleucine in conserved region 11 is replaced with other amino acids, preferentially with a leucine.

9. The method as claimed in claim 8, wherein the modified region 1 comprises the amino acid sequence G-E-I-T-T-N and/or the modified conserved region 2 comprises die amino acid sequence Q-S-P-D-I-N-Q-S-V-D and/or the modified conserved region 3 comprises the amino acid sequence Q-S-A-G-D-Q-G-L and/or the modified conserved region 4 comprises the amino acid sequence selected among P-I-T-Y-A-Y-R-L, L-T-T-Y-A-H-R-L and L-I-T-Y-A-Y-R-L, and/or the modified conserved region 5 comprises the amino acid sequence W-L-C-P-D and/or the modified conserved region 6 comprises the amino acid sequence selected among V-V-L-S-T-Q-Y, V-D-L-S-T-Q-H and V-D-L-S-T-Q-Y, and or the modified conserved region 7 comprises the amino acid sequence I-N-P-I-G-R-F and/or the modified conserved region 8 comprises the amino acid sequenc V-I-G-G-S-M-G and/or the modified conserved region 9 comprises the amino acid sequence I-V-D-T-Y-G-D and/or the modified conserved region 10 comprises the amino acid sequence K-V-D-R-F-A-A and/or the modified conserved region 11 comprises the amino acid sequence E-I-Q-V-S-Y-A-L-G-V-A.

10. The method as claimed in any one of claims 1 to 9, wherein the modified microorganism is selected among prokaryotes and eukaryotes, preferentially under prokaryotes and more specifically Escherichia coli or Coryne bacterium glutamicum.

11. A mutated homoserine transsuccinylase with reduced sensitivity for the feedback inhibitors S-adenosylmethioninc and methionine as defined in any one of claims 2 to 5.

12. A nucleotide sequence encoding a mutated homoserine transsuccinylase with reduced sensitivity for the feedback inhibitors S-adenosylmethionine and methionine as defined in claim 11 13. A mutated S-adenosylmethionine synthetase as defined in one of claims 6 to 9. 14. A nucleotide sequence encoding a mutated S-adenosylmethionine synthetase as defined in claim 13. 15. A microorganism comprising a nucleotide sequence of claim 12. 16. The microorganism as claimed in claim 15, further comprising a nucleotide sequence of claim 14. 17. The microorganism of claim 15 or 16, wherein the microorganism is selected among prokaryotes and eukaryotes, preferentially under prokaryotes and more specifically Escherichia coli or Corynebaclerium glutamicum. 18. The microorganism of one of claims 15 to 17, wherein the gene encoding aspartokinase/homoserine dehydrogenase is over expressed and/or the gene encoding methionine repressor metJ is deleted. 19. The microorganism of one of the claims 15 to 17, wherein the aspartokinase homoserine dehydrogenase is encoded by a feed-back deregulated thrA allele. 20. The microorganism of claim 19 in which the ThrA enzyme is feed-back deregulated by the mutation Phe318Ser.