Method and microorganism for methionine production by fermentation with improved methionine efflux

1. A recombinant Escherichia coli ( E. coli ) strain for the fermentative production of methionine, wherein in said recombinant strain:

the expression of ygaZH homologous genes of ygaZ and ygaH genes from E. coli is increased compared to a non-modified E. coli strain with the proviso that it is neither combined with overexpression of genes metH, and optionally of fldA and fpr from E. coli nor with attenuation of expression of at least one gene selected from the group consisting of metN, metI and metQ genes, and

said ygaZH homologous genes are selected from the group consisting of genes encoding the pair of YgaZH homologue defined respectively by: SEQ ID NO: 3 and SEQ ID NO: 4 from Citrobacter koseri , SEQ ID NO: 9 and SEQ ID NO: 10 from Enterobacter sp. (R4-368), SEQ ID NO: 17 and SEQ ID NO: 18 from Citrobacter youngae, SEQ ID NO: 19 and SEQ ID NO: 20 from Citrobacter freundii,

and wherein the increased expression is achieved by:

i) increasing the number of copies of the genes in the microorganism,

ii) using a promoter leading to a high level of expression of the genes, and/or

iii) attenuating the activity or the expression of a transcription repressor.

2. The recombinant microorganism of claim 1 , wherein the ygaZH homologous genes are expressed under control of an inducible promoter.

3. The recombinant microorganism of claim 1 , wherein the expression of at least one of the following genes is also increased: ptsG, pyc, pntAB, cysP, cysU, cysW, cysA, cysM, cyst, cysI, cysH, gcvT, gcvH, gcvP, lpd, serA, serB, serC, cysE, metF, metA, metA* allele encoding for an enzyme with reduced feed-back sensitivity to S-adenosylmethionine and/or methionine, thrA, or a thrA* allele encoding for an enzyme with reduced feed-back inhibition to threonine.

4. The recombinant microorganism of claim 3 , wherein at least one of said genes is under the control of an inducible promoter.

5. The recombinant microorganism of claim 1 , wherein the expression of at least one of the following genes is also attenuated: metJ, pykA, pykF, purU, ybdL, yncA, metE, dgsA or udhA.

6. The recombinant microorganism of claim 1 , wherein:

a. the ygaZ and ygaH homologous genes are overexpressed,

b. the expression of the genes metA*, cysPUWAM, cysJIH, gcvTHP, metF, serA, serB, serC, cysE, thrA*, ptsG and pyc are enhanced; and

c. the expression of the genes metJ, pykA, pykF, purU, dgsA, metE and yncA are attenuated.

7. A method for the fermentative production of methionine comprising:

a. culturing a recombinant Escherichia coli ( E. coli ) strain, wherein in said microorganism, the ygaZH homologous genes of ygaZ and ygaH genes from E. coli are overexpressed; with the provisio that this overexpression is neither combined with overexpression of metH, and optionally of fldA and fpr from E. coli or their homologous genes from C. glutamicum nor with attenuation of expression of at least one gene selected from the group consisting of metN, metI and metQ, said ygaZH homologous genes are selected from the group of genes encoding the pair of YgaZH homologue defined respectively by: SEQ ID NO: 3 and SEQ ID NO: 4 from Citrobacter koseri , SEQ ID NO: 9 and SEQ ID NO: 10 from Enterobacter sp. (R4-368), SEQ ID NO: 17 and SEQ ID NO: 18 from Citrobacter youngae, SEQ ID NO: 19 and SEQ ID NO: 20 from Citrobacter freundii in an appropriate culture medium comprising a fermentable source of carbon and a source of sulphur, and

b. recovering methionine from the culture medium,

wherein the overexpression is achieved by:

i) increasing the number of copies of the genes in the microorganism,

ii) using a promoter leading to a high level of expression of the genes, and/or

iii) attenuating the activity or the expression of a transcription repressor.

8. The method of claim 7 , wherein growth of the recombinant microorganism is subjected to limitation or deficiency for one or several inorganic substrate(s) in the culture medium.

9. The method of claim 7 , wherein recovering methionine comprises concentration of methionine in the fermentation broth.

10. A genetically modified Escherichia coli ( E. coli ) strain for the fermentative production of methionine, wherein in said genetically modified strain, ygaZH homologous genes of ygaZ and ygaH genes from E. coli are overexpressed said ygaZH homologous genes are selected from the group consisting of genes encoding the pair of YgaZH homologue defined respectively by: SEQ ID NO: 3 and SEQ ID NO: 4 from Citrobacter koseri , SEQ ID NO: 9 and SEQ ID NO: 10 from Enterobacter sp. (R4-368), SEQ ID NO: 17 and SEQ ID NO: 18 from Citrobacter youngae, SEQ ID NO: 19 and SEQ ID NO: 20 from Citrobacter freundii.

11. A method for the fermentative production of methionine comprising:

a. culturing a genetically modified Escherichia coli ( E. coli ) strain wherein in said microorganism, the ygaZH homologous genes of ygaZ and ygaH genes from E. coli are overexpressed in an appropriate culture medium comprising a fermentable source of carbon and a source of sulphur, said ygaZH homologous genes are selected from the group consisting of genes encoding the pair of YgaZH homologue defined respectively by: SEQ ID NO: 3 and SEQ ID NO: 4 from Citrobacter koseri , SEQ ID NO: 9 and SEQ ID NO: 10 from Enterobacter sp. (R4-368), SEQ ID NO: 17 and SEQ ID NO: 18 from Citrobacter youngae, SEQ ID NO: 19 and SEQ ID NO: 20 from Citrobacter freundii , and

b. recovering methionine from the culture medium.

12. The method of claim 7 , wherein growth of the recombinant microorganism is subjected to limitation or deficiency for phosphate and/or potassium in the culture medium.