Engineering the pathway for succinate production
This invention relates to biocatalysts for the efficient production of succinic acid and/or other products from renewable biological feedstocks. The biocatalysts have a very high efficiency for the growth-coupled production of succinic acid and/or other products from carbohydrate feed stocks as a result of both genetic manipulation and metabolic evolution. More specifically, certain biocatalysts of the present invention produce succinic acid at high titers and yields in mineral salts media during simple pH-controlled batch fermentation without the addition of any exogenous genetic material. The genetic manipulations of the present invention are concerned with energy-conserving strategies coupled with the elimination of alternative routes for NADH oxidation other than the routes for succinic acid production. The biocatalysts contain glucose-repressed gluconeogenic phosphoenolpyruvate carboxykinase (pck) derepressed by genetic modifications and a genetically-inactivated phosphotransferase system. In terms of succinic acid production efficiency, the biocatalysts of the present invention are functionally equivalent to succinate producing rumen bacteria such as Actinobacillus succinogens and Mannheimia succiniproducens , with one difference: that the biocatalysts are able to achieve this high level of succinic acid production in a minimal salt medium with carbohydrate source, as opposed to the requirement for a rich medium for succinic acid production by rumen bacteria.
1. A genetically modified E. coli cell comprising increased levels of phosphoenol pyruvate carboxykinase (pck) gene transcripts as compared to the corresponding unmodified E. coli cell, wherein the genetically modified E. coli cell comprises one or more genetic modifications to one or more endogenous genes leading to inactivation of the one or more endogenous genes, wherein the one or more endogenous genes are selected from the group consisting of:
a) a gene encoding a glycerol dehydrogenase (gldA),
b) a gene encoding a dihydroxyacetone kinase subunits K and L (dhaKL), and
c) a gene encoding a dihydroxyacetone kinase subunit M (dhaM),
and wherein said one or more genetic modifications leading to inactivation of the one or more genes comprise one or more of: (i) deleting the entire coding nucleotide sequence or a portion of the coding nucleotide sequence of said one or more genes, (ii) introducing a frame shift mutation into the coding nucleotide sequence of said one or more genes, (iii) introducing a missense mutation into the coding nucleotide sequence of said one or more genes or (iv) introducing a stop codon into the coding nucleotide sequence of said one or more genes.
2. The E. coli cell of claim 1 , said cell comprising a single point mutation in the promoter region of the endogenous pck gene that increases transcription of the pck gene.
3. The E. coli cell of claim 2 , wherein said single point mutation is a G to A transition at position −64 relative to the ATG start codon of the endogenous pck gene.
4. The E. coli cell of claim 1 , wherein said E. coli cell comprises a multicopy plasmid expressing a pck gene.
5. The genetically modified E. coli cell of claim 1 , wherein the endogenous gene is gldA.
6. The genetically modified E. coli cell of claim 1 , wherein the endogenous gene is dhaKL.
7. The genetically modified E. coli cell of claim 1 , wherein the endogenous gene is dhaM.
8. A method of producing succinic acid comprising:
a) culturing the genetically modified E. coli cell of claim 1 ;
b) proving a carbon source;
c) allowing the genetically modified E. coli cell to metabolize said carbon source;
and
d) isolating succinic acid.
9. The method of claim 8 , wherein said genetically modified E. coli cell is cultured in anaerobic condition, aerobic condition, microaerobic condition or combinations thereof.