Homofermentative production of D- or L-lactate in metabolically engineeredEscherichia coli RR1

We investigated metabolic engineering of fermentation pathways in Escherich ia coli for production of optically pure D- or L-lactate, Several pta mutan t strains were examined, and a pta mutant of E. coli RR1 which was deficien t in the phosphotransacetylase of the Pta-AckA pathway was found to metabol ize glucose to D-lactate and to produce a small amount of succinate by-prod uct under anaerobic conditions. An additional mutation in ppc made the muta nt produce D-lactate like a homofermentative lactic acid bacterium. When th e pta ppc double mutant was grown to higher biomass concentrations under ae robic conditions before it shifted to the anaerobic phase of D-lactate prod uction, more than 62.2 g of D-lactate per liter was produced in 60 h, and t he volumetric productivity was 1.04 g/liter/h. To examine whether the block ed acetate flux could be reoriented to a nonindigenous L-lactate pathway, a n L-lactate dehydrogenase gene from Lactobacillus casei was introduced into a pta ldhA strain which lacked phosphotransacetylase and D-lactate dehydro genase. This recombinant strain was able to metabolize glucose to L-lactate as the major fermentation product, and up to 45 g of L-lactate per liter w as produced in 67 h, These results demonstrate that the central fermentatio n metabolism of E. coli can be reoriented to the production of D-lactate, a n indigenous fermentation product, or to the production of L-lactate, a non indigenous fermentation product.