Homofermentative Production of d - or l -Lactate in Metabolically Engineered Escherichia coli RR1

Abstract

ABSTRACT We investigated metabolic engineering of fermentation pathways in Escherichia coli for production of optically pure d - or l -lactate. Several pta mutant strains were examined, and a pta mutant of E. coli RR1 which was deficient in the phosphotransacetylase of the Pta-AckA pathway was found to metabolize glucose to d -lactate and to produce a small amount of succinate by-product under anaerobic conditions. An additional mutation in ppc made the mutant produce d -lactate like a homofermentative lactic acid bacterium. When the pta ppc double mutant was grown to higher biomass concentrations under aerobic conditions before it shifted to the anaerobic phase of d -lactate production, more than 62.2 g of d -lactate per liter was produced in 60 h, and the volumetric productivity was 1.04 g/liter/h. To examine whether the blocked acetate flux could be reoriented to a nonindigenous l -lactate pathway, an l -lactate dehydrogenase gene from Lactobacillus casei was introduced into a pta ldhA strain which lacked phosphotransacetylase and d -lactate dehydrogenase. 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 was produced in 67 h. These results demonstrate that the central fermentation metabolism of E. coli can be reoriented to the production of d -lactate, an indigenous fermentation product, or to the production of l -lactate, a nonindigenous fermentation product.