Efficient Production of l-Lactic Acid by Metabolically Engineered Saccharomyces cerevisiae with a Genome-Integrated l-Lactate Dehydrogenase Gene

We developed a metabolically engineered yeast which produces lactic acid efficiently. In this recombinant strain, the coding region for pyruvate decarboxylase 1 (PDC1) on chromosome XII is substituted for that of thel-lactate dehydrogenase gene (LDH) through homologous recombination. The expression of mRNA for the genome-integratedLDHis regulated under the control of the nativePDC1promoter, whilePDC1is completely disrupted. Using this method, we constructed a diploid yeast transformant, with each haploid genome having a single insertion of bovineLDH. Yeast cells expressingLDHwere observed to convert glucose to both lactate (55.6 g/liter) and ethanol (16.9 g/liter), with up to 62.2% of the glucose being transformed into lactic acid under neutralizing conditions. This transgenic strain, which expresses bovineLDHunder the control of thePDC1promoter, also showed high lactic acid production (50.2 g/liter) under nonneutralizing conditions. The differences in lactic acid production were compared among four different recombinants expressing a heterologousLDHgene (i.e., either the bovineLDHgene or theBifidobacterium longum LDHgene): two transgenic strains with 2μm plasmid-based vectors and two genome-integrated strains.