We attempted to develop a metabolically engineered yeast that was able to efficiently produce D-lactic acid with high optical purity. To generate this recombinant strain, the pyruvate decarboxylase (PDC) 1 and 5 gene, which plays a key role in the major metabolic pathway for ethanol fermentation, was completely deleted, and six copies of D-lactate dehydrogenase gene from lactic acid bacteria were introduced under control of high expression type promoter. This recombinant strain showed high yield of D-lactic acid, however, led to decrease in cell growth and fermentation speed.
To solve these problems, we screened efficient D-lactic acid producing strains from pdc1 pdc5 double mutants by mutation breeding. The selected mutants recovered fermentation speed and further increased D-lactic acid yield. These mutants can produce D-lactic acid even in nutrient-poor medium.