Fermentation of sugar mixtures using Escherichia coli catabolite repression mutants engineered for production of L-lactic acid
Conversion of lignocellulose to lactic acid requires strains capable of fermenting sugar mixtures of glucose and xylose. Recombinant Escherichia coli strains were engineered to selectively produce L-lactic acid and then used to ferment sugar mixtures. Three of these strains were catabolite repression mutants (ptsG−) that have the ability to simultaneously ferment glucose and xylose. The best results were obtained for ptsG− strain FBR19. FBR19 cultures had a yield of 0.77 (g lactic acid/g added sugar) when used to ferment a 100 g/l total equal mixture of glucose and xylose. The strain also consumed 75% of the xylose. In comparison, the ptsG+ strains had yields of 0.47–0.48 g/g and consumed 18–22% of the xylose. FBR19 was subsequently used to ferment a variety of glucose (0–40 g/l) and xylose (40 g/l) mixtures. The lactic acid yields ranged from 0.74 to 1.00 g/g. Further experiments were conducted to discover the mechanism leading to the poor yields for ptsG+ strains. Xylose isomerase (XI) activity, a marker for induction of xylose metabolism, was monitored for FBR19 and a ptsG+ control during fermentations of a sugar mixture. Crude protein extracts prepared from FBR19 had 10–12 times the specific XI activity of comparable samples from ptsG+ strains. Therefore, higher expression of xylose metabolic genes in the ptsG− strain may be responsible for superior conversion of xylose to product compared to the ptsG+ fermentations.