(449c) A Novel Strategy to Overcome Glucose Repression In Mixed Sugar Fermentation In Saccharomyces Cerevisiae

Pretreatment of biomass by chemical or enzymatic methods yields a mixture of hexose sugars (C6 sugars, primarily glucose and mannose) and pentose sugars (C5 sugars, primarily D-xylose and L-arabinose). The fermentation of almost all the available C6 and C5 sugars to ethanol or other liquid biofuel is vital to the overall economics of these processes because this will maximize the yield and minimize the costs associated with waste disposal. However, glucose repression exists in the fermentation of mixed sugars, i.e. utilization of pentose sugars is inhibited in the presence of glucose. This is because transportation of pentose sugars is inhibited by glucose as pentose sugars are transported by glucose transporters and pentose metabolism may also be repressed by glucose. Here we report a novel strategy to overcome glucose repression in which a heterologous cellobiose transporter and a β-glucosidase are introduced into a S. cerevisiae strain capable of utilizing D-xylose. In both flask and bioreactor fermentation with 4% cellobiose and 5% xylose, the engineered S. cerevisiae strain showed greatly enhanced sugar consumption rate, cell growth rate, and ethanol production. In flasks, 41.4% increase in cell growth and 28.8% increase in ethanol production were found; while in bioreactors, 1.02-fold increase in cell growth and 4.4-fold increase in ethanol production were found.