S87: Enhanced Utilization Of Mixed Sugars From Cellulosic Biomass By Engineered Yeast

The lack of microbial strains capable of fermenting all sugars present in plant cell wall hydrolyzates to fuels and chemicals is a major challenge of industrial biotechnology. While naturally existing or engineered microorganisms can ferment mixed sugars (glucose, xylose, and galactose) in hydrolyzates from terrestrial or marine biomass, the preferential utilization of glucose to non-glucose sugars often results in lower conversion yield and productivity of fuels and chemicals. To overcome these bottlenecks, we engineered yeasts to co-ferment mixtures of cellobiose and xylose. After constructing an efficient xylose-fermenting strain of Saccharomyces cerevisiae through rational and combinatorial strategies, we introduced intracellular cellobiose utilizing pathways. Therefore, degradation of cellobiose takes place inside yeast cells through the action of an intracellular β-glucosidase or cellobiose phosphorylase following import by wild type or engineered cellodextrin transporters. The resulting yeasts not only co-fermented cellobiose and xylose simultaneously, but also exhibited improved ethanol yields as compared when either cellobiose or xylose was used as a sole carbon source. With a similar strategy, we demonstrated that co-fermentation of various sugar mixtures and enhanced production of value added chemicals from mixed sugars are also feasible. Our results suggest that this simultaneous co-fermentation of mixed sugars is a promising strategy for producing fuels and chemicals from plant biomass.