1-38: Efficient ethanol production by engineered Saccharomyces cerevisiae with simultaneous saccharification and co-fermentation of cellulose and xylose

Simultaneous saccharification and fermentation (SSF) has been proposed as an effective way for producing cellulosic biofuels because it improves both hydrolysis and fermentation synergistically. While lignocelluosic biomass consists of cellulose and hemicellulose, most SSF processes have focused on fermentation of a cellulose fraction rather than complete utilization of both cellulose and hemicellulose fractions. Here, we demonstrate a simultaneous saccharification and co-fermentation process (SSCoF) using an engineered Saccharomyces cerevisiae strain capable of simultaneously fermenting cellobiose and xylose. The engineered S. cerevisiae was developed by introducing heterologous xylose and cellobiose metabolic pathways. Specifically, three genes (XYL1, XYL2, and XYL3) coding for xylose metabolic enzymes from Pichia stipitis and two genes (cdt-1 and gh1-1) coding for a cellodextrin transporter and an intracellular β-glucosidase from Neurospora crassa were introduced into S. cerevisiae. Using the engineered strain, we investigated synergistic effects of co-fermentation on hydrolysis and fermentation of Avicel and xylose. When Avicel was used as a sole carbon source, the engineered strain showed 70% increase in ethanol production and 50% reduction in cellobiose accumulation as compared with a control strain that cannot ferment cellobiose. Moreover, the engineered strain exhibited less accumulation of cellobiose and efficient xylose fermentation when Avicel and xylose were used as carbon sources. As a result, we observed significant improvements in both ethanol yield and productivity by the engineered strain as compared to various control strains. These results suggest that SSCoF using an engineered strain capable of co-fermenting cellobiose and xylose is a promising strategy for producing cellulosic biofuels economically.