Background: Hydrolysis of cellulose requires the action of the cellulolytic enzymes endoglucanase,
cellobiohydrolase and b-glucosidase. The expression ratios and synergetic effects of these enzymes significantly
influence the extent and specific rate of cellulose degradation. In this study, using our previously developed
method to optimize cellulase-expression levels in yeast, we constructed a diploid Saccharomyces cerevisiae strain
optimized for expression of cellulolytic enzymes, and attempted to improve the cellulose-degradation activity and
enable direct ethanol production from rice straw, one of the most abundant sources of lignocellulosic biomass.