Simultaneous saccharification and fermentation by engineered Saccharomyces cerevisiae without supplementing extracellular β-glucosidase

Simultaneous saccharification and fermentation (SSF) has been considered a promising and economical process for cellulosic ethanol production. Further cost savings could be gained by reducing enzyme loading and engineering host strain for ethanol production. In this study, we demonstrate efficient ethanol production by SSF without supplementation of beta-glucosidase using an engineered Saccharomyces cerevisiae strain expressing a cellodextrin transporter and an intracellular beta-glucosidase from Neurospora crassa. Ethanol production profiles by the engineered yeast without supplementation of beta-glucosidase and by a parental strain with supplementation of beta-glucosidase were examined under various fermentation conditions. When initial cell mass concentrations were low, the traditional SSF with supplementation of beta-glucosidase showed better ethanol production than SSF with the engineered strain without supplementing beta-glucosidase. However, the engineered strain without supplementation of beta-glucosidase showed almost the same or even better ethanol productivity than the parental strain with supplementation of beta-glucosidase when initial cell mass concentrations were elevated. Our results suggest that efficient ethanol production by SSF could be achieved by engineered yeast capable of fermenting cellobiose without addition of extracellular beta-glucosidase, leading to economic production of cellulosic ethanol. (C) 2013 Elsevier B.V. All rights reserved.