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

• Engineered Saccharomyces cerevisiae strain capable of fermenting cellobiose can be used in SSF without extracellular supplementation of β-glucosidase.
• Unlike the traditional SSF process, higher initial cell concentration is critical for efficient ethanol production by engineered S. cerevisiae without supplementation of β-glucosidase.
• By increasing the initial concentration of engineered S. cerevisiae, efficient cellobiose utilization during SSF of cellulose can be achieved by the increased amount of intracellular 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 β-glucosidase using an engineered Saccharomyces cerevisiae strain expressing a cellodextrin transporter and an intracellular β-glucosidase from Neurospora crassa. Ethanol production profiles by the engineered yeast without supplementation of β-glucosidase and by a parental strain with supplementation of β-glucosidase were examined under various fermentation conditions. When initial cell mass concentrations were low, the traditional SSF with supplementation of β-glucosidase showed better ethanol production than SSF with the engineered strain without supplementing β-glucosidase. However, the engineered strain without supplementation of β-glucosidase showed almost the same or even better ethanol productivity than the parental strain with supplementation of β-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 β-glucosidase, leading to economic production of cellulosic ethanol.