Engineered <i>Saccharomyces cerevisiae</i> capable of simultaneous cellobiose and xylose fermentation

TLDR

Efficient biofuel production from lignocellulosic biomass requires simultaneous utilization of glucose and xylose, yet Saccharomyces cerevisiae strains used for ethanol fermentation can only consume glucose, and engineered xylose‑fermenting strains are slow and glucose‑repressed. The study engineered Saccharomyces cerevisiae to coferment xylose and cellobiose simultaneously. The engineered strains express a high‑affinity cellodextrin transporter and an intracellular β‑glucosidase that hydrolyzes imported cellobiose, thereby relieving glucose repression and permitting simultaneous xylose consumption. Cofermentation of cellobiose and xylose by the engineered strains yielded higher ethanol production and productivities than fermentations with either sugar alone, and demonstrated promise in simulated cellulosic hydrolyzates, marking a key advance toward economical biofuel production.

Abstract

The use of plant biomass for biofuel production will require efficient utilization of the sugars in lignocellulose, primarily glucose and xylose. However, strains of Saccharomyces cerevisiae presently used in bioethanol production ferment glucose but not xylose. Yeasts engineered to ferment xylose do so slowly, and cannot utilize xylose until glucose is completely consumed. To overcome these bottlenecks, we engineered yeasts to coferment mixtures of xylose and cellobiose. In these yeast strains, hydrolysis of cellobiose takes place inside yeast cells through the action of an intracellular β-glucosidase following import by a high-affinity cellodextrin transporter. Intracellular hydrolysis of cellobiose minimizes glucose repression of xylose fermentation allowing coconsumption of cellobiose and xylose. The resulting yeast strains, cofermented cellobiose and xylose simultaneously and exhibited improved ethanol yield when compared to fermentation with either cellobiose or xylose as sole carbon sources. We also observed improved yields and productivities from cofermentation experiments performed with simulated cellulosic hydrolyzates, suggesting this is a promising cofermentation strategy for cellulosic biofuel production. The successful integration of cellobiose and xylose fermentation pathways in yeast is a critical step towards enabling economic biofuel production.