Direct fermentation of cellulose to ethanol by Saccharomyces cerevisiae displaying a bifunctional cellobiohydrolase gene from Orpinomyces sp. Y102

Abstract

Abstract A cellobiohydrolase gene, cbhC16, isolated from the cDNA library of a rumen fungus. The recombinant CbhC16 demonstrated a specific activity of 7.0 U mg−1 protein against phosphoric acid-swollen avicel, and cellobiose was the main product in the reaction mixture. Glucose was further released from cellobiose by the action of CbhC16 and fermented to ethanol by Saccharomyces cerevisiae. This phenomenon, possessing activities of both cellobiohydrolase and β-glucosidase, is rarely observed in the hydrolysis by typical cellobiohydrolases. Yeast cells displaying CbhC16 were inoculated into YNB–CAA broth containing carboxymethyl cellulose, β-glucan, ammonia fibre explosion-treated rice straw, or wheat bran, and incubated at 30 °C for 3 d. Under the same conditions, 0.20, 0.61, 0.15, and 0.61 g/L of ethanol were detected in the culture supernatant of the strain with the recombinant plasmid containing cbhC16, whereas wild-type yeasts without the recombinant plasmid containing cbhC16 did not produce detectable ethanol levels. The results indicated that yeast strains with cbhC16 could directly saccharify and ferment cellulose to produce ethanol in singular step. This gene encoding both cellobiohydrolase and β-glucosidase activities will enable developing a simple approach for producing a consolidated microorganism that will directly convert cellulosic feedstocks to ethanol.