Expression of a codon-optimized β-glucosidase from Cellulomonas flavigena PR-22 in Saccharomyces cerevisiae for bioethanol production from cellobiose

Bioethanol is one of the main biofuels produced from the fermentation of saccharified agricultural waste; however, this technology needs to be optimized for profitability. Because the commonly used ethanologenic yeast strains are unable to assimilate cellobiose, several efforts have been made to express cellulose hydrolytic enzymes in these yeasts to produce ethanol from lignocellulose. The C. flavigenabglA gene encoding beta-glucosidase catalytic subunit was optimized for preferential codon usage in S. cerevisiae. The optimized gene, cloned into the episomal vector pRGP-1, was expressed, which led to the secretion of an active beta-glucosidase in transformants of the S. cerevisiae diploid strain 2-24D. The volumetric and specific extracellular enzymatic activities using pNPG as substrate were 155 IU L-1 and 222 IU g(-1), respectively, as detected in the supernatant of the cultures of the S. cerevisiae RP2-BGL transformant strain growing in cellobiose (20 g L-1) as the sole carbon source for 48 h. Ethanol production was 5 g L-1 after 96 h of culture, which represented a yield of 0.41 g g(-1) of substrate consumed (12 g L-1), equivalent to 76% of the theoretical yield. The S. cerevisiae RP2-BGL strain expressed the beta-glucosidase extracellularly and produced ethanol from cellobiose, which makes this microorganism suitable for application in ethanol production processes with saccharified lignocellulose.