Expression of Phanerochaete chrysosporium β-glucosidase in industrial Saccharomyces cerevisiae yeast for bioethanol production from lignocellulosic biomass

Introduction. Ethanol is currently being produced mostly by fermentation of sugars obtained from commodities that can also be used as food and feed. This competition makes first generation bioethanol unsustainable. Lignocellulose is one of the most promising alternative raw materials, due to its low cost and wide availability as a waste product. So far, little has been done in order to select robust strains that can both tolerate stressful industrial applications and efficiently ferment sugars. Further, to reduce production costs, a fermenting yeast, capable of producing one or more of the enzymes required for cellulose hydrolysis, is needed (van Rooyen et al. 2005). β-glucosidases, splitting cellobiose into glucose, play a major role in the enzymatic hydrolysis of cellulose by eliminating the inhibitory activity of cellobiose on other enzymes involved in the saccharification of cellulose (van Rooyen et al. 2005). Materials and methods. The fungal β-glucosidase BGL3 from Phanerochaete chrysosporium expresses high hydrolytic activities on cellobiose, when secreted by a Saccharomyces cerevisiae laboratory strain (Njokweni et al. 2012). BGL3 was then δ-integrated into the chromosome of two industrial S. cerevisiae strains, namely M2n and MEL2 (Viktor et al. 2013; Favaro et al. 2015), selected for their robustness and high ethanol performances. Results. Several mitotically stable recombinants were able to grow using cellobiose as the sole carbon source and their enzymatic activity was evaluated in vitro on p-nitrophenyl-β-D-glucopyranoside. The highest extracellular β-glucosidase activity was about 20 nkat per ml and cell-bound activity was also detected at high levels. Discussion. This study reports the successful construction of recombinant industrial S. cerevisiae strains capable of growing on cellobiose as sole carbon source, by expressing the fungal β-glucosidase BGL3 from P. crysosporium. Their fermenting abilities will be on cellobiose and native cellulosic substrates, with the addition of customized cellulases cocktails required to complete the hydrolysis of cellulose.