Comparing laboratory and industrial yeast platforms for the direct conversion of cellobiose into ethanol under simulated industrial conditions.

An engineered yeast producing all the cellulases needed for cellulose saccharification could produce ethanol from lignocellulose at a lower cost. This study aimed to express fungal ß-glucosidases in Saccharomyces cerevisiae to convert cellobiose into ethanol. Furthermore, two engineering platforms (laboratory vs industrial strain) have been considered towards the successful deployment of the engineered yeast under simulated industrial conditions. The industrial S. cerevisiae M2n strain was engineered through the δ-integration of the ß-glucosidase Pccbgl1 of Phanerochaete chrysosporium. The most efficient recombinant, M2n[pBKD2-Pccbgl1]-C1, was compared to the laboratory S. cerevisiae Y294[Pccbgl1] strain, expressing Pccbgl1 from episomal plasmids, in terms of cellobiose fermentation in a steam exploded sugarcane bagasse pre-hydrolysate. Saccharomyces cerevisiae Y294[Pccbgl1] was severely hampered by the pre-hydrolysate. The industrial M2n[pBKD2-Pccbgl1]-C1 could tolerate high inhibitors-loading in pre-hydrolysate under aerobic conditions. However, in oxygen limited environment, the engineered industrial strain displayed ethanol yield higher than the laboratory Y294[Pccbgl1] only when supplemented with supernatant containing further recombinant ß-glucosidase. This study showed that the choice of the host strain is crucial to ensure bioethanol production from lignocellulose. A novel cellobiose-to-ethanol route has been developed and the recombinant industrial yeast could be a promising platform towards the future consolidated bioprocessing of lignocellulose into ethanol.

Engineering of Saccharomyces cerevisiae to utilize xylan as a sole carbohydrate source by co-expression of an endoxylanase, xylosidase and a bacterial xylose isomerase.

Xylan represents a major component of lignocellulosic biomass, and its utilization by Saccharomyces cerevisiae is crucial for the cost effective production of ethanol from plant biomass. A recombinant xylan-degrading and xylose-assimilating Saccharomyces cerevisiae strain was engineered by co-expression of the xylanase (xyn2) of Trichoderma reesei, the xylosidase (xlnD) of Aspergillus niger, the Scheffersomyces stipitis xylulose kinase (xyl3) together with the codon-optimized xylose isomerase (xylA) from Bacteroides thetaiotaomicron. Under aerobic conditions, the recombinant strain displayed a complete respiratory mode, resulting in higher yeast biomass production and consequently higher enzyme production during growth on xylose as carbohydrate source. Under oxygen limitation, the strain produced ethanol from xylose at a maximum theoretical yield of ~90 %. This study is one of only a few that demonstrates the construction of a S. cerevisiae strain capable of growth on xylan as sole carbohydrate source by means of recombinant enzymes.