Engineering yeast for efficient cellulose degradation
Saccharomyces cerevisiae produces several β-1,3-glucanases, but lacks the multicomponent cellulase complexes that hydrolyse the β-1,4-linked glucose polymers present in cellulose-rich biomass as well as in haze-forming glucans in certain wines and beers. We have introduced into S. cerevisiae a functional cellulase complex for efficient cellulose degradation by cloning the Endomyces fibuliger cellobiase (BGL1) gene and co-expressing it with the Butyrivibrio fibrisolvens endo-β-1,4-glucanase (END1), the Phanerochaete chrysosporium cellobiohydrolase (CBH1) and the Ruminococcus flavefaciens cellodextrinase (CEL1) gene constructs in this yeast. The END1, CBH1 and CEL1 genes were inserted into yeast expression/secretion cassettes. Expression of END1, CBH1 and CEL1 was directed by the promoter sequences derived from the alcohol dehydrogenase II (ADH2), the phosphoglycerate kinase I(PKG1) and the alcohol dehydrogenase I (ADH1) genes, respectively. In contrast, BGL1 was expressed under the control of its native promoter. Secretion of End1p and Cel1p was directed by the signal sequence of the yeast mating pheromone α-factor (MFα1), whereas Cbh1p and Bgl1p were secreted using their authentic leader peptides. The construction of a fur1 ura3 S. cerevisiae strain allowed for the autoselection of this multicopy URA3-based plasmid in rich medium. S. cerevisiae transformants secreting biologically active endo-β-1,4-glucanase, cellobiohydrolase, cellodextrinase and cellobiase were able to degrade various substrates including carboxymethylcellulose, hydroxyethylcellulose, laminarin, barley glucan, cellobiose, polypectate, birchwood xylan and methyl-β-D-glucopyranoside. This study could lead to the development of industrial strains of S. cerevisiae capable of converting cellulose in a one-step process into commercially important commodities.