Lactic fermentation of cellobiose by a yeast strain displaying β-glucosidase on the cell surface

Abstract

The Aspergillus aculeatus β-glucosidase 1 (bgl1) gene was expressed in a lactic-acid-producing Saccharomyces cerevisiae strain to enable lactic fermentation with cellobiose. The recombinant β-glucosidase enzyme was expressed on the yeast cell surface by fusing the mature protein to the C-terminal half region of the α-agglutinin. The β-glucosidase expression plasmids were integrated into the genome. Three strong promoters of S. cerevisiae, the TDH3, PGK1, and PDC1 promoters, were used for β-glucosidase expression. The specific β-glucosidase activity varied with the promoter used and the copy number of the bgl1 gene. The highest activity was obtained with strain PB2 that possessed two copies of the bgl1 gene driven by the PDC1 promoter. PB2 could grow on cellobiose and glucose minimal medium at the same rate. Fermentation experiments were conducted in non-selective-rich media containing 95 g l⁻¹ cellobiose or 100 g l⁻¹ glucose as a carbon source under microaerobic conditions. The maximum rate of l-lactate production by PB2 on cellobiose (2.8 g l⁻¹ h⁻¹) was similar to that on glucose (3.0 g l⁻¹ h⁻¹). This indicates that efficient fermentation of cellobiose to l-lactate can be accomplished using a yeast strain expressing β-glucosidase from a mitotically stable genomic integration plasmid.