ABSTRACTSaccharomyces cerevisiaecannot utilize cellobiose, but this yeast can be engineered to ferment cellobiose by introducing both cellodextrin transporter (cdt-1) and intracellular β-glucosidase (gh1-1) genes fromNeurospora crassa. Here, we report that an engineeredS. cerevisiaestrain expressing the putative hexose transporter geneHXT2.4fromScheffersomyces stipitisandgh1-1can also ferment cellobiose. This result suggests that HXT2.4p may function as a cellobiose transporter whenHXT2.4is overexpressed inS. cerevisiae. However, cellobiose fermentation by the engineered strain expressingHXT2.4andgh1-1was much slower and less efficient than that by an engineered strain that initially expressedcdt-1andgh1-1. The rate of cellobiose fermentation by theHXT2.4-expressing strain increased drastically after serial subcultures on cellobiose. Sequencing and retransformation of the isolated plasmids from a single colony of the fast cellobiose-fermenting culture led to the identification of a mutation (A291D) in HXT2.4 that is responsible for improved cellobiose fermentation by the evolvedS. cerevisiaestrain. Substitutions for alanine (A291) of negatively charged amino acids (A291E and A291D) or positively charged amino acids (A291K and A291R) significantly improved cellobiose fermentation. The mutant HXT2.4(A291D) exhibited 1.5-fold higherKmand 4-fold higherVmaxvalues than those from wild-type HXT2.4, whereas the expression levels were the same. These results suggest that the kinetic properties of wild-type HXT2.4 expressed inS. cerevisiaeare suboptimal, and mutations of A291 into bulky charged amino acids might transform HXT2.4p into an efficient transporter, enabling rapid cellobiose fermentation by engineeredS. cerevisiaestrains.
Identification and Characterization of a Cellodextrin Transporter in Aspergillus niger
