Identification and characterization of an efficient D-xylose transporter in Saccharomyces cerevisiae.

D-xylose is the most abundant hemicellulosic monomer on earth, but wild-type Saccharomyces cerevisiae has very limited D-xylose uptake capacity. We conducted bioprospecting for new sugar transporters from the D-xylose-consuming filamentous fungus Trichoderma reesei and identified three candidates belonging to the major facilitator superfamily. When they were expressed in yeast and assayed for D-xylose uptake, one of them, Xltr1p, had D-xylose transport activity that was more efficient than that of Gal2p, an endogenous yeast transporter. Site-directed mutagenesis was used to examine the functional contributions of 13 amino acid residues for the uptake of D-xylose, and these experiments identified particular amino acids that function distinctly in D-xylose vs. glucose transport (e.g., F300). Excitingly, the yeast strain expressing the N326FXltr1p variant was able to carry a 'high efficiency' transporter for D-xylose but was completely unable to utilize glucose; in contrast, the strain with the F300AXltr1p variant grew on glucose but lost D-xylose transport activity.