Xylose uptake by yeasts.

摘要

Toward the goal of introducing a gene for a xylose transporter into the yeast Saccharomyces cerevisiae for the development of a strain appropriate for the production of fuel ethanol from hemicellulose, a xylose transporter in the yeast Pichia heedii has been characterized. At least two transport systems for the uptake of xylose, varying in their affinity for xylose, were demonstrated. The Km of high-affinity xylose uptake was similar to that of the efficient high-affinity glucose uptake system of Saccharomyces cerevisiae. At least two transport systems for glucose were also demonstrated. When P. heedii was grown on 2% (vs. 0.05%) xylose or glucose, low-affinity xylose uptake was enhanced and high-affinity uptake reduced. Elevated external glucose concentration (2% vs 0.05%) enhanced glucose transport. Both glucose and xylose transport by P. heedii were affected by the uncoupler sodium azide. Glucose uptake by P. heedii was not inhibited by a 100-fold molar excess of xylose and a mutant P. heedii strain defective in high affinity xylose but not in glucose transport has been isolated. This suggests that xylose uptake by P. heedii may be by carrier system(s) distinct from those responsible for glucose uptake. Overlaps between the functional or regulatory mechanism for glucose and xylose uptake may exist since some mutant strains found to be defective in xylose uptake were also defective in glucose transport. P. heedii was found to possess NADPH-dependent xylose reductase and NAD-dependent xylitol dehydrogenase activities at levels comparable to those of other xylose metabolizing species. Defects in these enzymes might influence transport. No mutants were identified with defects in either of these enzymes. The HXT2 gene (which encodes a sugar transporter from S. cerevisiae) hybridized to regions of the genome of P. heedii on Southern blot analysis. Thus, cloning sugar transporters from P. heedii by virtue of their homology to members of the multigene sugar transporter family of S. cerevisiae is feasible. Since the only carbohydrates P. heedii is reported to metabolize are glucose and xylose the identification of a gene for high affinity xylose transport should be facilitated.