Isolation and characterization of xyl mutants in a xylose-utilizing yeast,Pichia stipitis

Summary Mutants of the xylose-utilizing yeast, Pichia stipitis, unable to grow on xylose as the sole carbon source were isolated and characterized. The mutants were deficient in either xylose reductase or xylitol dehydrogenase. By immunological means and upon analysis of revertants, both mutant types could be attributed to the structural genes XYL1 and XYL2, which code for xylose reductase and xylitol dehydrogenase, respectively. These data support previous assumptions that both NADH- and NADPH-dependent xylose reductase activity are due to one protein or gene, respectively. Revertant analysis of xyl1 mutants has revealed the existence of a second xylose reductase gene in P. stipitis. This gene is very likely cryptic. Its corresponding xylose reductase activity is NADPH-dependent.