Transcriptional control of ADH genes in the xylose-fermenting yeast Pichiastipitis

We studied the expression of the genes encoding group I alcohol dehydrogena ses (PsADH1 and PsADH2) in the xylose-fermenting yeast Pichia stipitis CBS 6054. The cells expressed PsADH1 approximately 10 times higher under oxygen -limited conditions than under fully aerobic conditions when cultivated on xylose. Transcripts of PsADH2 were not detectable under either aeration con dition. We used a PsADH1:lacZ fusion to monitor PsADH1 expression and found that expression increased as oxygen decreased. The level of PsADH1 transcr ipt was repressed about 10-fold in cells grown in the presence of heme unde r oxygen-limited conditions. Concomitantly with the induction of PsADH1, Ps CYC1 expression was repressed. These results indicate that oxygen availabil ity regulates PsADH1 expression and that regulation may be mediated by heme . The regulation of PsADH2 expression was also examined in other genetic ba ckgrounds. Disruption of PsADH1 dramatically increased PsADH2 expression on nonfermentable carbon sources under fully aerobic conditions, indicating t hat the expression of PsADH2 is subject to feedback regulation under these conditions.