Control of the Saccharomyces cerevisiae regulatory gene PET494: transcriptional repression by glucose and translational induction by oxygen.

The product of the Saccharomyces cerevisiae nuclear gene PET494 is required to promote the translation of the mitochondrial mRNA encoding cytochrome c oxidase subunit III (coxIII). The level of cytochrome c oxidase activity is affected by several different environmental conditions, which also influence coxIII expression. We have studied the regulation of PET494 to test whether the level of its expression might modulate coxIII translation in response to these conditions. A pet494::lacZ fusion was constructed and used to monitor PET494 expression. PET494 was regulated by oxygen availability: expression in a respiration-competent diploid strain grown anaerobically was one-fifth the level of expression in aerobically grown cells. However, since PET494 mRNA levels did not vary in response to oxygen deprivation, regulation by oxygen appears to occur at the translational level. This oxygen regulation was not mediated by heme, and PET494 expression was independent of the heme activator protein HAP2. The regulation of PET494 expression by carbon source was also examined. In cells grown on glucose-containing medium, PET494 was expressed at levels four- to sixfold lower than in cells grown on ethanol and glycerol. However, addition of ethanol to glucose-containing medium induced PET494 expression approximately twofold. PET494 transcript levels varied over a fourfold range in response to different carbon sources. The effects on PET494 expression of mutations in the SNF1, SNF2, SSN6, and HXK2 genes were also determined and found to be minimal.