Stationary-phase regulation of the Saccharomyces cerevisiae SOD2 gene is dependent on additive effects of HAP2/3/4/5- and STRE-binding elements

SOD2, encoding manganese superoxide dismutase (MnSOD), is essential for stationary-phase survival of yeast cells. In addition, stationary-phase cells are more resistant to oxidative stress than exponential-phase cells. The use of a SOD2::lacZfusion construct in this study shows that transcription of SOD2 increases 6.5-fold as cells enter stationary phase in rich, glucose medium. The increase in SOD2 expression appears to be due to two phenomena - the switch to a non-fermentable carbon source and nutrient limitation. Analysis of SOD2 transcription in mutant Saccharomyces cerevisiae strains showed that the gene was negatively regulated by intracellular cAMP levels which decrease as cells enter stationary phase. Mutation of 'stress-responsive' (STRE) elements in the SOD2 promoter which respond to cAMP levels resulted in the loss of cAMP-dependent expression but only partially reduced the increase in expression as cells entered stationary phase. A putative Yap1p-binding site was found to be inactive and mutation of YAP1 had no effect on the STRE-mediated expression. To fully eliminate the stationary-phase response, it was necessary to mutate a HAP2/3/4/5 complex binding site in addition to the STRE elements. It is postulated that the effects of the STRE sites and the HAP2/3/4/5 complex binding site are additive.