In Saccharomyces cerevisiae, loss of cytosolic superoxide dismutase (Sod1) results in several air-dependent mutant phenotypes, including methionine auxotrophy and oxygen sensitivity. Here we report that these two sod1⌬ phenotypes were specifically suppressed by elevated expression of the TKL1 gene, encoding transketolase of the pentose phosphate pathway. The apparent connection between Sod1 and the pentose phosphate pathway prompted an investigation of mutants defective in glucose-6-phosphate dehydrogenase (Zwf1), which catalyzes the rate-limiting NADPH-producing step of this pathway. We confirmed that zwf1⌬ mutants are methionine auxotrophs and report that they also are oxygen-sensitive. We determined that a functional ZWF1 gene product was required for TKL1 to suppress sod1⌬, leading us to propose that increased flux through the oxidative reactions of the pentose phosphate pathway can rescue sod1 methionine auxotrophy. To better understand this methionine growth requirement, we examined the sulfur compound requirements of sod1⌬ and zwf1⌬ mutants, and noted that these mutants exhibit the same apparent defect in sulfur assimilation. Our studies suggest that this defect results from the impaired redox status of aerobically grown sod1 and zwf1 mutants, implicating Sod1 and the pentose phosphate pathway as being critical for maintenance of the cellular redox state.Aerobic organisms are constantly exposed to potentially harmful reactive oxygen species that are generated as byproducts of cellular metabolism. These oxygen free radicals have the potential to damage critical cellular components such as DNA, proteins, and lipids (see Refs. 1 and 2 for review). However, aerobic organisms have evolved with both enzymatic and non-enzymatic oxidant defense mechanisms that are designed for protection against such assaults. Non-enzymatic defenses include the tripeptide glutathione (GSH) and the small peptide thioredoxin (Trx), while enzymatic defenses include catalases, peroxidases, and superoxide dismutases (Ref.

The SIN3 global regulatory factor affects expression of many yeast genes, including the phospholipid biosynthetic gene, INO1. Mutations in the SIN3 gene result in elevated levels of INO1 expression under conditions that normally confer full repression of INO1 transcription, indicating that SIN3 is a negative regulator of INO1. In this study, the INO1 promoter was analyzed for sequences that play a role in responding to SIN3-mediated repression. Two distinct promoter elements, the upstream repression sequence (URS1) and the INO1 upstream activation sequence (UASINO) both were found to be involved in enabling SIN3 to repress INO1 expression.

The copper toxicity of yeast lacking the CUP1 metallothionein is suppressed by overexpression of the CRS4 gene. We now demonstrate that CRS4 is equivalent to SOD1, encoding copper/zinc superoxide dismutase (SOD). While overexpression of SOD1 enhanced copper resistance, a deletion of SOD1, but not SOD2 (encoding manganese SOD), conferred an increased sensitivity toward copper. This role of SOD1 in copper buffering appears unrelated to its superoxide scavenging activity, since the enzyme protected against copper toxicity in anaerobic as well as aerobic conditions. The distinct roles of SOD1 in copper and oxygen radical homeostasis could also be separated genetically: the pmr1, bsd2, and ATX1 genes that suppress oxygen toxicity in sod1 mutants failed to suppress the copper sensitivity of these cells. The Saccharomyces cerevisiae SOD1 gene is transcriptionally induced by copper and the ACE1 transactivator, and we demonstrate here that this induction of SOD1 promotes protection against copper toxicity but is not needed for the SOD1-protection against oxygen free radicals. Collectively, these findings indicate that copper/zinc SOD functions in the homeostasis of copper via mechanisms distinct from superoxide scavenging.