O-2-DEPENDENT METHIONINE AUXOTROPHY IN CU,ZN SUPEROXIDE DISMUTASE-DEFICIENT MUTANTS OF SACCHAROMYCES-CEREVISIAE

Mutant strains of the yeast Saccharomyces cerevisiae which lack functional Cu,Zn superoxide dismutase (SOD-1) do not grow aerobically unless supplemented with methionine. The molecular basis of this O2-dependent auxotrophy in one of the mutants, Dscd1-1C, has been investigated. Sulfate supported anaerobic but not aerobic mutant growth. On the other hand, cysteine and homocysteine supported aerobic growth while serine, O-acetylserine, and homoserine did not, indicating that the interconversion of cysteine and methionine (and homocysteine) was not impaired. Thiosulfate (S2O32-) and sulfide (S2-) also supported aerobic growth; the activities of thiosulfate reductase and sulfhydrylase in the aerobic mutant strain were at wild-type levels. Although the levels of SO42- and adenosine-5' (the first intermediate in the SO42- assimilation pathway) were elevated in the aerobically incubated mutant strain, this condition could be attributed to a decrease in protein synthesis caused by the defacto sulfur starvation and not to a block in the pathway. Therefore, the activation of SO42- (to form 3'-phosphoadenosine-5'-phosphosulfate) appeared to be O2 tolerant. Sulfite reductase activity and substrate concentrations ([NADPH] and [SO32-]) were not significantly different in aerobically grown mutant cultures and anaerobic cultures, indicating that SOD-1- mutant strains could reductively assimilate sulfur oxides. However, the mutant strain exhibited an O2-dependent sensitivity to SO32- concentrations of <50 μM not exhibited by any SOD-1+ strain or by SOD-1- strains supplemented with a cytosolic O2--scavenging activity. This result suggests that the aerobic reductive assimilation of SO42- at the level of SO42- may generate a cytotoxic compound(s) which persists in SOD-1- yeast strains.