title: O2-dependent methionine auxotrophy in Cu,Zn superoxide dismutase-deficient mutants of Saccharomyces cerevisiae.
abstract: 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 (S2O3(2-] 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 SO4(2-) and adenosine-5'-sulfate (the first intermediate in the SO4(2-) assimilation pathway) were elevated in the aerobically incubated mutant strain, this condition could be attributed to a decrease in protein synthesis caused by the de facto sulfur starvation and not to a block in the pathway. Therefore, the activation of SO4(2-) (to form 3'-phosphoadenosine-5'-phosphosulfate) appeared to be O2 tolerant. Sulfite reductase activity and substrate concentrations [( NADPH] and [SO3(2-)]) 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 SO3(2-) concentrations of less than 50 microM 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 SO4(2-) at the level of SO3(2-) may generate a cytotoxic compound(s) which persists in SOD-(1-) yeast strains.
authors.affiliation: Department of Biochemistry, School of Medicine, State University of New York, Buffalo 14214.
authors.forename: E C
authors.initials: EC
authors.lastname: Chang
authors.forename: D J
authors.initials: DJ
authors.lastname: Kosman
mesh_terms: D000332:Aerobiosis; D000693:Anaerobiosis; D003600:Cytosol / Q000378:metabolism; D008715:Methionine / Q000378:metabolism*; D008954:Models, Biological; D009154:Mutation; D050862:Oxidoreductases Acting on Sulfur Group Donors / Q000302:isolation & purification / Q000378:metabolism; D010101:Oxygen Consumption; D012441:Saccharomyces cerevisiae / Q000201:enzymology / Q000235:genetics* / Q000378:metabolism; D013431:Sulfates / Q000378:metabolism*; D013447:Sulfites / Q000494:pharmacology; D013461:Sulfur Oxides / Q000378:metabolism; D013482:Superoxide Dismutase / Q000235:genetics*
chemical_list: D013431:Sulfates; D013447:Sulfites; D013461:Sulfur Oxides; D008715:Methionine; D013482:Superoxide Dismutase; D050862:Oxidoreductases Acting on Sulfur Group Donors