Saccharomyces cerevisiae mutant strain DTY26, lacking ACE1, the protein mediator for the induction or metallothionein gene expression, is unable to increase Cu,Zn superoxide dismutase mRNA in response to copper. In the wild‐type strain DTY22 transcription or both Cu,Zn superoxide dismutase and metallothionein genes is induced by copper and silver, as expected on the basis of previous results indicating that ACE1 binds only Ag(I) besides Cu(I). We conclude that at the transcriptional level Cu,ZnSOD is co‐regulation with metallothionein. Furthermore, structural similarities between the two promoters were found, which could explain the co‐regulation effect and the quantitative differences in the response of the two genes to copper.

The regulation of Cu,Zn-and Mn-superoxide dismutases (SOD) was investigated by Northern blotting and gene fusions of SODI and SOD2 promoters with then-galactosida~ reporter gene. QZnSOD expression was increased 3-fold under glucose derepressing conditions, and decreased 4-to 6.fold by oxygen or heme deficiency. MnSOD expression was increased 5-fold by glucose derepression, and decreased 8-to lo-fold by anaerobiosis and 4-to S-fold by heme deficiency. Induction by paraquat was modest, about 50% for SOD1 and 100% for SODZ; it was apparently independent of the respiratory chain function.

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The Cu/Zn superoxide dismutase activity of Saccharomyces cerevisiae was found to be strictly related to the extent of oxygen metabolism, since cells grown under anaerobic or repressed conditions were found to contain 10% and 40% the activity of derepressed cells, respectively. The dependence of Cu/Zn superoxide dismutase on oxygen was found to be related to the availability of copper to the cells since the enzyme activity and immunoreactive protein measured under the various conditions was roughly proportional to the copper content of cells and in anaerobic cells a large fraction of the enzyme was found to be in the form of an inactive proenzyme which was activated by the addition of copper to cell extracts. The Cu/Zn superoxide dismutase mRNA did not parallel the dependence of the enzyme concentration on oxygen metabolism, suggesting that the gene expression was affected by copper also at the post-transcriptional level. However, under conditions of copper overloading, a more direct effect on transcription was observed and the presence of the inactive proenzyme in anaerobic cultures was associated with the over-expression of metallothionein.

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