Mutations in Saccharomyces cerevisiae iron-sulfur cluster assembly genes and oxidative stress relevant to Cu,Zn superoxide dismutase

Saccharomyces cerevisiae lacking Cu,Zn superoxide dismutase (SOD1) show several metabolic defects including aerobic blockages in methionine and lysine biosynthesis. We have previously shown that mutations in genes implicated in the formation of iron-sulfur clusters, designated seo (suppressors of endogenous oxidation), reverse the oxygen-dependent methionine and lysine auxotrophies of a sod1Δ strain. We now report the surprising finding that seo mutants do not reduce oxidative damage as shown by the lack of reduction of EPR-detectable "free" iron, which is characteristic of sod1Δ mutants. In fact, they exhibit increased oxidative damage as evidenced by increased accumulation of protein carbonyls. The seo class of mutants overaccumulates mitochondrial iron, and this iron accumulation is critical for suppression of the sod1Δ biosynthetic defects. Blocking overaccumulation of mitochondrial iron abolished the ability of the seo mutants to suppress the sod1A auxotrophies. By contrast, increasing the mitochondrial iron content of sod1Δ yeast using high copy MMT1, which encodes a mitochondrial iron transporter, was sufficient to mimic the seo mutants. Our studies indicated that suppression of the sod1Δ methionine auxotrophy was dependent on the pentose phosphate pathway, which is a major source of NADPH production. By comparison, the sod1Δ lysine auxotrophy appears to be reversed in the seo mutants by increased expression of genes in the lysine biosynthetic pathway, perhaps through sensing of mitochondrial damage by the retrograde response.