Abstract

Iron regulatory protein 1 (IRP1) is regulated through the assembly/disassembly of a [4Fe–4S] cluster, which interconverts IRP1 with cytosolic aconitase. A genetic screen to isolate Saccharomyces cerevisiae strains bearing mutations in genes required for the conversion of IRP1 to c-aconitase led to the identification of a previously uncharacterized, essential gene, which we call CFD1 (cytosolic Fe–S cluster deficient). CFD1 encodes a highly conserved, putative P-loop ATPase. A non-lethal mutation of CFD1 (cfd1-1) reduced c-aconitase specific activity in IRP1-transformed yeast by >90%, although IRP1 in these cells could be readily converted to c-aconitase in vitro upon incubation with iron alone. IRP1-transformed cfd1-1 yeast lacked EPR-detectable Fe–S clusters in c-aconitase, pointing to a defect in Fe–S cluster assembly. The specific activity of another cytosolic Fe–S protein, Leu1p, was also inhibited by >90% in cfd1-1 yeast, whereas activity of mitochondrial Fe–S proteins was not inhibited. Consistent with a cytosolic site of activity, Cfd1p was localized in the cytoplasm. To our knowledge, Cfd1p is the first cytoplasmic Fe–S cluster assembly factor described in eukaryotes.