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The YFH1 gene is the yeast homologue of the human FRDA gene, which encodes the frataxin protein. Saccharomyces cerevisiae cells lacking the YFH1 gene showed very low cytochrome content. In Deltayfh1 strains, the level of ferrochelatase (Hem15p) was very low, as a result of transcriptional repression of HEM15. However, the low amount of Hem15p was not the cause of haeme deficiency in Deltayfh1 cells. Ferrochelatase, a mitochondrial protein, able to mediate insertion of iron or zinc into the porphyrin precursor, made primarily the zinc protoporphyrin product. Zinc protoporphyrin instead of haeme accumulated during growth of Deltayfh1 mutant cells and, furthermore, preferential formation of zinc protoporphyrin was observed in real time. The method for these studies involved direct presentation of porphyrin to mitochondria and to ferrochelatase of permeabilized cells with intact architecture, thereby specifically testing the iron delivery portion of the haeme biosynthetic pathway. The studies showed that Deltayfh1 mutant cells are defective in iron use by ferrochelatase. Mössbauer spectroscopic analysis showed that iron was present as amorphous nano-particles of ferric phosphate in Deltayfh1 mitochondria, which could explain the unavailability of iron for haeme synthesis. A high frequency of suppressor mutations was observed, and the phenotype of such mutants was characterized by restoration of haeme synthesis in the absence of Yfh1p. Suppressor strains showed a normal cytochrome content, normal respiration, but remained defective in Fe-S proteins and still accumulated iron into mitochondria although to a lesser extent. Yfh1p and Hem15p were shown to interact in vitro by Biacore studies. Our results suggest that Yfh1 mediates iron use by ferrochelatase.

SummaryMagnetotactic bacteria form chains of intracellular membrane-enclosed, nanometre-sized magnetite crystals for navigation along the earth's magnetic field. The assembly of these prokaryotic organelles requires several specific polypeptides. Among the most abundant proteins associated with the magnetosome membrane of Magnetospirillum gryphiswaldense are MamB and MamM, which were implicated in magnetosomal iron transport because of their similarity to the cation diffusion facilitator family.

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