Apocytochrome was detected in the periplasm of a new mutant of , HN48, that is pleiotropically lacking type cytochromes, produces reduced levels of siderophores and carries a Tn insertion in the gene for which sequence data, along with that for the contiguous , are reported. A counterpart to the gene was found in an archaebacterium but could not be located in the yeast genome, whereas mitochondrial haem lyases in the latter were not present in an archaeobacterial or in eubacterial genomes. A topological analysis for CcmF is presented which indicates at least eleven transmembrane helices, suggesting a role as a transporter; evidence against the substrate being haem is presented but sequence similarity with γ-aminobutyric acid transporter was identified. Analysis by pulse-chase methodology has shown that, in this and another cytochrome--deficient mutant, the apo form of P. denitrificans cytochrome is much less stable than the holo form, directly demonstrating the presence of a periplasmic degradation system in that removes non-functional proteins. A variety of phenotypes are observed for mutated in different genes, thus indicating that the stability of the gene products does not require assembly of a complex of all the Ccm proteins.


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