Escherichia coli employs several c -type cytochromes, which are found in the periplasm or on the periplasmic side of the cytoplasmic membrane; they are used for respiration under different growth conditions. All E. coli c -type cytochromes are multiheme cytochromes; E. coli does not have a monoheme cytochrome c of the kind found in mitochondria. The attachment of heme to cytochromes c occurs in the periplasm, and so the apoprotein must be transported across the cytoplasmic membrane; this step is mediated by the Sec system, which transports unfolded proteins across the membrane. The protein CcmE has been found to bind heme covalently via a single bond and then transfer the heme to apocytochromes. It should be mentioned that far less complex systems for cytochrome c biogenesis exist in other organisms and that enterobacteria do not function as a representative model system for the process in general, although plant mitochondria use the Ccm system found in E. coli . The variety and distribution of cytochromes and their biogenesis systems reflect their significance and centrality in cellular bioenergetics, though the necessity for and origin of the diverse biogenesis systems are enigmatic.