Cytochromes c covalently bind their heme prosthetic groups through thioether bonds between the vinyl groups of the heme and the thiols of a CXXCH motif within the protein.In Gram-negative bacteria,this process is catalyzed by the Ccm (cytochrome c maturation) proteins,also called System I.The Ccm proteins are found in the bacterial inner membrane,but some (CcmE,CcmG,CcmH,and Ccml) also have soluble functional domains on the periplasmic face of the membrane.Elucidation of the mechanisms involved in the transport and relay of heme and the apocytochrome from the bacterial cytosol into the periplasm,and their subsequent reaction,has proved challenging due to the fact that most of the proteins involved are membrane-associated,but recent progress in understanding some key components has thrown up some surprises.In this Review,we discuss advances in our understanding of this process arising from a substrate’s point of view and from recent structural information about individual components.