New perspectives on assembling c-type cytochromes, particularly from sulphate reducing bacteria and mitochondria

► The Ccm system (System I) is a multicomponent system for attaching heme to the thiol groups of two cysteines on apoproteins. ► A novel cross linked by heme product between a component of the Ccm system (E) and an apocytochrome is discussed. ► Progress in understanding how System III recognises its target apocytochrome is summarised. ► A highly conserved phenylalanine is crucial for interaction between System III and its target. ► Sulphate reducing bacteria contain a variant of System I that poses many issues.