Required during biogenesis of c-type cytochromes (cytochrome c6 and cytochrome f) at the step of heme attachment. Probably not involved in heme transport or reduction as mutants deleted for the first 24 residues cannot be rescued by growth in the presence of heme or reductant. May interact with CcsA. Bacteria lacking the N-terminus of CcsB are able to grow anaerobically but not aerobically, suggesting its function is essential for aerobic growth. In these bacteria reduced amounts of some holo-c-type cytochromes are observed (cytochrome f and cytochrome c550); unprocessed precursor apocytochrome f accumulates but is not thought to be integrated into the cytochrome b6-f complex. Holo-cytochrome c553 (also known as cytochrome c6) does not accumulate however. Belongs to the Ccs1/CcsB family.