Required during biogenesis of c-type cytochromes (cytochrome c6 and cytochrome f) at the step of heme attachment. May interact with ccs1. By light. Cannot be insertionally inactivated suggesting it has an essential function (PubMed:9187367, PubMed:10542282). Belongs to the CcmF/CycK/Ccl1/NrfE/CcsA family.