Required during biogenesis of c-type cytochromes (cytochrome c6 and cytochrome f) at the step of heme attachment. May interact with ccsA. Algae lacking CCS1 are unable to grow phototrophically; they specifically accumulate apocytochrome c6 and apocytochrome f. A mutant with a small stroma insert (ccs1-ac206) does not produce holocytochromes. Belongs to the Ccs1/CcsB family.