In bacteria and energy generating organelles， -type cytochromes are a class of universal electron carriers with a heme cofactor covalently linked via one or two thioether bonds to a heme binding site. The covalent attachment of heme to apocytochromes is a catalyzed process， taking place via three evolutionarily distinct assembly pathways (Systems I， II， III). System II was discovered in the green alga through the genetic analysis of the mutants (ytochrome ynthesis)， which display a block in the apo- to holo- form conversion of cytochrome and ， the thylakoid lumen resident -type cytochromes functioning in photosynthesis. Here we show that the gene corresponding to the locus encodes a 1，719 amino acid polypeptide and identify the molecular lesions in the to alleles. The CCS2 protein displays seven degenerate amino acid repeats， which are variations of the ctatricoeptide-epeat motif (OPR) recently recognized in several nuclear-encoded proteins controlling the maturation， stability， or translation of chloroplast transcripts. A plastid site of action for CCS2 is inferred from the finding that GFP fused to the first 100 amino acids of the algal protein localizes to chloroplasts in . We discuss the possible functions of CCS2 in the heme attachment reaction.