CCS2, an Octatricopeptide-Repeat Protein, Is Required for Plastid Cytochrome c Assembly in the Green Alga Chlamydomonas reinhardtii

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 ctatrico eptide- 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.