Assembly of chloroplast cytochromes b and c

The synthesis of holocytochromes in plastids is a catalyzed process. Severa l proteins, including plastid CcsA, Ccs1, possibly CcdA and a thioredoxin, plus at least two additional Ccs factors, are required in sub-stoichiometri c amounts for the conversion of apocytochromes f and c(6) to their respecti ve holoforms. CcsA, proposed to be a heme delivery factor, and Ccs1, an apo protein chaperone, are speculated to interact physically in vivo. The forma tion of holocytochrome b(6) is a multi-step pathway in which at least four, as yet unidentified, Ccb factors are required for association of the b(H) heme. The specific requirement of reduced heme for in vitro synthesis of a cytochrome b(559)-derived holo-beta(2), suggests that cytochrome b synthesi s in PSII might also be catalyzed in vivo. (C) 2000 Societe francaise de bi ochimie et biologie moleculaire / Editions scientifiques et medicales Elsev ier SAS.