The synthesis of holocytochromes in plastids is a catalyzed process. Several proteins, including plastid CcsA, Ccs1, possibly CcdA and a thioredoxin, plus at least two additional Ccs factors, are required in sub-stoichiometric amounts for the conversion of apocytochromes f and c 6 to their respective holoforms. CcsA, proposed to be a heme delivery factor, and Ccs1, an apoprotein chaperone, are speculated to interact physically in vivo. The formation 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 5 5 9 -derived holo-β 2 suggests that cytochrome b synthesis in PSII might also be catalyzed in vivo.