Photoreactions of Cytochromes in Algal Chloroplasts

Light-induced cytochrome redox reactions were investigated with algal chloroplasts capable of high rates of electron transport coupled to phosphorylation. The electron-donor pool preceding photosystem I consists of membrane-bound cytochrome f-554.5 and a soluble cytochrome c-553; the latter replaces the function of plastocyanin of higher-plant chloroplasts. Both cytochromes are reduced by photosystem II and oxidized by photosystem I. A site of energy conservation precedes these c-type cytochromes. The data obtained with respect to the function of b-type cytochromes are comparable to those obtained with higher-plant chloroplasts. Cyclic electron transport is mediated by cytochrome b-563 in a photosystem-I-dependent reaction. In addition, cytochrome b-563 may be reduced by photosystem II, in accordance with recent findings with intact spinach chloroplasts. It therefore appears that cytochrome b-563 is a member of both cyclic and non-cyclic electron transport. In contrast to higher-plant chloroplasts, redox reactions of cytochrome b-559 are observable without any pretreatments. Cytochrome b-559, high-potential, is reduced by photosystem II through plastoquinone. In the presence of carbonyl cyanide m-chlorophenylhydrazone a cytochrome-b-559 oxidation by photosystem II is measured.