The role of cytochrome b6 and cytochrome f in cyclic electron flow in a blue-green alga.

Cytochrome b₆ can be both photooxidized and photoreduced by Photosystem I in a cell-free preparation from the blue-green alga Nostoc muscorum. The cytochrome appears to have an oxidation-reduction potential near 0.0 V. The reduction of cytochrome b₆ when ferredoxin is added during Photosystem I illumination suggests that the cytochrome may function as a component of a ferredoxin-catalyzed cyclic electron transport pathway. In the presence of ferredoxin, the addition of ADP in the light results in oxidation of cytochrome b₆ and reduction of cytochrome f, suggesting the existence of a coupling site between the two cytochromes. An acceleration of the rate of the dark reduction of photooxidized cytochrome b₆ also observed on addition of ADP raises the possibility of a second coupling site on the reducing side of cytochrome b₆.