Light-induced oxidation-reduction reactions in a cell-free preparation from the blue-green alga Nostoc muscorum: The role of cytochrome f, cytochrome b558, C550, and P700 in noncyclic electron transport

1. Cytochrome f (λ_max = 554 nm, E_m = +0.35 V) and cytochrome b₅₅₈ (λ_max = 558 nm, E_m = +0.35 V) were photooxidized by Photosystem I and photoreduced by Photosystem II in a cell-free preparation from the blue-green alga Nostoc muscorum. The steady-state oxidation levels of both cytochromes were affected by noncyclic electron acceptors and by inhibitors of noncyclic electron transport. These results are consistent with the hypothesis that the mechanism of NADP reduction by water involves a Photosystem II and a Photosystem I light reaction operating in series and linked by a chain of electron carriers that includes cytochrome f and cytochrome b₅₅₈.2. Phosphorylation cofactors shifted the steady-state of cytochrome f to a more reduced level under conditions of noncyclic electron transport but had no effect on cytochrome b₅₅₈. These observations suggest that the noncyclic phosphorylation site lies before cytochrome f (on the Photosystem II side) and that cytochrome f is closer to this site than is cytochrome b₅₅₈.3. A Photosystem II photoreduction of C550 at 77 °K was observed, suggesting that in blue-green algae, as in other plants, C550 is closely associated with the primary electron acceptor for Photosystem II. A Photosystem I photooxidation of P700 at 77 °K was observed, consistent with P700 serving as the primary electron donor of Photosystem I.