Photooxidation and photoreduction of exogenous cytochrome c by photosystem II preparations after various modifications of the water-oxidizing complex

The redox interaction of exogenous cytochrome c 550 (Cyt) with PSII isolated from spinach was studied. Illumination of PSII particles in the presence of Cyt led to: ( 1 ) Cyt photooxidation by PSII reaction center (demonstrated at the first time), ( 2 ) Cyt photoreduction via O 2 −• photoproduced on the acceptor side of PSII, and ( 3 ) Cyt photoreduction by reduced electron carriers of PSII. A step-by-step removal of components of water-oxidizing complex was accompanied by the appearance of Cyt photooxidation, an increase in the superoxide dismutase (SOD)-dependent Cyt photoreduction (related to O 2 −• formation), and a decrease in the SOD-independent Cyt photoreduction. Re-addition of PsbO protein diminished the Cyt-induced restoration of electron transfer in PSII. Addition of diuron led to inhibition of these photoprocesses, while exogenous Mn 2+ inhibited only the Cyt c photooxidation. The results can be important for correct measurements of O 2 −• photoproduction in PSII and for elucidation of the role of cytochrome c 550 in cyanobacterial PSII.