Photooxidation and photoreduction of exogenous cytochrome <italic>c</italic> by photosystem II preparations after various modifications of the water-oxidizing complex.

The redox interaction of exogenous cytochrome <italic>c</italic>550 (Cyt) with PSII isolated from spinach was studied. Illumination of PSII particles in the presence of Cyt led to: (<italic>1</italic>) Cyt photooxidation by PSII reaction center (demonstrated at the first time), (<italic>2</italic>) Cyt photoreduction <italic>via</italic> O2−• photoproduced on the acceptor side of PSII, and (<italic>3</italic>) 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 O2−• 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 Mn2+ inhibited only the Cyt <italic>c</italic> photooxidation. The results can be important for correct measurements of O2−• photoproduction in PSII and for elucidation of the role of cytochrome <italic>c</italic>550 in cyanobacterial PSII. [ABSTRACT FROM AUTHOR]