Cytochrome b559 (cyt b559) is an intrinsic and essential component of the photosystem II (PSII) protein complex, but its function, stoichiometry, and electron-transfer kinetics in the physiological system are not well-defined. In this study, we have used flash-detection optical spectroscopy to measure the kinetics and yields of photooxidation and dark reduction of cyt b559 in untreated, O2-evolving PSII-enriched membranes at room temperature. The dark redox states of cyt b559 and the primary electron acceptor, QA, were determined over the pH range 5.0-8.5. Both the fraction of dark-oxidized cyt b559 and dark-reduced QA increased with increasing acidity. Consistent with these results, an acid-induced drop in pH from 8.5 to 4.9 in a dark-adapted sample caused the oxidation of cyt b559, indicating a shift in the redox state during the dark reequilibration. As expected from the dark redox state of cyt b559, the rate and extent of photooxidation of cyt b559 during continuous illumination decreased toward more acidic pH values. After a single, saturating flash, the rate of photooxidation of cyt b559 was of the same order of magnitude as the rate of S2 QA-charge recombination. In untreated PSII samples at pH 8.0 with 42% of cyt 6559 oxidized and 15% of QA reduced in the dark, 4.7% of one copy of cyt b559 was photooxidized after one flash with a t1/2 of 540 ± 90 ms. On the basis of our previous work [Buser, C.A., Thompson, L.K., Diner, B.A., & Brudvig, G. W (1990) Biochemistry 29, 8977] and the data presented here, we conclude that Sn+1, Yz*, and P680+are in redox equilibrium and cyt b559 (and Yd) are oxidized via P680+. After a period of illumination sufficient to fully reduce the plastoquinone pool, we also observed the pH-dependent dark reduction of photooxidized cyt 6559, where the rate of reduction decreased with decreasing pH and was not observed at pH <6.4. To determine the direct source of reductant to oxidized cyt 6559, we studied the dark reduction of cyt b559 and the reduction of the PQ pool as a function of 3-(3, 4-dichlorophenyl)-1, 1-dimethylurea (DCMU) concentration. We find that DCMU inhibits the reduction of cyt b559 under conditions where the plastoquinone pool and QA are reduced. We conclude that QB-∙(H+) or QBH2 is the most likely source of the electron required for the reduction of oxidized cyt b559. On the basis of our results and their mechanistic implications, we suggest that cyt b559 mediates cyclic electron transfer within PSII, where cyt b559 is oxidized by P680+via a monomer chlorophyll and is reduced by a reducing equivalent from the QB site.
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