Changes in Light-Induced Electron Transfer Through Cytochrome b-563 Depending on the Oxidation-Reduction Conditions in Intact Cells of the Cyanobacterium Synechocystis PCC 6714

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Flash-induced redox changes of cytochrome b-563 were studied in intact cells of the cyanobacterium Synechocystis PCC 6714. The redox reactions of this cytochrome depended on the redox state of the electron transport system, as determined by a balance between the influx of electron from substrates and the efflux to molecular oxygen. Under aerobic conditions, flash-induced changes in cytochrome b-563 were almost insignificant in the photoheterotrophic cells grown in the presence of DCMU (0.1 mM) and glucose (60 mM), whereas a rapid reduction of the cytochrome, followed by a re-oxidation, was clearly observed when the respiratory oxidase was inhibited by KCN (1 mM). Under anaerobic conditions, when cytochrome b-563 was fully reduced in the dark, a prominent oxidation of the cytochrome was observed after a flash; the subsequent re-reduction was very slow. The oxidation of cytochrome b-563 was inhibited by the addition of 2-heptyl-4-hydroxyquinoline N-oxide (HQNO). These results suggest that electron transfer in the cytochrome bf complex occurs by a modified Q-cycle mechanism, the reaction sequence of which depends on the redox states of cytochrome b-563 and plastoquinone. Flash-induced oxidation and reduction of cytochrome b-563 in cells grown under photoautotrophic conditions were less prominent, but the patterns of the reactions were consistent with the suggested interpretation.
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