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

Flash-induced redox changes of cytochrome b-563 were studiedin intact cells of the cyanobacterium Synechocystis PCC 6714.The redox reactions of this cytochrome depended on the redoxstate of the electron transport system, as determined by a balancebetween the influx of electron from substrates and the effluxto molecular oxygen. Under aerobic conditions, flash-inducedchanges in cytbchrome b-563 were almost insignificant in thephotoheterotrophic cells grown in the presence of DCMU (0.1mM) and glucose (60 mM), whereas a rapid reduction of the cytochrome,followed by a re-oxidation, was clearly observed when the respiratoryoxidase was inhibited by KCN (1 mM). Under anaerobic conditions,when cytochrome b-563 was fully reduced in the dark, a prominentoxidation of the cytochrome was observed after a flash; thesubsequent re-reduction was very slow. The oxidation of cytochromeb-563 was inhibited by the addition of 2-heptyl-4-hydroxyquinolineN-oxide (HQNO). These results suggest that electron transferin the cytochrome bf complex occurs by a modified Q-cycle mechanism,the reaction sequence of which depends on the redox states ofcytochrome b-563 and plastoquinone. Flash-induced oxidationand reduction of cytochrome b-563 in cells grown under photoautotrophicconditions were less prominent, but the patterns of the reactionswere consistent with the suggested interpretation. (Received May 9, 1988; Accepted August 15, 1988)