FLASH-INDUCED REDUCTION OF CYTOCHROME B-559 BY Q(B)OVER-BAR IN CHLOROPLASTS IN THE PRESENCE OF PROTONOPHORES

Flash-induced, fast (t1/2 almost-equal-to 1 ms), reversible reduction of the high potential cytochrome b-559 (cyt b-559HP) was observed in c hloroplasts in the presence of 2 muM protonophore, FCCP (carbonylcyani de p-trifluoromethoxyphenylhydrazone), CCCP (carbonylcyanide 3-chlorop henylhydrazone) or SF 6847 (2,6-di-(t-butyl)-4-(2',2'-dicyanovinyl)phe nol). These protonophores promote autooxidation of cyt b-559HP in the dark (Arnon and Tang 1988, Proc Natl Acad Sci USA 85: 9524). No fast p hotoreduction could, however, be observed if the molecules were oxidiz ed with ferricyanide in the absence of protonophores. This suggests th at the molecules must be deprotonated to be capable for fast photoredu ction. Photoreduction of cyt b-559HP was largely insensitive to DBMIB (2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone), but was inhibited b y DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea). With a train of flas hes, no oscillation could be observed in the amplitudes of photoreduct ion. These data strongly suggest that cyt b-559HP is reduced by the se mireduced secondary quinone acceptor (Q(B)-) of Photosystem 2.