Redox changes of cytochrome b559 in the presence of plastoquinones

Abstract

We have found that short chain plastoquinones effectively stimulated photoreduction of the low potential form of cytochrome b559 and were also active in dark oxidation of this cytochrome under anaerobic conditions in Triton X-100-solubilized photosystem II (PSII) particles. It is also shown that molecular oxygen competes considerably with the prenylquinones in cytochrome b559 oxidation under aerobic conditions, indicating that both molecular oxygen and plastoquinones could be electron acceptors from cytochrome b559 in PSII preparations, α-Tocopherol quinone was not active in the stimulation of cytochrome photoreduction but efficiently oxidized it in the dark. Both the observed photoreduction and dark oxidation of the cytochrome were not sensitive to 3-(3,4-dichlorophenyl)-1,1-dimethylurea. It was concluded that both quinone-binding sites responsible for the redox changes of cytochrome b559 are different from either the QA or QB site in PSII and represent new quinone-binding sites in PSII.