PH-DEPENDENT PHOTOREACTIONS OF THE HIGH-POTENTIAL AND LOW-POTENTIAL FORMS OF CYTOCHROME B559 IN SPINACH PS-II-ENRICHED MEMBRANES

Cytochrome b559 (Cyt b559) is a well-known intrinsic component of Phot osystem II (PS II) reaction center in all photosynthetic oxygen-evolvi ng organisms, but its physiological role remains unclear. This work re ports the response of the two redox forms of Cyt b559 (i.e. the high- (HP) and low-potential (LP) forms) to inhibition of the donor or accep tor side of PS II. The photooxidation of HP Cyt b559 induced by red li ght at room temperature was pi-I-dependent under conditions in which e lectron flow from water was diminished. This photooxidation was observ ed only at pH values higher than 7.5. However, in the presence of 1 mu M CCCP, a limited oxidation of HP Cyt b559 was observed at acidic pH. At pH 8.5 and in the presence of the protonophore, this photooxidatio n of the HP form was accompanied by its partial transformation into th e LP form. On the other hand, a partial photoreduction of LP Cyt b559 was induced by red light under aerobic conditions when electron transf er through the primary quinone acceptor Q(A) was impaired by strong ir radiation in the presence of DCMU. This photoreduction was enhanced at acidic pH values. To the best of our knowledge, this is the first tim e that both photoreduction and photooxidation of Cyt b559 is described under inhibitory conditions using the same kind of membrane preparati ons. A model accommodating these findings is proposed.