EFFECTS OF LIGHT STRESS ON REDOX POTENTIAL FORMS OF CYT B-559 IN PHOTOSYSTEM-II MEMBRANES DEPLETED OF WATER-OXIDIZING COMPLEX

Effects of photoinhibition on the redox properties of Cyt b-559 were s tudied with NH2OH-treated PSII membranes, which are depleted of the wa ter-oxidizing complex. The membranes contained three redox forms (HP-, IP- and LP-forms) of Cyt b-559, with E(m) values of +435, +237 and +4 5 mV, respectively. A novel intermediate-potential form of Cyt b-559 w as generated during photoinhibition on the donor side of PSII: photoin hibitory illumination (7,000 mu E m(-2) s(-1)) for 1 min induced a 30% decrease in the level of the HP-form, with concomitant generation of the intermediate-potential (IP'-) form whose E(m) value was about +350 mV. Prolonged illumination (10 min) resulted in complete loss of the HP-form and an apparent increase in the level of the IP-form. After fu rther photoinhibitory treatment (60 min), complete loss of the IP'-for m was observed and levels of the IP- and LP-forms each increased to ab out 50% of the total amount of Cyt b-559. Kinetic analysis of these da ta led to the conclusion that the HP-form is converted to the LP-form via two intermediate-potential forms (IP' and IP), and that IP'-form a ppears only at the early phase of photoinhibition.