Abstract

Effects of photoinhibition on the redox properties of Cyt b-559 were studied with NH2OH treated PSII membranes, which are depleted of the water-oxidizing complex. The membranes contained three redox forms (HP-, IP- and LP-forms) of Cyt b-559, with Em values of +435, +237 and +45 mV, respectively. A novel intermediate-potential form of Cyt b-559 was generated during photoinhibition on the donor side of PSII: photoinhibitory illumination (7,000 μE 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 Em value was about +350mV. Prolonged illumination (10 min) resulted in complete loss of the HP-form and an apparent increase in the level of the IPform. After further photoinhibitory treatment (60 min), complete loss of the IP′-form was observed and levels of the IP- and LP-forms each increased to about 50% of the total amount of Cyt b-559. Kinetic analysis of these data 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 appears only at the early phase of photoinhibition.

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