Restoration of the high-potential form of cytochrome b559 of photosystem II occurs via a two-step mechanism under illumination in the presence of manganese ions

Spinach photosystem II membranes that had been depleted of the Mn cluster contained four forms of cytochrome (Cyt) b559, namely, high-potential (HP), HP', intermediate-potential (IP) and low-potential (LP) forms that exhibited the redox potentials of +400, +310, +170 and +35 mV, respectively, in potentiometric titration. When the membranes were illuminated with flashing light in the presence of 0.1 mM Mn²⁺, the IP form was converted to the HP' form by two flashes and then the HP' form was converted to the HP form by an additional flash. The quantum efficiency of the first conversion appeared to be quite high since the conversion was almost complete after two flashes. By contrast, the second conversion proceeded with low quantum efficiency and 40 flashes were required for completion. The effects of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) suggested that the first conversion did not require electron transfer from Q(A) to Q(B) while the second conversion had an absolute requirement for it. It was also suggested that the first conversion involved the reduction of the heme of Cyt b559, probably by Q(A)^-, and we propose that direct reduction by Q(A)^- induces a shift in the redox potential of the heme. The second conversion was also accompanied by the reduction of heme but it appeared that this conversion did not necessarily involve the reduction. The effects of DCMU on the reduction of heme suggested that the heme became reducible by Q(B)^- after the first conversion had been completed. This observation implies that the efficiency of electron transfer from Q(A) to Q(B) increased upon the conversion of the IP form to the HP' form, and we propose that restoration of the high-potential forms of Cyt b559 itself acts to make the acceptor side of photosystem II functional. Copyright (C) 1999 Elsevier Science B.V.