COUPLED ACTIVATION OF THE DONOR AND THE ACCEPTOR SIDE OF PHOTOSYSTEM-II DURING PHOTOACTIVATION OF THE OXYGEN-EVOLVING CLUSTER

Photoactivation of photosystem II has been studied in the FUD 39 mutan t of Chlamydomonas reinhardtii that lacks the 23 kDa extrinsic subunit of photosystem II. We have taken advantage of the slow photoactivatio n rate of FUD 39, earlier demonstrated in Rova, E. M., et al. [(1996) J. Biol. Chem. 271, 28918-28924], to study events in photosystem II du ring intermediate stages of the process. By measuring the EPR multilin e signal, the decay of the variable fluorescence after single flashes, and electron transfer from water to the Q(B) site, we found a good co rrelation between the building of a tetrameric Mn cluster, longer reco mbination times between Q(A)(-) and the donor side of photosystem II, and the achievement of water splitting ability. An increased rate of e lectron transfer from Q(A)(-) to the Q(B) Site on the acceptor side of photosystem II, mainly due to enhanced efficiency of binding of Q(B) to its Site, was found to precede the building of the Mn cluster. We a lso showed that Tyro was oxidized simultaneously with this increase in electron-transfer rate. Thus, it appears that photoactivation is sequ ential, with an increased rate of electron transfer on the acceptor si de occurring together with the oxidation of Tyro in the first step, fo llowed by the assembly of the Mn cluster. We suggest that a conformati onal change of photosystem II is induced early in the photoactivation process facilitating electron transfer from the primary donor to the a cceptor side. As a consequence, Tyr(D), an auxiliary electron donor to P-680(+)/Tyr(z)(.), is oxidized. That this occurs before the Mn clust er is fully functional serves to protect photosystem II against donor side induced photodamage.