Contrasted effects of inhibitors of cytochrome b(6)f complex on state transitions in Chlamydomonas reinhardtii - The role of Q(o) site occupancy in LHCII kinase activation

We have investigated the relationship between the occupancy of the Q(o) sit e in the cytochrome b(6)f complex and the activation of the LHCII protein k inase that controls state transitions. To this aim, fluorescence emission a nd LHCII phosphorylation patterns were studied in whole cells of Chlamydomo nas reinhardtii treated with different plastoquinone analogues. The analysi s of fluorescence induction at room temperature indicates that stigmatellin consistently prevented transition to State 2, whereas 2,5-dibromo-3-methyl -6-isopropyl-p-benzoquinone behaved as an inhibitor of state transitions on ly after the cells were preilluminated, The same effects were observed on t he phosphorylation patterns of the LHCII proteins, while subunit V of the c ytochrome b(6)f complex showed a different behavior. These findings are dis cussed on the basis of a dynamic structural model of cytochrome b(6)f that relates the activation of the LHCII kinase to the occupancy of the Q(o) sit e and the movement of the Rieske protein.