Contrasted effects of inhibitors of cytochrome b6f complex on state transitions in Chlamydomonas reinhardtii: the role of Qo site occupancy in LHCII kinase activation

We have investigated the relationship between the occupancy of the Q(o) site in the cytochrome b(6)f complex and the activation of the LHCII protein kinase that controls state transitions. To this aim, fluorescence emission and LHCII phosphorylation patterns were studied in whole cells of Chlamydomonas reinhardtii treated with different plastoquinone analogues. The analysis 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 only after the cells were preilluminated. The same effects were observed on the phosphorylation patterns of the LHCII proteins, while subunit V of the cytochrome b(6)f complex showed a different behavior. These findings are discussed 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) site and the movement of the Rieske protein.