The importance of the hydrophilic region of PsbL for the plastoquinone electron acceptor complex of Photosystem II

The PsbL protein is a 4.5 kDa subunit at the monomer–monomer interface of Photosystem II (PS II) consisting of a single membrane-spanning domain and a hydrophilic stretch of ~ 15 residues facing the cytosolic (or stromal) side of the photosystem. Deletion of conserved residues in the N-terminal region has been used to investigate the importance of this hydrophilic extension. Using Synechocystis sp. PCC 6803, three deletion strains: ?(N6–N8), ?(P11–V12) and ?(E13–N15), have been created. The ?(N6–N8) and ?(P11–V12) strains remained photoautotrophic but were more susceptible to photodamage than the wild type; however, the ?(E13–N15) cells had the most severe phenotype. The Δ(E13–N15) mutant showed decreased photoautotrophic growth, a reduced number of PS II centers, impaired oxygen evolution in the presence of PS II-specific electron acceptors, and was highly susceptible to photodamage. The decay kinetics of chlorophyll a variable fluorescence after a single turnover saturating flash and the sensitivity to low concentrations of PS II-directed herbicides in the Δ(E13–N15) strain indicate that the binding of plastoquinone to the Q_B-binding site had been altered such that the affinity of Q_B is reduced. In addition, the PS II-specific electron acceptor 2,5-dimethyl-p-benzoquinone was found to inhibit electron transfer through the quinone-acceptor complex of the ?(E13–N15) strain. The PsbL Y20A mutant was also investigated and it exhibited increased susceptibility to photodamage and increased herbicide sensitivity. Our data suggest that the N-terminal hydrophilic region of PsbL influences forward electron transfer from Q_A through indirect interactions with the D–E loop of the D1 reaction center protein. Our results further indicate that disruption of interactions between the N-terminal region of PsbL and other PS II subunits or lipids destabilizes PS II dimer formation. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: Keys to Produce Clean Energy.