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

Abstract

The PsbL protein is a 4.5kDa 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 QB-binding site had been altered such that the affinity of QB 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 QA 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.

Keywords: Electron transfer; Herbicide binding; Photosystem II; PsbL; Q(B); Synechocystis sp. PCC 6803.