Stabilization of Photosystem II by the PsbT protein impacts photodamage, repair and biogenesis.

Photosystem II (PS II) catalyzes the light-driven process of water splitting in oxygenic photosynthesis. Four core membrane-spanning proteins, including D1 that binds the majority of the redox-active co-factors, are surrounded by 13 low-molecular-weight (LMW) proteins. We previously observed that deletion of the LMW PsbT protein in the cyanobacterium Synechocystis sp. PCC 6803 slowed electron transfer between the primary and secondary plastoquinone electron acceptors QA and QB and increased the susceptibility of PS II to photodamage. Here we show that photodamaged ∆PsbT cells exhibit unimpaired rates of oxygen evolution if electron transport is supported by HCO3- even though the cells exhibit negligible variable fluorescence. We find that the protein environment in the vicinity of QA and QB is altered upon removal of PsbT resulting in inhibition of QA- oxidation in the presence of 2,5-dimethyl-1,4-benzoquinone, an artificial PS II-specific electron acceptor. Thermoluminescence measurements revealed an increase in charge recombination between the S2 oxidation state of the water-oxidizing complex and QA- by the indirect radiative pathway in ∆PsbT cells and this is accompanied by increased 1O2 production. At the protein level, both D1 removal and replacement, as well as PS II biogenesis, were accelerated in the ∆PsbT strain. Our results demonstrate that PsbT plays a key role in optimizing the electron acceptor complex of the acceptor side of PS II and support the view that repair and biogenesis of PS II share an assembly pathway that incorporates both de novo synthesis and recycling of the assembly modules associated with the core membrane-spanning proteins.