Characterisation of the role of the PsbT subunit in photosystem II in Synechocystis sp. PCC 6803

The PsbT subunit of photosystem II (PSII) is a single α-helical trans-membrane protein found at the monomer-monomer interface of PSII dimers. During oxygen evolution by PSII in the cyanobacterium Synechocystis sp. PCC 6803, efficient electron transport between the primary and secondary plastoquinone electron acceptors QA and QB is supported by a non-heme iron coordinated by four protein ligands and a bicarbonate ion. However, in the absence of PsbT, the coordination of the non-heme ion appears to be impaired by the addition of formate. The putative bicarbonate ligand to the non-heme iron is proposed to be the target site of formate inhibition. This is the first observation of a formate-induced inhibition of QA-oxidation in intact cells at atmospheric concentrations of CO2. Furthermore, this inhibition is reversible by the addition of bicarbonate. Additionally, the susceptibility to photoinhibition of ΔPsbT cells was evaluated. An increased affinity for the PSII-specific electron acceptor 2,5-dimethyl-p-benzoquinone at the QA or QB binding sites, revealed through the kinetics of QA-reoxidation, is proposed to inhibit electron transport and induce a susceptibility of these cells to photoinhibition. This was further supported by the addition of bicarbonate, which removed the photoinhibition effect from ΔPsbT and ΔPsbT:ΔPsb27 cells. Previous studies have shown a relationship between PsbT and dimerisation of PSII; however, it is uncertain whether this is the case in Synechocystis sp. PCC 6803 since ΔPsbT cells exhibited a change of PSII to PSI stoichiometry leading to an altered PSII:chlorophyll a ratio. Many experiments are dependent on the PSII:chlorophyll a ratio for quantification of PSII. Therefore the role of PsbT derived from this study appears to be the stabilisation of the reaction centre structure, facilitating the efficient electron transport on the acceptor side of PSII