The PsbT protein modifies the bicarbonate-binding environment of Photosystem II

Photosystem II is the pigment–protein complex that oxidises water and reduces plastoquinone in oxygenic photosynthesis. Each Photosystem II complex contains four core proteins surrounded by 13 low-molecular-weight proteins. The 5-kDa PsbT protein is one of the subunits found at the interface of functional Photosystem II dimers. We previously observed that deletion of PsbT in the cyanobacterium Synechocystis sp. PCC 6803 slowed electron transfer and increased the susceptibility of Photosystem II to photodamage and that the addition of bicarbonate (or HCO3−) could prevent this photodamage. Bicarbonate is an essential Photosystem II cofactor that binds to a non-heme iron found between the QA and QB plastoquinone electron acceptors. Formate can bind to the non-heme iron thereby displacing bicarbonate and this results in blocked electron transfer and impaired oxygen evolution. We show Synechocystis sp. PCC 6803 cells lacking PsbT exhibit an enhanced sensitivity to formate addition, while formate inhibition of oxygen evolution in these cells is reversed by the addition of bicarbonate. Our results suggest that HCO3− binding is weakened in ΔPsbT cells and that PsbT plays a key role in maintaining the HCO3–-iron-quinone electron-acceptor complex of Photosystem II and consequently moderates the susceptibility of Photosystem II to high-light-induced photodamage.