Abstract
Photosystem II (PS II) catalyses the oxidation of water and reduction of plastoquinone in oxygenic photosynthesis. The PS II complex is found in the thylakoid membrane as a dimer where each monomer consists of 20 protein subunits: these consist of the D1 and D2 subunits that contain the reaction centre, two chlorophyll-binding antenna proteins CP43 and CP47, 13 low-molecular-weight (LMW) proteins, and three hydrophilic subunits that are found on the lumenal side of the photosystem. The cytochrome b559 (Cyt b559) protein, composed of the PsbE and PsbF LMW proteins are located in close proximity to the LMW PsbJ subunit, and different cofactors (mobile plastoquinone acceptor (QB), β-carotene, and phosphatidylglycerol (PG)). Despite considerable study, the exact role of Cyt b559 in PS II assembly and electron transfer has not been established. Using the cyanobacterium Synechocystis sp. PCC 6803 the function of Cyt b559 was studied by introducing targeted mutations. Introducing mutations in PsbE at the acceptor side of PS II suppressed the expression of PsbE resulting in fewer active PS II centres in the cells of the F10A and S11A mutants. Losing contacts with PG due to these mutational changes led to impaired forward electron transfer and reduced PS II activity. Notably, the mutational change at Phe10 introduced light sensitivity to the cells which has led to the proposal of a model for the light-dependent recovery from photodamage in Synechocystis sp. PCC 6803. In addition, mutating several residues (Pro64, Ile65, Arg69, and Asn73) of PsbE that interact with the D2 subunit at the donor side of PS II altered the acceptor side electron transfer as well the stoichiometry between PS II and PS I. The PsbJ transmembrane protein was also studied and also appeared to be involved in maintaining the balance between PS II and PS I along with influencing steady-state electron flow and photoprotection of PS II. Furthermore, introducing point mutational changes in PsbJ targeting putative interactions with a previously uncharacterised Mg2+ cofactor revealed that the Mg2+ ion plays a key role in the ability of PsbJ to support the optimal function of PS II.