The effect of removing PsbJ in Synechocystis sp. PCC 6803 mutants that lack the extrinsic proteins of Photosystem II

Photosystem II (PS II) is the light-driven water-plastoquinone oxidoreductase of photosynthesis. PS II consists of 20 protein subunits including four core complexes forming the reaction centre, 13 additional low-molecular-weight (LMW) intrinsic proteins and up to 5 membrane-peripheral, extrinsic proteins whose functions during biogenesis are not yet established. PsbJ is a single α-helical LMW transmembrane protein found near cytochrome b559 that is formed by the PsbE and PsbF LMW PS II subunits. When and where PsbJ associates with the intermediate assembly subcomplexes that form the mature PS II complex is not known. The three main extrinsic proteins, PsbO, PsbU, and PsbV are found at the lumenal side of PS II and a role in stabilising and maintaining the Mn4CaO5 cluster has been suggested. It has been observed that the putative extrinsic protein CyanoQ facilitates the binding of PsbV while CyanoP, although considered as extrinsic subunit, may in fact be an assembly factor not present in the mature complex. In this study, the role of PsbJ and extrinsic proteins during biogenesis was investigated in the model cyanobacterium Synechocystis sp. PCC 6803 by creating single or double gene knockout mutants to remove specific subunits. The removal of either PsbO or PsbV in PsbJ-lacking cells had detrimental effects on the strain as they were unable to grow photoautotrophically despite having PS II monomers containing reaction centre subunits. While not as detrimental, the removal of PsbU also made the effect of the absence of PsbJ on photoautotrophic growth rate, PS II assembly, forward electron transfer between QA and QB worse than seen in either of the corresponding single mutants. In particular, the acceptor side of the ΔPsbJ:ΔPsbU strain was impaired as artificial quinones were unable to accept electrons and little oxygen was evolved, and there was no back electron flow to the oxidised Mn cluster of the electron donor. Removing CyanoP and CyanoQ in the PsbJ-lacking strain had little effect on photoautotrophic growth rate and oxygen-evolving activity; however, the electron transfer within PS II was affected and the formation of PS II dimers was reduced. The current study suggested that the PsbJ and the extrinsic proteins are required for the formation of either stable inactive monomers or stable dimers, and it is speculated that these proteins might bind at early PS II biogenesis steps