Abstract
The Photosystem II complexes of cyanobacteria possess lipoproteins which assist in biogenesis, repair, and stabilisation of the oxygen-evolving complex. In Synechocystis sp. PCC 6803 these include CyanoP (PsbP), CyanoQ (PsbQ), Psb27, and Ycf48; which are lipidated at a conserved lipobox sequence found near their N-terminus. Single deletion strains of the CyanoP, CyanoQ and Psb27 show only slight reduction in PS II function; however, deletion of Ycf48 disrupts PS II biogenesis and function substantially. Furthermore, deletion of the low molecular weight, intrinsic protein, PsbJ also results in significant disruption of PS II biogenesis, function and repair. To further elucidate the roles of the lipoproteins associated with PS II, lipoprotein deletion strains were characterised in a psbJ deletion background. Low temperature emission spectroscopy reveals an increased fluorescence at the 685 nm peak for the ΔPsbJ:CyanoP and ΔPsbJ:ΔCyanoQ strains, when excited with a 580 nm wavelength. Despite this, PS II centre formation is unaffected, as indicated by a 440 nm excitation wavelength. The oxygen-evolving rate for the ΔPsbJ strain is significantly reduced compared to the wild-type strain in the presence of 2,5-dimethy1-1,4-benzoquinone. Surprisingly, in the presence of bicarbonate the effect is reversed with the ΔPsbJ strain outperforming the wildtype strain. This recovery of oxygen-evolving rate was also seen for the ΔYcf48 strain, although to a lesser extent. The ΔPsbJ:ΔPsb27 strain also has a higher oxygen-evolving rate than wild type, although it is reduced compared to the ΔPsbJ strain. The oxygen-evolving rate is reduced back to the wild-type rate, for the ΔPsbJ:ΔCyanoP and ΔPsbJ:ΔCyanoQ deletion strains. The ΔPsbJ strain shows intolerance to high-light conditions but recovers its oxygenevolving ability in low light. Likewise the ΔPsbJ:ΔCyanoP, ΔPsbJ:ΔCyanoQ and ΔPsbJ:ΔPsb27 strains are sensitive to high-light conditions but recover slightly more slowly than the ΔPsbJ strain. Interestingly, the addition of bicarbonate confers high-light tolerance to these strains. Lastly, the ΔPsbJ:ΔYcf48 strain cannot grow photo-autotrophically, whereas the ΔPsbJ and ΔYcf48 deletion strains show only reduced growth in comparison to the wild type. The data show that PsbJ or Ycf48 deletion result in destabilisation of CP43 and an altered bicarbonate binding affinity. The data also show that deletion of Psb27 in conjunction with PsbJ results in an altered QB-binding site for the secondary plastoquinone electron acceptor of PS II.