Abstract
Photosystem II (PS II) is the water-plastoquinone oxido-reductase responsible for splitting water in photosynthesis. The core of the enzyme is surrounded by 13 low-molecular-weight (LMW) proteins (<10 kDa). In this study, the role of three LMW proteins (PsbM, PsbX and PsbY) associating with the CP47 pre-assembly complex was investigated in the model cyanobacterium Synechocystis sp. PCC 6803. Gene knockouts were used to create strains lacking these proteins in single and double mutants. The PS II-specific electron acceptor 2,5-dimethyl-1,4-benzoquinone (DMBQ) was found to exhibit altered binding in these mutants while 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), a PS II specific inhibitor, failed to block oxygen evolution in the presence of 2,6-dichloro-1,4-benzoquinone (DCBQ). These observations indicated that the native quinone binding site of PS II was altered in the mutants. The PsbM-lacking strain showed destabilisation of PS II dimers while the absence of PsbY was observed to be not so detrimental to PS II activity and assembly. The ∆PsbM:∆PsbY double mutant exhibited a combination of features observed in the corresponding single mutants increasing the susceptibility of PS II to high light (HL). Removing PsbX caused inhibition of electron transport and increased the susceptibility of PS II centres to HL intensity. Further analysis of the photodamaged PS II centres using 35S-labelled methionine to probe repair process in PsbX-lacking cells indicated a susceptibility of the D2 protein to photodamage under HL. The enhanced susceptibility to D2 was explored by mutating two N-terminal Arg residues (24 and 26). The characterisation of the D2 point mutants indicated the possible role of Arg24 and Arg26 in PS II assembly. The current study indicates Arg24 and Arg26 residues of D2 are necessary for PS II assembly and suggests such residues can be used as a tool for arresting PS II assembly, which can potentially be helpful in identifying intermediate steps of PS II assembly.