Abstract
Photosystem (PS) II is a photochemical oxidoreductase enzyme capable of splitting water and initiating photosynthesis. PS II is a multimeric complex. Therefore assembly and repair would not be achievable without numerous interactions in a stepwise manner. This thesis investigates the role of the highly conserved and well-studied lipoprotein Psb27 in the model cyanobacterium Synechocystis sp. PCC 6803 (Synechocystis 6803). Psb27 is an ~11 kDa protein that forms a four-helix bundle and has a dual role in both PS II assembly and repair. What makes Psb27 unique in Synechocystis 6803 is that the psb27 gene encoding Psb27 overlaps the slr1646 gene, encoding a putative RNase III, by four nucleotides and hence forms a psb27:slr1646 operon. This study focused on characterising psb27 mutant strains created using a mutagenesis system that introduced a spectinomycin-resistance cassette as a selectable marker within the slr1646 open-reading frame. Fluorescence-based assays were used to study PS II function in a series of mutants carrying targeted mutations that modified conserved surface residues on Psb27 that were hypothesised to interact with PS II during biogenesis. Two charge swap mutants were investigated where Arg54 or Arg94 had been changed to a Glu. The R54E and R94E strains exhibited an unexpected high PS II-specific fluorescence yield and altered PS II:PS I stoichiometry. A putative interaction between Lys63 of Psb27 and Asp231 of the PS II chlorophyll-binding core antenna protein CP43 was also studied. During this project, several of the strains underwent a secondary mutation in their psbA gene encoding the D1 reaction centre protein of PS II. The D1 protein is known to have a high turnover rate to enable PS II to recover from light-induced damage that occurs as a result of its water-splitting chemistry. The results of this study are consistent with the hypothesis that the psb27:slr1646 operon has a role in D1 turnover and protection from photodamage in mutants having altered levels or forms of Psb27