Abstract
Photosystem II (PSII) is the solar powered membrane-protein water-plastoquinone oxidoreductase complex responsible for driving the metabolism of cyanobacteria and plants. Understanding the biogenesis, assembly, and repair of PSII, a complex membrane-protein with around twenty-one subunits, remains a challenge. Three of the low molecular weight (LMW) proteins – PsbJ, PsbX, and PsbY – alongside an assembly factor – Ycf48 – were selected for double knockout experimentation to ascertain their roles and interactions within PSII. The single mutant knockouts of the LMW proteins and Ycf48 were characterised first to provide insights into their individual phenotypes. Their phenotypes matched the work done by others in the past and, as such, were able to be utilised as controls for the double mutants. The deletion of the psbJ and ycf48 genes in a strain resulted in a strain that does not grow photoautotrophically, appeared to have large accumulations of both early- and late-stage assembly complexes, and struggled to evolve oxygen except when rescued with bicarbonate incubation. The deletion of the psbX and ycf48 genes produced a strain that had a substantial decrease in the number of functional PSII centres, and, while the rate of PSII biogenesis seemed minimally impacted, the reduction in centres appeared to be due to an increased rate of PSII turnover from excessive ROS production via backreactions. The deletion of the psbY and ycf48 genes had little effect that could not be explained by a summation of the single mutants – the strain grew slower, evolved less oxygen, and had fewer functional centres. The mutant did seem less capable of photo-protection and had an increased rate of backreactions and side reactions; however, the increase did not seem to be so drastic that the cells could not maintain photoautotrophic growth. This thesis made some novel observations, the first being that PsbJ seemed critical for late-stage assembly – specifically, the addition of the bicarbonate onto the non-heme iron, which facilitates the dissociation Ycf48, allowing the oxygen evolving complex (OEC) to bind and promote creation of the active mature PSII complex. Secondly, that PsbX and Ycf48 appear critical for the proper alignment of the antenna modules to the reaction centre (RC), which facilitates efficient exciton energy transfer. The third being that Ycf48 likely stays bound to PSII till late in assembly to protect the OEC from binding and early activation of PSII occurring