Photosystem I monomers and trimers in cyanobacteria
Photosystem I (PSI) is one of the chlorophyll-protein complexes located in the thylakoid membranes of photosynthetic organisms. It catalyses the oxidation of reduced plastocyanin or cytochrome C6 and reduction of ferredoxin or flavodoxin during photosynthesis. The complex is composed of two core subunits, PsaA and PsaB, which bind electron-carrying cofactors, and numerous low molecular weight subunits. The precise composition of small subunits varies between prokaryotic and eukaryotic PSI and the function of many of these subunits remains unclear. This thesis describes studies of cyanobacterial mutants lacking small subunits in order to clarify the function of the PsaL and PsaE subunits. The subunit PsaE has been implicated in the interaction of PSI with ferredoxin and in cyclic electron transport. In cyanobacteria a trimeric form of PSI predominates, with PsaL acting as the point of contact between three monomer units. Cyanobacterial mutants lacking this subunit therefore produce only monomeric PSI. No evidence of trimerisation has been found for PSI in eukaryotic organisms. This study has established growth conditions in which PsaL-lacking mutants are disadvantaged compared to wild type. An investigation into the interaction of monomeric PSI with light-harvesting structures such as the phycobilisomes and the iron stress-induced CP43' antenna ring revealed that trimerisation of PSI is not required for interaction with either structure. However, the rates of state transitions and phycobilisome diffusion were measured and compared to those of wild type cells, revealing an increase in rate in both cells when PSI is monomeric. This suggests a change in the stability of the phycobilisome-PSI interaction. EPR analysis of electron transfer processes in cyanobacterial mutants lacking the PsaE subunit and of PSI in either monomeric (PsaL-lacking) or trimeric (wild type) forms has been conducted. Possible functional roles for PSI trimerisation are discussed.