The assembly and repair of cyanobacterial Photosystem II

Abstract

Photosystem II (PSII) is the multi-subunit membrane-anchored light-driven water:plastoquinone oxidoreductase located in the thylakoid membranes of chloroplasts and cyanobacteria. This macrocomplex harvests solar energy to power oxygenic photosynthesis, a reaction that renews atmospheric oxygen, supplies fixed carbon for the food chain and maintains the global carbon level. The D1 subunit is one of the two reaction centre proteins of PSII and provides amino-acid ligands to the Mn4CaO5 cluster involved in water oxidation. CP43, which plays a light-harvesting role in PSII, also provides a ligand to the cluster. In the structurally related photosystem I (PSI) complex, the equivalent to CP43 and D1 are fused and synthesised as a single subunit. In this work, a CP43-D1 fusion strain was successfully constructed in the cyanobacterium Synechocystis sp. PCC 6803 and shown to assemble a functional PSII complex capable of water oxidation. However, PSII activity in vivo was sensitive to high irradiances, and photoinhibition analyses showed impaired PSII repair in the fusion mutant and slower degradation of the CP43-D1 fusion protein. This work supports the hypothesis that CP43 and D1 are synthesised as separate subunits to allow prompt and efficient repair of PSII following damage by light. I have shown by phylogenetic analysis that PSII repair mediated by FtsH proteases might have also played an important role in the evolution of oxygenic photosynthesis. There are at least three orthologous groups of FtsH in the tree of life, with the FtsH involved in PSII repair diverging earlier than the radiation of FtsH from all other bacteria. Additionally, I carried out a structural conservation analysis on Psb29, an interacting partner of FtsH, to reveal potential interacting sites on this protein that are of interest for future studies for understanding the regulation of the PSII repair mechanism.