What makes plants, green algae and cyanobacteria so remarkable is their ability to convert light energy to chemical energy and at the same time evolve oxygen. In order to do so, they have developed a complex molecular machinery involving pigment and co-factor binding multi-protein complexes. Photosystem II is one of the protein complexes responsible for trapping photons and carrying out this energy conversion in a process where water is oxidized and molecular oxygen is released.

In this thesis, I have studied the organizational, functional and regulatory aspects of photosystem II. The results from these investigations can be summarized as follows: (i) A simple method was developed to isolate a highly pure, intact and active photosystem II complex containing the light-harvesting LHCII directly from thylakoid membranes. (ii) This so-called photosystem II supercomplex has properties similar to the membrane fragments enriched in photosystem II (BBY), but contain less of other non-photosystem II proteins, hence providing a better experimental alternative to these membrane fragments. (iii) A CP47 containing photosystem II reaction center with neither extrinsic proteins nor CP43 could be photoactivated to evolve oxygen, demonstrating that the latter proteins are not required for water oxidation. (iv) The re-association of the cyclophilin-like protein (TLP40) with the lumenal surface of the thylakoid membranes, which regulates dephosphorylation and turnover of the photosystem II reaction center protein, is dependent on both temperature and pH. (v) The association between TLP40 and the photosynthetic membranes may also involve the cytochrome b6/f complex.

Finally, a model is presented discussing the possibilities that a GTP-33-kDa complex could regulate the interaction between TLP40 and the thylakoid membranes in light.