Assembly and Repair of Photosystem II in Chlamydomonas reinhardtii.

Oxygenic photosynthetic organisms use Photosystem II (PSII) to oxidize water and reduce plastoquinone. Here, we review the mechanisms by which PSII is assembled and turned over in the model green alga Chlamydomonas reinhardtii. This species has been used to make key discoveries in PSII research due to its metabolic flexibility and amenability to genetic approaches. PSII subunits originate from both nuclear and chloroplastic gene products in Chlamydomonas. Nuclear-encoded PSII subunits are transported into the chloroplast and chloroplast-encoded PSII subunits are translated by a coordinated mechanism. Active PSII dimers are built from discrete reaction center complexes in a process facilitated by assembly factors. The phosphorylation of core subunits affects supercomplex formation and localization within the thylakoid network. Proteolysis primarily targets the D1 subunit, which when replaced, allows PSII to be reactivated and completes a repair cycle. While PSII has been extensively studied using Chlamydomonas as a model species, important questions remain about its assembly and repair which are presented here.

Conformational changes in a Photosystem II hydrogen bond network stabilize the oxygen-evolving complex.

The Mn4CaO5 oxygen-evolving complex (OEC) in Photosystem II (PSII) is assembled in situ and catalyzes water oxidation. After OEC assembly, the PsbO extrinsic subunit docks to the lumenal face of PSII and both stabilizes the OEC and facilitates efficient proton transfer to the lumen. D1 residue R334 is part of a hydrogen bond network involved in proton release during catalysis and interacts directly with PsbO. D1-R334 has recently been observed in different conformations in apo- and holo-OEC PSII structures. We generated a D1-R334G point mutant in Synechocystis sp. PCC 6803 to better understand this residue's function. D1-R334G PSII is active under continuous light, but the OEC is unstable in darkness. Isolated D1-R334G core complexes have little bound PsbO and less manganese as the wild type control. The S2 intermediate is stabilized in D1-R334G indicating that the local environment around the OEC has been altered. These results suggest that the hydrogen bond network that includes D1-R334 exists in a different functional conformation during PSII biogenesis in the absence of PsbO.

Chloride facilitates Mn(III) formation during photoassembly of the Photosystem II oxygen-evolving complex.

The Mn4Ca oxygen-evolving complex (OEC) in Photosystem II (PSII) is assembled in situ from free Mn2+, Ca2+, and water. In an early light-driven step, Mn2+ in a protein high-affinity site is oxidized to Mn3+. Using dual-mode electron paramagnetic resonance spectroscopy, we observed that Mn3+ accumulation increases as chloride concentration increases in spinach PSII membranes depleted of all extrinsic subunits. At physiologically relevant pH values, this effect requires the presence of calcium. When combined with pH studies, we conclude that the first Mn2+ oxidation event in OEC assembly requires a deprotonation that is facilitated by chloride.