The growth of plants, algae, and cyanobacteria relies on the catalytic activity of the oxygen-evolving PSII complex, which uses solar energy to extract electrons from water to feed into the photosynthetic electron transport chain. PSII is proving to be an excellent system to study how large multi-subunit membrane-protein complexes are assembled in the thylakoid membrane and subsequently repaired in response to photooxidative damage. Here we summarize recent developments in understanding the biogenesis of PSII, with an emphasis on recent insights obtained from biochemical and structural analysis of cyanobacterial PSII assembly/repair intermediates. We also discuss how chlorophyll synthesis is synchronized with protein synthesis and suggest a possible role for PSI in PSII assembly. Special attention is paid to unresolved and controversial issues that could be addressed in future research.

• MeSH
• Chlorophyll metabolism   MeSH
• Photosynthesis MeSH
• Photosystem II Protein Complex * metabolism   MeSH
• Cyanobacteria * metabolism   MeSH
• Thylakoids metabolism   MeSH
• Publication type
• Journal Article MeSH
• Review MeSH
• Names of Substances
• Chlorophyll MeSH
• Photosystem II Protein Complex * MeSH

Assembly of photosystem II (PSII), a water-splitting catalyst in chloroplasts and cyanobacteria, requires numerous auxiliary proteins which promote individual steps of this sequential process and transiently associate with one or more assembly intermediate complexes. In this study, we focussed on the role of a PSII-associated protein encoded by the ssl1498 gene in the cyanobacterium Synechocystis sp. PCC 6803. The N-terminal domain of this protein, which is here called Psb34, is very similar to the N-terminus of HliA/B proteins belonging to a family of high-light-inducible proteins (Hlips). Psb34 was identified in both dimeric and monomeric PSII, as well as in a PSII monomer lacking CP43 and containing Psb28. When FLAG-tagged, the protein is co-purified with these three complexes and with the PSII auxiliary proteins Psb27 and Psb28. However, the preparation also contained the oxygen-evolving enhancers PsbO and PsbV and lacked HliA/B proteins even when isolated from high-light-treated cells. The data suggest that Psb34 competes with HliA/B for the same binding site and that it is one of the components involved in the final conversion of late PSII assembly intermediates into functional PSII complexes, possibly keeping them free of Hlips. Unlike HliA/B, Psb34 does bind to the CP47 assembly module before its incorporation into PSII. Analysis of strains lacking Psb34 indicates that Psb34 mediates the optimal equilibrium of HliA/B binding among individual PSII assembly intermediates containing CP47, allowing Hlip-mediated photoprotection at all stages of PSII assembly.

• Keywords
• CP47, High-light-inducible protein, Photosynthesis, Photosystem II,
• MeSH
• Bacterial Proteins metabolism   MeSH
• Photosynthesis MeSH
• Photosystem II Protein Complex metabolism   MeSH
• Tumor Necrosis Factor Ligand Superfamily Member 14 metabolism   MeSH
• Synechocystis * metabolism   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Bacterial Proteins MeSH
• Photosystem II Protein Complex MeSH
• Tumor Necrosis Factor Ligand Superfamily Member 14 MeSH

High-light-inducible proteins (Hlips) are single-helix transmembrane proteins that are essential for the survival of cyanobacteria under stress conditions. The model cyanobacterium Synechocystis sp. PCC 6803 contains four Hlip isoforms (HliA-D) that associate with Photosystem II (PSII) during its assembly. HliC and HliD are known to form pigmented (hetero)dimers that associate with the newly synthesized PSII reaction center protein D1 in a configuration that allows thermal dissipation of excitation energy. Thus, it is expected that they photoprotect the early steps of PSII biogenesis. HliA and HliB, on the other hand, bind the PSII inner antenna protein CP47, but the mode of interaction and pigment binding have not been resolved. Here, we isolated His-tagged HliA and HliB from Synechocystis and show that these two very similar Hlips do not interact with each other as anticipated, rather they form HliAC and HliBC heterodimers. Both dimers bind Chl and β-carotene in a quenching conformation and associate with the CP47 assembly module as well as later PSII assembly intermediates containing CP47. In the absence of HliC, the cellular levels of HliA and HliB were reduced, and both bound atypically to HliD. We postulate a model in which HliAC-, HliBC-, and HliDC-dimers are the functional Hlip units in Synechocystis. The smallest Hlip, HliC, acts as a 'generalist' that prevents unspecific dimerization of PSII assembly intermediates, while the N-termini of 'specialists' (HliA, B or D) dictate interactions with proteins other than Hlips.

• Keywords
• CP47, Chlorophyll, High-light-inducible proteins, Photosystem II, Synechocystis,
• MeSH
• Bacterial Proteins metabolism   MeSH
• Photosystem II Protein Complex metabolism   MeSH
• Tumor Necrosis Factor Ligand Superfamily Member 14 metabolism   MeSH
• Light-Harvesting Protein Complexes * metabolism   MeSH
• Synechocystis * metabolism   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Bacterial Proteins MeSH
• Photosystem II Protein Complex MeSH
• Tumor Necrosis Factor Ligand Superfamily Member 14 MeSH
• Light-Harvesting Protein Complexes * MeSH

The repair of photosystem II is a key mechanism that keeps the light reactions of oxygenic photosynthesis functional. During this process, the PSII central subunit D1 is replaced with a newly synthesized copy while the neighbouring CP43 antenna with adjacent small subunits (CP43 module) is transiently detached. When the D2 protein is also damaged, it is degraded together with D1 leaving both the CP43 module and the second PSII antenna module CP47 unassembled. In the cyanobacterium Synechocystis sp. PCC 6803, the released CP43 and CP47 modules have been recently suggested to form a so-called no reaction centre complex (NRC). However, the data supporting the presence of NRC can also be interpreted as a co-migration of CP43 and CP47 modules during electrophoresis and ultracentrifugation without forming a mutual complex. To address the existence of NRC, we analysed Synechocystis PSII mutants accumulating one or both unassembled antenna modules as well as Synechocystis wild-type cells stressed with high light. The obtained results were not compatible with the existence of a stable NRC since each unassembled module was present as a separate protein complex with a mutually similar electrophoretic mobility regardless of the presence of the second module. The non-existence of NRC was further supported by isolation of the His-tagged CP43 and CP47 modules from strains lacking either D1 or D2 and their migration patterns on native gels.

• Keywords
• CP43, CP47, No reaction centre complex, Photosynthesis, Photosystem II,
• MeSH
• Bacterial Proteins genetics   metabolism   MeSH
• Photosystem II Protein Complex metabolism   MeSH
• Oxygen metabolism   MeSH
• Synechocystis * genetics   metabolism   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Bacterial Proteins MeSH
• Photosystem II Protein Complex MeSH
• Oxygen MeSH