title: A reaction center-dependent photoprotection mechanism in a highly robust photosystem II from an extremophilic red alga, Cyanidioschyzon merolae.
abstract: Members of the rhodophytan order Cyanidiales are unique among phototrophs in their ability to live in extremely low pH levels and moderately high temperatures. The photosynthetic apparatus of the red alga Cyanidioschyzon merolae represents an intermediate type between cyanobacteria and higher plants, suggesting that this alga may provide the evolutionary link between prokaryotic and eukaryotic phototrophs. Although we now have a detailed structural model of photosystem II (PSII) from cyanobacteria at an atomic resolution, no corresponding structure of the eukaryotic PSII complex has been published to date. Here we report the isolation and characterization of a highly active and robust dimeric PSII complex from C. merolae. We show that this complex is highly stable across a range of extreme light, temperature, and pH conditions. By measuring fluorescence quenching properties of the isolated C. merolae PSII complex, we provide the first direct evidence of pH-dependent non-photochemical quenching in the red algal PSII reaction center. This type of quenching, together with high zeaxanthin content, appears to underlie photoprotection mechanisms that are efficiently employed by this robust natural water-splitting complex under excess irradiance. In order to provide structural details of this eukaryotic form of PSII, we have employed electron microscopy and single particle analyses to obtain a 17 Å map of the C. merolae PSII dimer in which we locate the position of the protein mass corresponding to the additional extrinsic protein stabilizing the oxygen-evolving complex, PsbQ'. We conclude that this lumenal subunit is present in the vicinity of the CP43 protein, close to the membrane plane.
authors.affiliation: Department of Plant Molecular Physiology, Faculty of Biology, University of Warsaw, Miecznikowa 1, 02-096 Warsaw, Poland.
authors.forename: Tomasz
authors.initials: T
authors.lastname: Krupnik
authors.forename: Eva
authors.initials: E
authors.lastname: Kotabová
authors.forename: Laura S
authors.initials: LS
authors.lastname: van Bezouwen
authors.forename: Radoslaw
authors.initials: R
authors.lastname: Mazur
authors.forename: Maciej
authors.initials: M
authors.lastname: Garstka
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Nixon
authors.forename: James
authors.initials: J
authors.lastname: Barber
authors.forename: Radek
authors.initials: R
authors.lastname: Kaňa
authors.forename: Egbert J
authors.initials: EJ
authors.lastname: Boekema
authors.forename: Joanna
authors.initials: J
authors.lastname: Kargul
mesh_terms: D004795:Enzyme Stability; D006358:Hot Temperature; D006863:Hydrogen-Ion Concentration; D008027:Light; D010449:Peptide Mapping; D045332:Photosystem II Protein Complex / Q000737:chemistry* / Q000378:metabolism; D000461:Rhodophyta / Q000201:enzymology*
chemical_list: D045332:Photosystem II Protein Complex
keywords: Algae; Cyanidoschyzon merolae; Fluorescence; Photoprotection; Photosynthesis; Photosynthetic Pigments; Photosystem II; Single Particle Analysis