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      The Roles of Cytochrome b 559 in Assembly and Photoprotection of Photosystem II Revealed by Site-Directed Mutagenesis Studies

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          Abstract

          Cytochrome b 559 (Cyt b 559) is one of the essential components of the Photosystem II reaction center (PSII). Despite recent accomplishments in understanding the structure and function of PSII, the exact physiological function of Cyt b 559 remains unclear. Cyt b 559 is not involved in the primary electron transfer pathway in PSII but may participate in secondary electron transfer pathways that protect PSII against photoinhibition. Site-directed mutagenesis studies combined with spectroscopic and functional analysis have been used to characterize Cyt b 559 mutant strains and their mutant PSII complex in higher plants, green algae, and cyanobacteria. These integrated studies have provided important in vivo evidence for possible physiological roles of Cyt b 559 in the assembly and stability of PSII, protecting PSII against photoinhibition, and modulating photosynthetic light harvesting. This mini-review presents an overview of recent important progress in site-directed mutagenesis studies of Cyt b 559 and implications for revealing the physiological functions of Cyt b 559 in PSII.

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          Most cited references55

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          Cyanobacterial photosystem II at 2.9-A resolution and the role of quinones, lipids, channels and chloride.

          Photosystem II (PSII) is a large homodimeric protein-cofactor complex located in the photosynthetic thylakoid membrane that acts as light-driven water:plastoquinone oxidoreductase. The crystal structure of PSII from Thermosynechococcus elongatus at 2.9-A resolution allowed the unambiguous assignment of all 20 protein subunits and complete modeling of all 35 chlorophyll a molecules and 12 carotenoid molecules, 25 integral lipids and 1 chloride ion per monomer. The presence of a third plastoquinone Q(C) and a second plastoquinone-transfer channel, which were not observed before, suggests mechanisms for plastoquinol-plastoquinone exchange, and we calculated other possible water or dioxygen and proton channels. Putative oxygen positions obtained from a Xenon derivative indicate a role for lipids in oxygen diffusion to the cytoplasmic side of PSII. The chloride position suggests a role in proton-transfer reactions because it is bound through a putative water molecule to the Mn(4)Ca cluster at a distance of 6.5 A and is close to two possible proton channels.
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            Complete genome structure of the thermophilic cyanobacterium Thermosynechococcus elongatus BP-1.

            The entire genome of a thermophilic unicellular cyanobacterium, Thermosynechococcus elongatus BP-1, was sequenced. The genome consisted of a circular chromosome 2,593,857 bp long, and no plasmid was detected. A total of 2475 potential protein-encoding genes, one set of rRNA genes, 42 tRNA genes representing 42 tRNA species and 4 genes for small structural RNAs were assigned to the chromosome by similarity search and computer prediction. The translated products of 56% of the potential protein-encoding genes showed sequence similarity to experimentally identified and predicted proteins of known function, and the products of 34% of these genes showed sequence similarity to the translated products of hypothetical genes. The remaining 10% lacked significant similarity to genes for predicted proteins in the public DNA databases. Sixty-three percent of the T. elongatus genes showed significant sequence similarity to those of both Synechocystis sp. PCC 6803 and Anabaena sp. PCC 7120, while 22% of the genes were unique to this species, indicating a high degree of divergence of the gene information among cyanobacterial strains. The lack of genes for typical fatty acid desaturases and the presence of more genes for heat-shock proteins in comparison with other mesophilic cyanobacteria may be genomic features of thermophilic strains. A remarkable feature of the genome is the presence of 28 copies of group II introns, 8 of which contained a presumptive gene for maturase/reverse transcriptase. A trace of genome rearrangement mediated by the group II introns was also observed.
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              Reversible and irreversible intermediates during photoinhibition of photosystem II: stable reduced QA species promote chlorophyll triplet formation.

              Photoinhibition of photosynthesis was studied in isolated photosystem II membranes by using chlorophyll fluorescence and electron paramagnetic resonance (EPR) spectroscopy combined with protein analysis. Under anaerobic conditions four sequentially intermediate steps in the photoinhibitory process were identified and characterized. These intermediates show high dark chlorophyll fluorescence (Foi) with typical decay kinetics (fast, semistable, stable, and nondecaying). The fast-decaying state has no bound QB but possesses a single reduced QA species with a 30-s decay half-time in the dark (QB, second quinone acceptor; QA, first quinone acceptor). In the semistable state, Q-A is stabilized for 2-3 min, most likely by protonation, and gives rise to the Q-A Fe2+ EPR signal in the dark. In the stable state, QA has become double reduced and is stabilized for 0.5-2 hr by protonation and a protein conformational change. The final, nondecaying state is likely to represent centers where QA H2 has left its binding site. The first three photoinhibitory states are reversible in the dark through reestablishment of QA to QB electron transfer. Significantly, illumination at 4 K of anaerobically photoinhibited centers trapped in all but the fast state gives rise to a spinpolarized triplet EPR signal from chlorophyll P680 (primary electron donor). When oxygen is introduced during anaerobic illumination, the light-inducible chlorophyll triplet is lost concomitant with induction of D1 protein degradation. The results are integrated into a model for the photoinhibitory process involving initial loss of bound QB followed by stable reduction and subsequent loss of QA facilitating chlorophyll P680 triplet formation. This in turn mediates light-induced formation of highly reactive and damaging singlet oxygen.
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                Author and article information

                Contributors
                Journal
                Front Plant Sci
                Front Plant Sci
                Front. Plant Sci.
                Frontiers in Plant Science
                Frontiers Media S.A.
                1664-462X
                12 January 2016
                2015
                : 6
                : 1261
                Affiliations
                [1]Institute of Plant and Microbial Biology – Academia Sinica Taipei, Taiwan
                Author notes

                Edited by: Julian Eaton-Rye, University of Otago, New Zealand

                Reviewed by: Pavel Pospíšil, Palacký University, Czech Republic; Gary Brudvig, Yale University, USA

                *Correspondence: Hsiu-An Chu, chuha@ 123456gate.sinica.edu.tw

                This article was submitted to Plant Cell Biology, a section of the journal Frontiers in Plant Science

                Article
                10.3389/fpls.2015.01261
                4709441
                85af3a04-f6f6-43a8-9f1c-50590854d539
                Copyright © 2016 Chu and Chiu.

                This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

                History
                : 03 November 2015
                : 24 December 2015
                Page count
                Figures: 2, Tables: 0, Equations: 0, References: 59, Pages: 7, Words: 0
                Categories
                Plant Science
                Mini Review

                Plant science & Botany
                photosynthesis,photosystem ii,cytochrome b559,site-directed mutagenesis,photoprotection,photoinhibition

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