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      PGR5 is involved in cyclic electron flow around photosystem I and is essential for photoprotection in Arabidopsis.

      Cell

      Light, Amino Acid Sequence, genetics, Arabidopsis, cytology, metabolism, Arabidopsis Proteins, isolation & purification, radiation effects, Base Sequence, Carbon Dioxide, Cell Respiration, Chlorophyll, Electron Transport, Gene Expression Regulation, Plant, physiology, adverse effects, Light-Harvesting Protein Complexes, Macromolecular Substances, Molecular Sequence Data, Mutation, NADP, Oxidation-Reduction, Oxygen, Photic Stimulation, Photosynthesis, Photosynthetic Reaction Center Complex Proteins, Photosystem I Protein Complex, Photosystem II Protein Complex, Thylakoids

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          Abstract

          During photosynthesis, plants must control the utilization of light energy in order to avoid photoinhibition. We isolated an Arabidopsis mutant, pgr5 (proton gradient regulation), in which downregulation of photosystem II photochemistry in response to intense light was impaired. PGR5 encodes a novel thylakoid membrane protein that is involved in the transfer of electrons from ferredoxin to plastoquinone. This alternative electron transfer pathway, whose molecular identity has long been unclear, is known to function in vivo in cyclic electron flow around photosystem I. We propose that the PGR5 pathway contributes to the generation of a Delta(pH) that induces thermal dissipation when Calvin cycle activity is reduced. Under these conditions, the PGR5 pathway also functions to limit the overreduction of the acceptor side of photosystem I, thus preventing photosystem I photoinhibition.

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          12176323

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