Natural Variants of Photosystem II Subunit D1 Tune Photochemical Fitness to Solar Intensity |
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Authors: | David J Vinyard Javier Gimpel Gennady M Ananyev Mario A Cornejo Susan S Golden Stephen P Mayfield G Charles Dismukes |
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Institution: | From the ‡Department of Chemistry and Chemical Biology and ;§Waksman Institute of Microbiology, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854.;the ¶Department of Chemistry, Princeton University, Princeton, New Jersey 08540, and ;the ‖San Diego Center for Algae Biotechnology, Division of Biological Sciences, University of California, San Diego, La Jolla, California 92093 |
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Abstract: | Photosystem II (PSII) is composed of six core polypeptides that make up the minimal unit capable of performing the primary photochemistry of light-driven charge separation and water oxidation in all oxygenic phototrophs. The D1 subunit of this complex contains most of the ligating amino acid residues for the Mn4CaO5 core of the water-oxidizing complex (WOC). Most cyanobacteria have 3–5 copies of the psbA gene coding for at least two isoforms of D1, whereas algae and plants have only one isoform. Synechococcus elongatus PCC 7942 contains two D1 isoforms; D1:1 is expressed under low light conditions, and D1:2 is up-regulated in high light or stress conditions. Using a heterologous psbA expression system in the green alga Chlamydomonas reinhardtii, we have measured growth rate, WOC cycle efficiency, and O2 yield as a function of D1:1, D1:2, or the native algal D1 isoform. D1:1-PSII cells outcompete D1:2-PSII cells and accumulate more biomass in light-limiting conditions. However, D1:2-PSII cells easily outcompete D1:1-PSII cells at high light intensities. The native C. reinhardtii-PSII WOC cycles less efficiently at all light intensities and produces less O2 than either cyanobacterial D1 isoform. D1:2-PSII makes more O2 per saturating flash than D1:1-PSII, but it exhibits lower WOC cycling efficiency at low light intensities due to a 40% faster charge recombination rate in the S3 state. These functional advantages of D1:1-PSII and D1:2-PSII at low and high light regimes, respectively, can be explained by differences in predicted redox potentials of PSII electron acceptors that control kinetic performance. |
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Keywords: | Algae Chlamydomonas Cyanobacteria Photosynthesis Photosystem II Biomass Crop Yield Oxygen Evolution Solar Energy Conversion Water Oxidation Efficiency |
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