Exogenous quinones inhibit photosynthetic electron transfer in Chloroflexus aurantiacus by specific quenching of the excited bacteriochlorophyll c antenna.
In the photosynthetic green filamentous bacterium Chloroflexus aurantiacus, excitation energy is transferred from a large bacteriochlorophyll (BChl) c antenna via smaller BChl a antennas to the reaction center. The effects of substituted 1,4-naphthoquinones on BChl c and BChl a fluorescence and on flash-induced cytochrome c oxidation were studied in whole cells under aerobic conditions. BChl c fluorescence in a cell suspension with 5.4 microM BChl c was quenched to 50% by addition of 0.6 microM shikonin ((R)-2-(1-hydroxy-4-methyl-3-pentenyl)-5,8-dihydroxy-1, 4-naphthoquinone), 0.9 microM 5-hydroxy-1,4-naphthoquinone, or 4 microM 2-acetyl-3-methyl-1,4-naphthoquinone. Between 25 and 100 times higher quinone concentrations were needed to quench BChl a fluorescence to a similar extent. These quinones also efficiently inhibited flash-induced cytochrome c oxidation when BChl c was excited, but not when BChl a was excited. The quenching of BChl c fluorescence induced by these quinones correlated with the inhibition of flash-induced cytochrome c oxidation. We concluded that the quinones inhibited electron transfer in the reaction center by specifically quenching the excitation energy in the BChl c antenna. Our results provide a model system for studying the redox-dependent antenna quenching in green sulfur bacteria because the antennas in these bacteria inherently exhibit a sensitivity to O(2) similar to the quinone-supplemented cells of Cfx. aurantiacus.
Frigaard
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N
Department of Biology, Tokyo Metropolitan University, Minami-ohsawa 1-1, Hachioji, 192-0397, Tokyo, Japan. nxf10@psu.edu
Tokita
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S
Matsuura
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K
D001426:Bacterial Proteins / Q000378:metabolism; D001429:Bacteriochlorophylls; D002478:Cells, Cultured; D019414:Chlorobi / Q000187:drug effects* / Q000378:metabolism; D003574:Cytochrome c Group / Q000378:metabolism; D004579:Electron Transport / Q000187:drug effects; D005453:Fluorescence; D045342:Light-Harvesting Protein Complexes; D015394:Molecular Structure; D010084:Oxidation-Reduction; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism; D011809:Quinones / Q000737:chemistry / Q000494:pharmacology*
D001426:Bacterial Proteins; D001429:Bacteriochlorophylls; D003574:Cytochrome c Group; D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D011809:Quinones; C002084:bacteriochlorophyll c