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 f lash-induced cytochrome c oxidation were studied in whole cells under aerob ic conditions. BChl c fluorescence in a cell suspension with 5.4 mu M BChl c was quenched to 50% by addition of 0.6 mu M shikonin ((R)-2-(1-hydroxy-4- methyl-3-pentenyl)-5,8-dihydroxy-1, 1-naphthoquinone), 0.9 mu M 5-hydroxy-1 ,4-naphthoquinone, or 4 mu M 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 inhibite d flash-induced cytochrome c oxidation when BChl c was excited, but not whe n BChl a was excited. The quenching of BChl c fluorescence induced by these quinones correlated with the inhibition of flash-induced cytochrome c oxid ation, We concluded that the quinones inhibited electron transfer in the re action center by specifically quenching the excitation energy in the BChl I I c antenna. Our results provide a model system for studying the redox-depe ndent antenna quenching in green sulfur bacteria because the antennas in th ese bacteria inherently exhibit a sensitivity to O-2 Similar to the quinone -supplemented cells of Cfx. aurantiacus. (C) 1999 Elsevier Science B.V. All rights reserved.