QUINONES IN CHLOROSOMES OF GREEN SULFUR BACTERIA AND THEIR ROLE IN THE REDOX-DEPENDENT FLUORESCENCE STUDIED IN CHLOROSOME-LIKE BACTERIOCHLOROPHYLL-C AGGREGATES

The light-harvesting chlorosome antennae of anaerobic, photosynthetic green sulfur bacteria exhibit a highly redox-dependent fluorescence su ch that the fluorescence intensity decreases under oxidizing condition s. We found that chlorosomes from Chlorobium tepidum contain three iso prenoid quinone species (chlorobiumquinone, menaquinone-7, and an unid entified quinone that probably is a chlorobiumquinone derivative) at a total concentration of approximately 0.1 mol per mol bacteriochloroph yll c. Most of the cellular chlorobiumquinone was found in the chloros omes and constituted about 70% of the total chlorosome quinone pool. W hen the quinones were added to artificial, chlorosome-like bacteriochl orophyll c aggregates in an aqueous solution, a high redox dependency of the fluorescence was observed. Chlorobiumquinones were most effecti ve in this respect. A lesser redox dependency of the fluorescence was still observed in the absence of quinones, probably due to another uni dentified redox-active component. These results suggest that quinones play a significant, but not exclusive role in controlling the fluoresc ence and in inhibiting energy transfer in chlorosomes under oxic condi tions. Chlorosomes from Chloroflexus aurantiacus contained menaquinone in an amount similar to that of total quinone in Chlorobium tepdium c hlorosomes, but did not contain chlorobiumquinones. This may explain t he much lower redox-dependent fluorescence observed in Chloroflexus ch lorosomes.