Studies of the location and function of isoprenoid quinones in chlorosomesfrom green sulfur bacteria

The chlorosome antenna of the green sulfur bacterium Chlorobium tepidum ess entially consists of aggregated bacteriochlorophyll (BChl) c enveloped in a glycolipid monolayer. Small amounts of protein and the isoprenoid quinones chlorobiumquinone (CK) and menaquinone-7 (MK-7) are also present. Treatmen t of isolated chlorosomes from Cb. tepidum with sodium dodecyl sulfate (SDS ) did not affect the quinones, demonstrating that these are located in a si te which is inaccessible to SDS, probably in the interior of the chlorosome s. About half of the quinones were removed by Triton X-100. The non-ionic c haracter of Triton probably allowed it to extract components from within th e chlorosomes. MK-10 in chlorosomes from the green filamentous bacterium Ch loroflexus aurantiacus was likewise found to be located in the chlorosome i nterior. The excitation transfer in isolated chlorosomes from Cb. tepidum i s redox-regulated. We found a ratio of BChl c fluorescence intensity under reducing conditions (F-red) to that under oxidizing conditions (F-ox) of ap proximately 40. The chlorosomal BChl a fluorescence was also redox-regulate d. When the chlorosomal BChl c-BChl c interactions were disrupted by 1-hexa nol, the BChl c F-red/F-ox ratio decreased to approximately 3. When CK and MK-7 were extracted from isolated chlorosomes with hexane, the BChl c F-red /F-ox ratio also decreased to approximately 3. A BChl c F-red/F-ox ratio of 3-5 was furthermore observed in aggregates of pure BChl c and in chlorosom es from Cfx. aurantiacus which do not contain CK. We therefore suggest that BChl c aggregates inherently exhibit a small redox-dependent fluorescence (F-red/F-ox approximate to 3) and that the large redox-dependent fluorescen ce observed in chlorobial chlorosomes (F-red/F-ox approximate to 40) is CK- dependent.