Assembly of plasmid DNA and chromatophore in Rhodospirillum rubrum

Rhodospirillum rubrum a photosynthetic bacterium, contains many photosynthe tic vesicular membranous structures called chromatophores. The organism con tains a 55 kb specific plasmid which is essential for photosynthesis, but t he exact relationship between the chromatophore and the plasmid is uncertai n. In this study we examined the precise localization of the plasmids, espe cially in relation to the chromatophores. Fluorescence in situ hybridizatio n indicated that there are several copies of the plasmid per cell and that some plasmids are localized close to the cellular envelope. In situ hybridi zation at the electron-microscopic level further revealed that the plasmid localized to the periphery of the chromatophore close to the envelope. More over, when the chromatophore fraction was purified from cells, the plasmid DNA was observed as a cluster around the chromatophore vesicles. The assemb ly of the plasmid and chromatophore may be related to chromatophore formati on by invagination of cell membrane.