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.