Covalently closed, circular DNA in kappa endosymbionts ofParamecium

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Abstract
SUMMARY: Caedobacter taeniospiralis(kappa), a bacterial endosymbiont isolated fromParamecium tetraureliastock 51, contains, in addition to the bacterial chromosome, covalently closed circular DNA molecules as shown by isolation on dye-buoyant-density gradients. The closed circular molecule has a contour length of 13·75 ± 0·04 µm with a buoyant density of 1·698 g/cm3. The buoyant density of the bacterial chromosome is 1·700–1·701 g/cm3. Kappa of the 51 group isolated from stock 298 and stock 6g2,P. tetraurelia, also contain the closed circular DNA. Two forms of kappa coexist in paramecia: brights and nonbrights. Examination by density-gradient centrifugation of the DNA of brights and nonbrights shows the extrachromosomal DNA to be associated mainly with brights. It is suggested that the extrachromosomal DNA might be the determinant for the refractile bodies and the helical phage-like structures found in brights.