Carlton B C, Helinski D R
Proc Natl Acad Sci U S A. 1969 Oct;64(2):592-9. doi: 10.1073/pnas.64.2.592.
DNA preparations from two strains of Bacillus megaterium, one which produces megacin and another that is megacin sensitive, were analyzed by dye-buoyant density centrifugation, cesium chloride gradient equilibrium centrifugation in the absence of dye, zone sedimentation in sucrose, and electron microscopy. Both strains were found to contain a substantial proportion of their total DNA as circular, covalently closed, double helical molecules. These DNA elements were heterogeneous in size and appeared slightly less dense than chromosomal DNA. The physiological role and origin of these elements is unknown at present.
