Characterization of the polydisperse closed circular deoxyribonucleic acid molecules of Bacillus megaterium

Abstract

The polydisperse circular deoxyribonucleic acid (DNA) molecules which comprise up to 30% of the total extractable DNA of Bacillus megaterium strain 216 have been purified and partially characterized. Banding in cesium chlorideethidium bromide by "gradient relaxation" in a fixed-angle rotor provided good resolution of circular and chromosomal DNAs for preparative separations. Renaturation studies on purified circular DNA failed to reveal a rapidly renaturing fraction, and DNA-DNA hybridization studies indicated that the majority of the chromosomal nucleotide sequences are represented in the heterogeneous-size population of circular molecules. It is concluded that the circular DNA of B. megaterium does not represent typical bacterial plasmid DNA. The possibility that the circular DNA molecules are the result of the expression of a defective bacteriophage is discussed.