Specific Labeling and Physical Characterization of R-Factor Deoxyribonucleic Acid in Escherichia coli

The molecular nature of R-factor deoxyribonucleic acid (DNA) was examined inEscherichia coli by using a method for the specific labeling of the derepressed R factor, R1, in a female cell after conjugation. Sixty minutes after mating, the R factor was isolated as a single molecule with a molecular weight of 65 × 106 daltons. This single molecular species sedimented as either a covalently closed molecule or a “nicked” circle. When the single R-factor component was centrifuged in a CsCl density gradient, only a single homogeneous species with a buoyant density of 1.711 g/cm3 was observed. R-factor DNA was also isolated directly from exponentially growing cells ofE. coli as a covalently closed single molecular species comprising about 1% of the total cellular DNA. Previous studies inProteus show that R1 factor DNA components of buoyant density 1.709, 1.711, and 1.716 g/cm3 can be identified as distinct replicons. It is suggested that the single molecule of R1 observed inE. coli is most simply explained as a composite structure resulting from a recombinational assemblage of a 1.709 and 1.716 g/cm3 replicon.