title: Specific labeling and physical characterization of R-factor deoxyribonucleic acid in Escherichia coli.
abstract: The molecular nature of R-factor deoxyribonucleic acid (DNA) was examined in Escherichia 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 x 10(6) 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/cm(3) was observed. R-factor DNA was also isolated directly from exponentially growing cells of E. coli as a covalently closed single molecular species comprising about 1% of the total cellular DNA. Previous studies in Proteus show that R1 factor DNA components of buoyant density 1.709, 1.711, and 1.716 g/cm(3) can be identified as distinct replicons. It is suggested that the single molecule of R1 observed in E. coli is most simply explained as a composite structure resulting from a recombinational assemblage of a 1.709 and 1.716 g/cm(3) replicon.
mesh_terms: D002499:Centrifugation, Density Gradient; D003433:Crosses, Genetic; D004261:DNA Replication; D004269:DNA, Bacterial / Q000096:biosynthesis; D004925:Escherichia / Q000528:radiation effects; D004926:Escherichia coli / Q000378:metabolism; D005827:Genetics, Microbial; D006824:Hybridization, Genetic; D008970:Molecular Weight; D011830:Radiation Effects; D013941:Thymine / Q000378:metabolism; D014316:Tritium; D014466:Ultraviolet Rays
chemical_list: D004269:DNA, Bacterial; D014316:Tritium; D013941:Thymine