Accumulation of replicating bacterial plasmid DNA during thymine limitation or hydroxyurea treatment

The rate of DNA chain elongation of a thymine auxotroph of Proteus mirabilis harboring an R factor was reduced by either thymine limitation or treatment with hydroxyurea. Using CsCl density gradient centrifugation, sedimentation in sucrose gradients, and electron microscopy, it has been shown that the fraction of total R factor DNA which is in the process of replication is greatly increased when the rate of DNA chain elongation is reduced while the initiation of plasmid replication continues at the normal rate. This makes possible the isolation and characterization of replicating plasmid DNA. Electron microscopy revealed that replicating R factor DNA consists of double-branched (theta type) circular molecules.