Replication of the single-stranded DNA of the male-specific bacteriophage M13: circular forms of the replicative DNA

Equilibrium centrifugation in caesium chloride, band sedimentation in alkaline solutions and electron microscopy have been used to study purified preparations of the M13 replicative DNA. The buoyant density of the M13 replicative DNA in a CsCl density gradient is 1.701, compared to a density of 1-710 for Escherichia coli DNA. When the replicative DNA is sedimented in alkaline solutions of CsCl (p = 1.35), two components are observed with uncorrected s-values of 48.6 +/- 0.2 and 15.2 +/- 0.5. Treatment of the replicative DNA with pancreatic DNase reduces the relative amount of the fast component in alkaline CsCl and similarly increases the relative amount of the slow component. Electron microscopic observation of the replicative DNA also shows two different forms of DNA, extended circles and condensed forms of DNA. The relative amount of condensed forms of DNA in a preparation of replicative DNA is equal to the relative amount of the fast component in alkaline velocity sedimentation. The average contour length of the extended circles is 2.19 +/- 0.07 micro.