Replication of the Minicircular DNA of Escherichia coli 15: Properties of Newly Replicated Open Circular DNA Molecules

During the replication of the minicircular DNA of Escherichia coli 15 an open circular form II has been identified as a precursor of superhelical form I. Pulse‐labeled form II DNA contains a newly synthesized linear strand which is non‐covalently associated with a circular template strand. Equilibrium centrifugation of isolated linear strands in alkaline CsCI shows that the incorporation of label occurs with equal frequency in either of the complementary strands indicating a symmetrical mechanism of replication. Labeling of replicative intermediates is observed in short pulses at 25 °C. Upon alkali denaturation these molecules yield small labeled DNA fragments (4–6 S) suggesting a discontinuous mode of strand synthesis.