Electron microscopy study of early lytic replication forms of bacteriophage P1 DNA.

P1 replication intermediates were isolated from the intracellular DNA of lytically infected cells and analyzed by electron microscopy. At early times in infection replication intermediates were mainly of two types, circular theta- and sigma-shaped molecules plus a small proportion of linear bubble-shaped molecules. At later times in infection sigma molecules were the predominant replicating form. In contrast, sigma molecules were rarely found in recombination deficient, recA, infected cells. These observations show that early P1 DNA synthesis occurs, in part, by a circular mode of replication and imply that transition to a later, probably rolling circle, phase of replication is controlled by the bacterial general recombination system.