Recombinational circularization of Salmonella phage P22 DNA.

The development of phage P22 following infection involves a mandatory recombination step, the timing of which is the same in both the lytic and lysogenic pathways. Covalently circular molecules of phage DNA were identified by alkaline sucrose sedimentation of infected cell lysates. The time of appearance of these structures corresponds to the time of the essential recombination step. On infection of retcells, the action of the P22 recombination function, erf, is necessary for the formation of covalent circles. The amount of covalently circular parental phage DNA observed in lytically infected cells is lower than are the levels found in cells destined for lysogeny. Evidence is presented that the lower levels in the former case are due to the conversion of covalently circular molecules to some other structure by a replicational process.