Lateral transduction is inherent to the life cycle of the archetypical Salmonella phage P22

Lysogenic induction ends the stable association between a bacteriophage and its host, and the transition to the lytic cycle begins with early prophage excision followed by DNA replication and packaging (ERP). This temporal program is considered universal for P22-like temperate phages, though there is no direct evidence to support the timing and sequence of these events. Here we report that the long-standing ERP program is an observation of the experimentally favored Salmonella phage P22 tsc(2)29 heat-inducible mutant, and that wild-type P22 actually follows the replication-packaging-excision (RPE) program. We find that P22 tsc(2)29 excises early after induction, but P22 delays excision to just before it is detrimental to phage production. This allows P22 to engage in lateral transduction. Thus, at minimal expense to itself, P22 has tuned the timing of excision to balance propagation with lateral transduction, powering the evolution of its host through gene transfer in the interest of self-preservation. During the transition from lysogeny (a stable association between a phage and its bacterial host) to the lytic cycle, prophage excision can be followed or preceded by DNA replication and packaging. Here, the authors show that prophage excision is delayed in Salmonella phage P22, thus allowing the packaging and transfer of large fragments of host DNA via lateral transduction.