Involvement of DNA replication in phage lambda Red-mediated homologous recombination

Crosses between a non-replicating linear bacteriophage lambda chromosome and a replicating plasmid bearing a short cloned segment of lambda DNA were monitored by extracting DNA from infected cells, and analysing it via restriction endonuclease digestion and Southern blots. Recombinant formation resulting from the action of the Red homologous recombination system, observed directly in this way, was found to be fast, efficient, independent of the bacterial recA function and highly dependent upon replication of the target plasmid. These features of the experimental system faithfully model Red-mediated recombination in a lytically infected cell in which phage DNA replication is occurring. Neither of the previously established mechanisms by which the Red system can operate – strand annealing or strand invasion – accounts well for these findings. A third mechanism, replisome invasion, involving replication directly in the recombination mechanism, is invoked as an alternative.