Architectural flexibility in lambda site‐specific recombination: three alternate conformations channel the attL site into three distinct pathways

Background :  In the phage lambda life cycle, the Integrase (Int) protein carries out recombination between two different sets of DNA substrates: attP and attB in integration, attL and attR in excision. In each case, the partners are very different in structure from each other and the recombination reaction between them is effectively irreversible. For comparison, we have studied the recombination mediated by Int between two identical attL sites. Both in vitro and in vivo , recombination between two attL sites can be mediated inefficiently by Int alone. But, while IHF can stimulate recombination 5–10‐fold in vivo (to the level of excision and integration), this stimulation is not observed under standard conditions in vitroResults :  We find that IHF can stimulate the in vitro recombination between two attL s that are modified to be defective in one of the high affinity binding sites for Int, P′1. With such substrates, the efficiency of IHF‐stimulated recombination is comparable to that seen in vivo . The requirements for this reaction distinguish it from other lambda recombination pathways, as does the performance of several mutant Int proteins. Recombination of attL sites on intracellular plasmids suggests that this pathway is effective in vivo , but that some unknown factor or condition permits it to operate on wild‐type as well as mutated attL sites. Conclusions :  The recombination pathway described in this work apparently uses a unique attL architecture, one which requires bending by IHF and is inhibited by Int bound at the P′1 site. In addition to demonstrating the architectural flexibity of the lambda system, this pathway should be a valuable resource for separating the basic requirements of strand exchange chemistry from the features which impart directionality.