Half-att site substrates reveal the homology independence and minimal protein requirements for productive synapsis in lambda excisive recombination.

The early events in site-specific excisive recombination were studied with phage lambda half-att sites that have no DNA to one side of the strand exchange region; they carry a single core-type integrase binding site and either P or P' arm flanking DNA. These half-attR and half-attL sites exhibit normal properties for the initial (covalent) top-strand transfer and form stable intermediates independent of later steps in the reaction. With these novel substrates we show that Xis specifically promotes the first strand exchange and that attL enhances Int cleavage at the top-strand site of attR. It is also shown that synapsis and initial strand transfers do not require DNA-DNA pairing but are mediated by protein-protein and protein-DNA interactions. These involve the two top-strand Int binding sites (required for the first strand exchange) and, in addition, one of the two bottom-strand sites (C') responsible for the second strand exchange.