Determinants of correct res site alignment in site-specific recombination by Tn3 resolvase

In site-specific recombination reactions catalyzed by Tn3 resolvase, the right and left arms of the res site are always religated to the correct partner. This poses the problem of how resolvase aligns the two sites correctly for the cleavage/religation reaction. We show that the "accessory" binding subsites II and III of res are important for correct alignment of the adjoining crossover subsite (subsite I). Deletion of subsites II and III from one of the two res sites removes a barrier to recombination between incorrectly aligned crossover subsites. Correct alignment does not require any DNA sequence asymmetry in the crossover subsite, DNA supercoiling, or covalent linkage of the two res sites. Our results suggest that correct subsite I alignment is determined by local, resolvase-mediated interactions of subsites II and III of both partners, consistent with a current model of the synapse. Surprisingly, the topological selectivity for intramolecular resolution in a supercoiled substrate does not require subsites II and III in both recombination partners.