Topology of Xer recombination on catenanes produced by lambda integrase

Xer site-specific recombination at the psi site from plasmid pSC101 display s topological selectivity, such that recombination normally occurs only bet ween directly repeated sites on the same circular DNA molecule. This intram olecular selectivity is important for the biological role of psi, and is im posed by accessory proteins PepA and ArcA acting at accessory DNA sequences adjacent to the core recombination site. Here we show that the selectivity for intramolecular recombination at psi can be bypassed in multiply interl inked catenanes. Xer site-specific recombination occurred relatively effici ently between antiparallel psi sites located on separate rings of right-han ded torus catenanes containing six or more nodes. This recombination introd uced one additional node into the catenanes. Antiparallel sites on four-nod ed right-handed catenanes, the normal product of Xer recombination at psi, were not recombined efficiently. Furthermore, parallel psi sites on right-h anded torus catenanes were not substrates for Xer recombination. These find ings support a model in which psi sites are plectonemically interwrapped, t rapping a precise number of supercoils that are converted to four catenatio n nodes by Xer strand exchange. (C) 1999 Academic Press.