TOPOLOGICAL SELECTIVITY IN XER SITE-SPECIFIC RECOMBINATION

The product topology of Xer-mediated site-specific recombination at pl asmid sites has been determined. The product of deletion at pSC101 psi is a right-handed antiparallel 4-noded catenane. The ColE1 cer deleti on product has an identical topology, except that only one pair of str ands is exchanged. These specific product topologies imply that the pr oductive synaptic complex and the strand exchange mechanism have fixed topologies. Further analysis suggests that synapsis traps exactly thr ee negative supercoils between recombining sites, and that strand exch ange introduces a further negative topological node in the deletion re action. We present a model in which the requirement for a specific syn aptic stucture, with two recombination sites interwrapped around the a ccessory proteins ArgR and PepA, ensures that recombination only occur s efficiently between directly repeated sites on the same DNA molecule .