Site-specific recombination by Tn3 resolvase: topological changes in the forward and reverse reactions

Site-specific recombination catalyzed by Tn3 resolvase proceeds with a linkage change, delta Lk, of +4 in the forward resolution reaction and -4 in the catenane fusion reverse reaction. The reverse reaction occurs only at low superhelical densities and gives unknotted circular products, consistent with plectonemic and not solenoidal wrapping of the two recombination sites. The strand exchange topologies are consistent with a mechanism in which resolvase cleaves all four DNA strands and religates them after a 180 degrees rotation of two duplex partners in a right-handed sense for the "forward" reaction, and in a left-handed sense for the "reverse" action. This could be achieved by a 180 degrees rotation of two resolvase subunits within a tetramer with D2 symmetry; we suggest that a different symmetry applies to phage lamda integrase catalysis.