The linkage change of a knotting reaction catalysed by Tn3 resolvase

Tn3 resolvase catalyses recombination between a pair of identical res sites, directly repeated on a supercoiled plasmid molecule. The normal in vitro product is a two-noded catenane. A plasmid with a single base substitution at one res site is converted to non-recombinant knots by resolvase; the first major product is a four-noded knot. The linkage change, delta Lk, on formation of the four-noded knot is +4. The formation of the observed knots from the mutant substrate, and this value of delta Lk were predicted by and in agreement with a "simple rotation" model for catalysis of strand exchange by resolvase. A number of alternative mechanisms for knotting by resolvase are inconsistent with the observed topological changes.