Mutants of Tn3 resolvase which do not require accessory binding sites for recombination activity

Tn3 resolvase promotes site-specific recombination between two res sites, e ach of which has three resolvase dimer-binding sites. Catalysis of DNA-stra nd cleavage and rejoining occurs at binding site I, but binding sites II an d III are required for recombination, We used an ill who screen to detect r esolvase mutants that were active on res sites with binding sites II and II I deleted (that is, only site I remaining). Mutations of amino acids Asp102 (D102) or Met103 (M103) were sufficient to permit catalysis of recombinati on between site I and a full res, but not between two copies of site I. A d ouble mutant resolvase, with a D102Y mutation and an additional activating mutation at Glu124 (E124Q), recombined substrates containing only two copie s of site I, ill vivo and lit vitro, In these novel site Ixsite I reactions , product topology is no longer restricted to the normal simple catenane, i ndicating synapsis by random collision. Furthermore, the mutants have lost the normal specificity for directly repeated sites and supercoiled substrat es; that is, they promote recombination between pairs of res sites in linea r molecules, or in inverted repeat in a supercoiled molecule, or in separat e molecules.