Mutants of the gamma delta resolvase: a genetic analysis of the recombination function

The resolvase protein encoded by the gamma delta transposon has two functions. It catalyzes a site-specific recombination, and it negatively regulates the expression of two transposon genes. Both functions involve the action of resolvase at the res site. To define regions of resolvase that are involved specifically in the recombination reaction, we have isolated and characterized mutants that are defective in cointegrate resolution but retain the ability to bind to res (as measured by regulatory activity). Nine independent mutants were found to contain six different amino acid substitutions among just four distinct residues. The altered residues all lie within the 140 amino acid amino-terminal domain of resolvase and fall within two clusters of amino acids that are highly conserved in other related recombinases. The regulatory properties of the mutants suggest that one of these clusters may be involved in the interaction of the catalytic domain with the crossover site.