Analysis of gamma delta resolvase mutants in vitro: evidence for an interaction between serine-10 of resolvase and site I of res.

The resolvase encoded by the transposon gamma delta mediates a site-specific recombination between the two copies of gamma delta in a cointegrate molecule to yield the final products of transposition. Several mutants of resolvase that lack recombinational activity have been isolated previously, and one of these, which has a serine-to-leucine change at position 10, we have characterized in vitro. We also have constructed a serine-to-cysteine change at position 10 by in vitro mutagenesis and have analyzed this mutant protein in vitro. We find that the cysteine-10 mutant is defective in recombinational activity but binds to the recombinational site, res, similarly to wild-type, as assayed by gel electrophoresis of the protein-DNA complexes. In contrast, the leucine-10 mutant has a specific defect in complex formation with site I, which contains the recombinational crossover point, although it can bind and provide the ancillary functions of resolvase at sites II and III. It has been shown that a phosphoserine linkage is made to the DNA during recombination; because serine-10 is absolutely conserved amongst the family of homologous site-specific recombination proteins, it is a good candidate for the active-site serine. The properties of these resolvase mutants with substitutions at position 10 are consistent with this hypothesis and suggest that serine-10 is in close contact with the DNA at site I.