Uncoupling of the recombination and topoisomerase activities of the γδ resolvase by a mutation at the crossover point

Abstract

In several well-characterized site-specific recombination systems it has been shown that, for efficient recombination, the two recombining sites must have identical DNA sequences across the region between the staggered points of exchange. The precise DNA sequence of this overlap region, however, appears to be of little importance1–6 (with the exception of one position in the loxP site of bacteriophage PI (ref. 6)). In this report we characterize a mutant recombination site for the site-specific recombination enzyme γδ resolvase (encoded by the γδ transposon), in which the dinucleotide at the crossover point is changed from AT to CT. Our results indicate that identity of thfe two overlap regions is not sufficient for recombination. Although resolvase binds normally to the mutant site and induces the structural deformation characteristic of the wild-type recombination site, catalysis at the crossover point (cutting and rejoining of DNA strands) is effectively limited to just one of the two strands, allowing resolvase to act as a topoisomerase but not as a recombinational enzyme.