Xis Protein Binding to the Left Arm Stimulates Excision of Conjugative Transposon Tn 916

Abstract

ABSTRACT Tn 916 and related conjugative transposons are clinically significant vectors for the transfer of antibiotic resistance among human pathogens, and they excise from their donor organisms using the transposon-encoded integrase ( Tn 916 Int) and excisionase ( Tn 916 Xis) proteins. In this study, we have investigated the role of the Tn 916 Xis protein in stimulating excisive recombination. The functional relevance of Tn 916 Xis binding sites on the arms of the transposon has been assessed in vivo using a transposon excision assay. Our results indicate that in Escherichia coli the stimulatory effect of the Tn 916 Xis protein is mediated by sequence-specific binding to either of its two binding sites on the left arm of the transposon. These sites lie in between the core and arm sites recognized by Tn 916 Int, suggesting that the Tn 916 Xis protein enhances excision in a manner similar to the excisionase protein of bacteriophage λ, serving an architectural role in the stabilization of protein-nucleic acid structures required for strand synapsis. However, our finding that excision in E. coli is significantly enhanced by the host factor HU, but does not depend on the integration host factor or the factor for inversion stimulation, defines clear mechanistic differences between Tn 916 and bacteriophage λ recombination.