The emergence of multiple antibiotic resistant bacteria is a major global health problem. A key contributor to the spread of resistance is mobile genetic elements, including transposons. The res hunter family of transposons is of particular interest as one subgroup of its members contains an integron (int) module which can serve as a gene capture and expression system and can facilitate accumulation and expression of drug-resistance cassettes. The other subgroup contains a Hg(II)-resistance module in place of the integron. Transposons that belong to this family exhibit an insertional and orientational specificity for a single target in plasmid RP1 located within the resolution (res) site of the par operon in the presence of its cognate resolvase (ParA). Tn502 a member of the Hg(II)-resistance res hunter family, contains a distinctive transposition module (tniABQ-res-tniR & tniM) and differs from other members as it is able to transpose in the absence of the preferred site at a reduced frequency.
The transposition mechanism of these transposons is not well understood. In this study, we constructed deletion mutants of each tni gene revealing that deletions of tniA, tniB or tniQ completely abolished transposition whilst deletion of tniM reduced transposition frequency. To further elucidate the transpositional mechanism of Tn502, we expressed the following proteins, TniA, TniB, TniQ, TniM and ParA and performed protein-protein interaction assays. Our results reveal a complex array of interactions among the Tni proteins and that all Tni proteins and the external ParA resolvase are involved in the transposition mechanism. This study suggests that TniQ is likely to be a scaffold protein in the transposition complex and an external resolvase (ParA) has a pivotal role in transposition to the preferred site while TniM enhances the transpositional activity. Elucidating the molecular mechanisms of res hunter family will inform efforts to control the spread of antibiotic resistance.