Restoration and functional analysis of the SGI1 resolution system – SGI1 multimers are eliminated by the reactivated resolution

SGI1 and the related elements that are specifically mobilized by the IncA- and IncC-family plasmids are efficient agents in the dissemination of multi-resistance in Gammaproteobacteria. The In104 gene cluster responsible for multi-resistance in these genomic islands is generally integrated into the conserved SGI1 backbone, upstream of a resolvase gene, presumably by res-hunting transposition events. In this work we demonstrate that precise deletion of In104 cluster with one copy of its flanking direct repeats restores the res site belonging to the resolvase gene, leading to an active Tn3-like resolution system. The entire res site and its subsites have been identified and the resolvase activity has been demonstrated in plasmid-based recombination assays. The major effect of the reactivated resolution system seems to be the rapid elimination of SGI1 multimers in SGI1 transconjugants. It has been shown that wt SGI1-C and the resolvase-deleted SGI1ΔIn104 variant produce significantly more concatemers, which persist for longer periods in transconjugants, than SGI1ΔIn104 with a functional resolution system. High prevalence of inactivated res systems among the multidrug-resistant members of SGI1-family suggests that the ability to produce more and more stable multimers in SGI1 transconjugants may confer evolutionary advantage to these elements.