Evolution of Pseudomonas R-plasmids: consequences of Tn1 insertion and resultant partial diploidy to chromosome and Tra- R-plasmid mobilization

Abstract

Tn1 transposes from pRO161, a Tra- derivative of RP1, to P. aeruginosa sex factor FP2. The acquisition of Tn1 by FP2 results in its ability to mobilize pRO161 to other bacteria. Genetic evidence presented here suggests 2 sequential mechanisms. Initially, transposition of Tn1 results in trans-diploidy for the Tra+ and Tra- plasmids. This subsequently allows mobilization of the Tra- R-plasmid dependent on a host recombination mechanism. Transconjugants from this mating contain either stable cointegrate R-plasmids or aggregates resulting from dissociation of the cointegrates into a Tra+ and Tra- plasmid. These aggregates have lost at least part of Tn1 from their parent FP2::Tn1 component, but now they mobilize the Tra- R-plasmid from a recombination-deficient (Rec-) genetic background and from Rec+ donor strains. Transconjugants from these retransfer matings are aggregates. These results suggest a contribution of transposons to R-plasmid evolution and dissemination beyond the mere acquisition of resistance to a given antibiotic.