p39R861-4, a type 2 A/C2 plasmid carrying a segment from the A/C1 RA1

11 March 2016

The largest plasmid in the strain 39R861, which is used as a plasmid size standard, was recovered by conjugation and sequenced to determine its exact size. Plasmid p39R861-4 transferred at high frequency. Though reported to be the A/C1 plasmid RA1, p39R861-4 is a 155794 bp Type 2 A/C2 plasmid in which a 39 kb segment derived from RA1 that includes a relative of the RA1 resistance island replaces 26.5 kb of the Type 2 backbone. p39R861-4 includes a single copy of IS10 and two resistance islands with a CR2-sul2 region in each of them. The 84 kb of backbone between the resistance islands is inverted relative to other known A/C plasmids and this inversion has arisen via recombination between the CR2-sul2 regions that are inversely oriented. The resistance islands prior to inversion were one related to but longer than that found in RA1, and a form of the ARI-B island identical to one found in the A/C2 plasmid R55. They contain genes conferring resistance to tetracycline (tetA(D)), sulphonamides (sul2) and florfenicol and chloramphenicol (floR). The tet(D) determinant is flanked by two IS26 in a transposon-like structure named Tntet(D). Both resistance islands contain remnants of the two ends of the integrative element GIsul2, consistent with the sul2 gene being mobilized by GIsul2 rather than by CR2.