p39R861-4, A Type 2 A/C2 Plasmid Carrying a Segment from the A/C1 Plasmid RA1

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. Although reported to be the A/C 1 plasmid RA1, p39R861-4 is a 155794-bp Type 2 A/C 2 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 IS 10 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 through recombination between the CR2- sul2 regions that are inversely oriented. The two resistance islands present before this inversion occurred were one related to but longer than that found in RA1, and one that is a form of the ARI-B island and identical to ARI-B in the A/C 2 plasmid R55. They contain genes conferring resistance to tetracycline ( tetA (D)), sulfonamides ( sul2 ), and florfenicol and chloramphenicol ( floR ). The tet (D) determinant is flanked by two IS 26 in a transposon-like structure named Tn tet (D). Both resistance islands contain remnants of the two ends of the integrative element GI sul2 , consistent with the sul2 gene being mobilized by GI sul2 rather than by CR2.