Complex multiple antibiotic and mercury resistance region derived from the r-det of NR1 (R100)

The sequence of the 45.2-kb multidrug and mercury resistance region of pRMH760, a large plasmid from a clinical isolate of Klebsiella pneumoniae collected in 1997 in Australia, was completed. Most of the modules found in the resistance determinant (r-det), or Tn2670, region of NR1 (also known as R100), isolated from a Shigella flexneri strain in Japan in the late 1950s, were present in pRMH760 but in a different configuration. The location was also different, with the Tn2670-derived region flanked by the transposition module of Tn1696 and a mercury resistance module almost identical to one found in the plasmid pDU1358. This arrangement is consistent with a three-step process. First, the r-det was circularized via homologous recombination between the IS1 elements and reincorporated at a new location, possibly in a different plasmid, via homologous recombination between the 5'-conserved (5'-CS) or 3'-CS of the In34 integron in the r-det and the same region of a second class I integron in a Tn1696 relative. Subsequently, resolvase-mediated recombination between the res sites in the r-det and a second mercury resistance transposon removed one end of the Tn1696-like transposon and part of the second transposon. Other events occurring within the r-det-derived portion have also contributed to the formation of the pRMH760 resistance region. Tn2 or a close relative that includes the bla(TEM-1b) gene had moved into the Tn21 mercury resistance module with subsequent deletion of the adjacent sequence, and all four 38-bp inverted repeats corresponding to Tn21 family transposon termini have been interrupted by an IS4321-like element.