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The IncC plasmid pB2-1, from a Klebsiella pneumoniae isolate recovered in Brisbane prior to 1995, belongs to a subtype of type 1 IncC plasmids, here designated type 1a, that includes those carrying carbapenem resistance genes such as bla NDM and bla KPC . pB2-1 carries a 2358 bp deletion in the rhs1 gene found in four other type 1a IncC plasmids. pB2-1 confers resistance to ampicillin, gentamicin, kanamycin, neomycin, tobramycin, sulfamethoxazole, tetracycline and trimethoprim. It transferred at a frequency of 4.7 × 10 − 3 transconjugants per donor, similar to that of another type 1a plasmid pDGO100 but ten-fold lower than for its closest relative pRMH760. This difference may be due to a single amino acid substitution in TraL. pB2-1 has an ISEc52 insertion in the dsbC gene, demonstrating that dsbC is not essential for transfer. pB2-1 lacks the ARI-B insertion and hence the sul2 gene. The resistance genes sul1 , dfrA10 , aphA1a , bla TEM , aadB , and tetA (B) are all in the ARI-A island, in a configuration that has evolved from ARI-A of pRMH760 in two steps. A 10.3 kb segment extending from the catA1 gene to the end of pDU mer module was lost via homologous recombination between two copies of IS 4321 . In addition, a 5.3 kb segment extending from IS 1326 to the left end of Tn 4352 B was replaced with an 18.7 kb tet (B)-containing segment bounded on one end by IS 1 and on the other by IS 26 . The IS 26 -bounded transposon Tn 4352 B was shown to be stable in K. pneumoniae in contrast to the high instability observed in E. coli . [ABSTRACT FROM AUTHOR]