Direct involvement of IS26 in an antibiotic resistance operon.

The plasmid pBWH77, originally found in an isolate of Klebsiella pneumoniae, harbors a new antibiotic resistance operon containing two resistance genes transcribed from an IS26-hybrid promoter, as shown by nucleotide sequencing, mRNA mapping, and the effect of inserting a transcription terminator within the promoter-proximal gene. The nucleotide sequence of this region revealed that the operon (IAB) is made up of three sections that are closely related to previously described genetic elements. The -35 region of the promoter, together with the adjacent sequence, is identical to sequences of the IS26 element. One of the resistance genes, aphA7, which is located next to the hybrid promoter, confers assistance to neomycin and structurally related aminoglycosides. This aphA7 gene is highly homologous to aphA1 of Tn903, with five nucleotide differences. The second gene, blaS2A, encodes an evolved SHV-type beta-lactamase with a pI of 7.6 that confers resistance to the broad-spectrum cephalosporins cefotaxime and ceftizoxime. The deduced amino acid sequence of SHV-2A shows that amino acid 238 is a serine, a residue reported to confer resistance to cefotaxime. We discuss how the operon may have evolved by a combination of insertion sequence-mediated genetic rearrangements and acquisitive evolution. Using phylogenetic parsimony, we show that aphA7 in the IAB operon evolved from an ancestral form similar to aphA1 in Tn903 and that blaS2A evolved from an ancestral form similar to blaS1.