Genetic organization of antibiotic resistance genes (aac(6')-Ib, aadA, and oxa9) in the multiresistance transposon Tn1331.

The multiresistance transposon Tn1331 has two 520 bp direct repeats flanking a DNA region similar to the variable portion of the integrons. This region harbors three antibiotic resistance genes, aac(6')-Ib, aadA, and a beta-lactamase gene which encodes an enzyme that was characterized as an oxacillinase-carbenicillinase and was named OXA-9. Within the direct repeat located upstream of these genes there is a copy of the TEM beta-lactamase promoter. Determination of ampicillin resistance levels of Escherichia coli harboring various recombinant clones and an insertion mutant showed that all three genes are transcribed from this promoter in a polycistronic mRNA. These results also indicated that the oxa9 gene possesses another functional promoter, located immediately upstream of this gene. The transcription start site of the mRNA driven by this promoter was identified by primer extension. Within the DNA sequence harboring all three antibiotic resistance genes it is possible to distinguish insert units as defined before in the variable region of the integrons (R. Hall, D. Brookes, and H. W. Stokes (1991) Mol. Microbiol. 5, 1941-1959).