Characterization of In53, a Class 1 Plasmid- and Composite Transposon-Located Integron ofEscherichia coliWhich Carries an Unusual Array of Gene Cassettes

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Abstract
Further characterization of the genetic environment of the gene encoding theEscherichia coliextended-spectrum β-lactamase,blaVEB-1, revealed the presence of a plasmid-located class 1 integron, In53, which carried eight functional resistance gene cassettes in addition toblaVEB-1. While theaadBand thearr-2gene cassettes were identical to those previously described, the remaining cassettes were novel: (i) a novel nonenzymatic chloramphenicol resistance gene of thecmlAfamily, (ii) aqacallele encoding a member of the small multidrug resistance family of proteins, (iii) a cassette,aacA1b/orfG,which encodes a novel 6′-N-acetyltransferase, and (iv) a fused gene cassette,oxa10/aadA1, which is made of two cassettes previously described as single cassettes. In addition,oxa10andaadA1genes were expressed from their own promoter sequence present upstream of theoxa10cassette.arr-2coded for a protein that shared 54% amino acid identity with the rifampin ADP-ribosylating transferase encoded by thearr-1gene fromMycobacterium smegmatisDSM43756. While inM. smegmatis, the main inactivated compound was 23-ribosyl-rifampin, the inactivated antibiotic recovered fromE. coliculture was 23-O-ADP-ribosyl-rifampin. The integrase gene of In53 was interrupted by an IS26insertion sequence, which was also present in the 3′ conserved segment. Thus, In53 is a truncated integron located on a composite transposon, named Tn2000, bounded by two IS26elements in opposite orientations. Target site duplication at both ends of the transposon indicated that the integron likely was inserted into the plasmid through a transpositional process. This is the first description of an integron located on a composite transposon.