Characterization of In53, a class 1 plasmid- and composite transposon-located integron of Escherichia coli which carries an unusual array of gene cassettes

Further characterization of the genetic environment of the gene encoding th e Escherichia coli extended-spectrum beta -lactamase, bla(VEB-1), revealed the presence of a plasmid-located class I integron, In53, which carried eig ht functional resistance gene cassettes in addition to bla,,. While the aad B and the arr-2 gene cassettes were identical to those previously described , the remaining cassettes were novel: (i) a novel nonenzymatic chlorampheni col resistance gene of the cmlA family, (ii) a qac allele encoding a member of the small multidrug resistance family of proteins, (iii) a cassette, aa cAlb/orfG, which encodes a novel 6'-N-acetyltransferase, and (iv) a fused g ene cassette, oxa10/aadA1, which is made of two cassettes previously descri bed as single cassettes. In addition, oxa10 and aadA1 genes were expressed from their own promoter sequence present upstream of the oxa10 cassette. ar r-2 coded for a protein that shared 54% amino acid identity with the rifamp in ADP-ribosylating transferase encoded by the arr-1 gene from Mycobacteriu m smegmatis DSM43756. While in M. smegmatis, the main inactivated compound was 23-ribosyl-rifampin, the inactivated antibiotic recovered from E. coil culture was 23-O-ADP-ribosyl-rifampin. The integrase gene of In53 was inter rupted by an IS26 insertion sequence, which was also present in the 3' cons erved segment. Thus, In53 is a truncated integron located on a composite tr ansposon, named Tn2000, bounded by two IS26 elements in opposite orientatio ns. Target site duplication at both ends of the transposon indicated that t he integron likely was inserted into the plasmid through a transpositional process. This is the first description of an integron located on a composit e transposon.