Analysis of two genes encoding heat-labile toxins and located on a single Ent plasmid from Escherichia coli

A clinical isolate of Escherichia coli harbored two copies of the heat-labile toxin (LT)-encoding gene (elt) on a 157-kb plasmid. The arrangement of the gene copies is different from the cholera toxin-encoding gene duplication described for some strains of Vibrio cholerae. The nucleotide sequences of the elt alleles are not identical (differing by 2 bp) and the duplicated region flanking the alleles extends 314 bp on one side and 1122 bp on the other side of each copy. Different partial copies of IS600 were identified 280 bp 3' to the stop codon of each allele. Partial and complete copies of other IS were also identified near the elt alleles. The data suggest that the regions surrounding the genes are hot spots for IS which would account for the observed heterogeneity in DNA flanking elt genes.