The E. coli gene encoding heat stable toxin is a bacterial transposon flanked by inverted repeats of IS1.

Restriction endonuclease subclones of the Escherichia coli gene encoding the heat stable (ST) toxin exhibit a stem and loop structure similar to those seen in many procaryotic transposons. An EcoRI DNA fragment encoding tetracycline (Tc) resistance but no transposition functions was spliced into the ST gene in one of these subclones. By monitoring Tcr, we were able to show that the ST gene transposes. Restriction and DNA sequence data strongly suggest that the ST transposon, Tn 1681, is flanked by inverted repeats of IS1.