Reactivation of insertionally inactivated shiga toxin 2 genes of Escherichia coli O157 : H7 caused by nonreplicative transposition of the insertion sequence

IS1203v is an insertion sequence which has been found in inactivated Shiga toxin 2 genes of Escherichia coli O157:H7. We analyzed the transpositional mechanism of IS1203v in order to investigate whether the Shiga toxin 2 gene s inactivated by IS1203v could revert to the wild type. When the transposas e activity of IS1203v was enhanced by artificial frameshifting, IS1203v was obviously excised from the Shiga toxin 2 gene in a circular form, The IS12 03v circle consisted of the entire IS1203v, but an extra 3-bp sequence (ATC ) intervened between the 5' and 3' ends of IS1203v, The extra 3-bp sequence was identical to a direct repeat which was probably generated upon inserti on. Moreover, we detected the Shiga toxin 2 gene with a precise excision of IS1203v, In the wild-type situation, the transposition products of IS1203v could be observed by PCR amplification, These results show that IS1203v ca n transpose in a nonreplicative manner and that the Shiga toxin gene inacti vated by this insertion sequence can revert to the wild type.