Almost all clinical enterohaemorrhagic (EHEC) O157: H7 isolates harbour a large virulence plasmid designated pO157. In this study, pO157 of EHEC O157:H7 reference strain EDL 933 was characterized at the molecular level. A restriction map was constructed by using seven restriction enzymes, with appropriate gel electrophoretic and hybridization methods. The molecular size of pO157 was determined to be 93.6 kb. By sequencing the DNA region extending in the 3′-direction of the previously described EHEC and genes, two further genes were discovered and analysed; these were termed EHEC and EHEC The newly discovered genes together with the EHEC and genes constitute a typical RTX (Repeats in ToXin) determinant (EHEC operon) with the gene order The map position of the operon was determined by hybridization experiments. Analysis of a DNA fragment carrying the downstream flanking region of the EHEC operon revealed an open reading frame which was highly homologous to of RepFIB, a basic replicon of IncF plasmids. It was located close to the EHEC operon.


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