SUMMARY: Phages coding for production of Vero cytotoxins VT1 or VT2 in strains of serotype O157.H7 or O157. H were morphologically indistinguishable. Their genome size and restriction enzyme digests of the phage DNA were similar. These phages were clearly different in these respects from a VT1-encoding phage isolated from a strain of O26. H11 (H19). However the VT1 region cloned from the phage originating in the O157.H7 strain was identical to the VT1 region previously cloned from the phage carried by H19. Sequences encoding VT2 that were cloned from the phage in O157. H have been mapped and the VT2 region identified by transposon insertion. The cloned regions coding for VT1 or VT2 production had no similarities in the presence of restriction enzyme sites over a distance of about 2 kb, and two VT1-specific probes spanning a region of about 1·4 kb did not hybridize under stringent conditions with cloned VT2 DNA. A 2 kb cll fragment contained the VT2 genes but hybridized to VT1-encoding phages and recombinant plasmids via flanking phage DNA. A 0·85 kb l fragment was a specific probe for VT2 sequences and did not hybridize under stringent conditions to phages or plasmid recombinants encoding VT1.


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