Complete nucleotide sequence of the prophage VT1-Sakai carrying the Shiga toxin 1 genes of the enterohemorrhagic Escherichia coli O157 : H7 strain derived from the Sakai outbreak

Shiga toxins 1 and 2 (Stx1 and Stx2) are encoded by prophages lysogenized i n enterohemorrhagic Escherichia coli (EHEC) O157:H7 strains. Lytic growth o f the phage particles carrying the stx1 genes (stx1A and stx1B) of the EHEC O157:H7 strain RIMD 0509952, which was derived from the Sakai outbreak in 1996 in Japan, was induced after treatment with mitomycin C, but the plaque formation of the phage was not detected. We have determined the complete n ucleotide sequence of the prophage VT1-Sakai. The integration site of the p rophage was identified within the yeh V gene at 47.7 min on the chromosome. The stx1 genes were downstream of the Q gene in the prophage genome, sugge sting that their expression was regulated by the Q protein, the regulator o f the late gene expression of the phage, which is similar to that of the st x1 or stx2 genes carried by the lambdoid phages reported previously. The se quences of the N gene and its recognition sites, nutL and nutR, were not ho mologous to those of the phages carrying the six genes thus far reported, b ut they were very similar to those of bacteriophage phi 21. The sequences o f the repressor proteins, CI and Cro, that regulate expression of the early genes had low similarities with those of the known repressors of other pha ges, and their operator sequences were different from any sequence reported . These data suggest that multiple genetic recombination among bacteriophag es with different immunities took place to generate the prophage VT1-Sakai. Comparison between the sequences of VT1-Sakai and lambda suggests that the ancestor of VT1-Sakai was produced by illegitimate excision, like lambda g al and bio phages. (C) 2000 Elsevier Science B.V. All rights reserved.