Shiga toxins even when different are encoded at identical positions in the genomes of related temperate bacteriophages

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Abstract
The nucleotide sequence of an 11,142-bp region including the stx 2 operon in the genome of the temperate bacteriophage 933W in the EDL933 strain of Escherichia coli O157 was determined and compared to the respective regions derived from other lambdoid bacteriophages. In phage 933W, a region of ORFs interlinked by overlapping start-stop codons (ATGA) was detected preceding the toxin gene. These ORFs show a high degree of sequence identity to genes of the nin region of phage λ. Immediately downstream of these nin genes we identified an ORF that may code for an antiterminator similar to the λ Q protein. It is concluded that toxin expression is directly associated with the initiation of cell lysis. Downstream of the stx 2 operon we identified an ORF that is homologous to the holin gene S of bacteriophage PA-2. PCR primers were designed, which, based on a comparison of the phage sequences, appeared to be common to both stx 1 - and stx 2 -harbouring phages. However, only seven of the 22 STEC strains investigated from serogroups O157, O26, O103 and O111 yielded the expected PCR amplification product. The data reported here may be useful in developing new strategies for inhibiting the expression of Stx and for developing universal diagnostic primers for use in tracking the origin and evolution of Shiga toxins and the phages that carry them.

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