The operator-early promoter regions of Shiga-toxin bearing phage H-19B

Genes (stx) encoding Shiga toxins (Stx), major virulence factors in some pathogenic strains of Escherichia coli (STEC), are located in prophages of the lambda family. Agents that induce prophages lead to high levels of Stx, suggesting a role for the prophage in stx expression. Activation of the phage regulatory cascade has been shown to contribute to Stx production and release. Therefore, repressor-operator interactions that maintain prophage repression appear important in regulating expression of a major bacterial virulence factor. To determine if the operators of an stx-bearing phage have distinctive features, we characterized the operator regions of H-19B, a lambdoid phage carrying stx1 genes. H-19B mutants that grow in the presence of repressor (classically called virulent mutants) were selected and the mutations definitively identified the operators. The H-19B operators, as those in other lambdoid phages, comprise variations of an inverted repeat. Four repeats were identified in O-R rather than the three found in each of the operators of other lambdoid phages. Primer extensions identified the transcription start sites of P-R and P-RM, the two promoters in O-R regulated by repressor.