Role for a Phage Promoter in Shiga Toxin 2 Expression from a Pathogenic Escherichia coli Strain

Shiga toxins (Stxs), encoded by the stxA and stxB genes, are important contributors to the virulence of Escherichia coli O157:H7 and other Stx-producing E. coli (STEC) strains. The stxA and stxB genes in STEC strains are located on the genomes of resident prophages of the λ family immediately downstream of the phage late promoters ( p R′ ). The phage-encoded Q proteins modify RNA polymerase initiating transcription at the cognate p R′ promoter which creates transcription complexes that transcend a transcription terminator immediately downstream of p R′ as well as terminator kilobases distal to p R′ . To test if this Q-directed processive transcription plays a role in stx 2 AB expression, we constructed a mutant prophage in an O157:H7 clinical isolate from which p R′ and part of Q were deleted but which has an intact p Stx, the previously described stx 2 AB -associated promoter. We report that production of significant levels of Stx2 in this O157:H7 isolate depends on the p R′ promoter. Since transcription initiating at p R′ ultimately requires activation of the phage lytic cascade, expression of stx 2 AB in STEC depends primarily on prophage induction. By showing this central role for the prophage in stx 2 AB expression, our findings contradict the prevailing assumption that phages serve merely as agents for virulence gene transfer.