The Operator and Early Promoter Region of the Shiga Toxin Type 2-Encoding Bacteriophage 933W and Control of Toxin Expression

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Abstract
The genes encoding Shiga toxin (Stx), the major virulence factor of Shiga toxin-producing Escherichia coli, are carried in the genomes of bacteriophages that belong to the lambdoid family of phages. Previous studies demonstrated that induction of prophages encoding stx significantly enhances the production and/or release of Stx from the bacterium. Therefore, factors that regulate the switch between lysogeny and lytic growth, e.g., repressor, operator sites, and associated phage promoters, play important roles in regulating the production and/or release of Stx. We report the results of genetic and biochemical studies characterizing these elements of the Stx-encoding bacteriophage 933W. Like λ, 933W has three operator repeats in the right operator region (OR), but unlike λ and all other studied lambdoid phages, which have three operator repeats in the left operator region (OL), 933W only has two operator repeats in OL. As was observed with λ, the 933W OR and OL regions regulate transcription from the early PR and PL promoters, respectively. A lysogen carrying a 933W derivative encoding a noncleavable repressor fails to produce Stx, unlike a lysogen carrying a 933W derivative encoding a cleavable repressor. This finding provides direct evidence that measurable expression of the stx genes encoded by a 933W prophage requires induction of that prophage with the concomitant initiation of phage gene expression.