Spontaneous tandem amplification and deletion of the Shiga toxin operon inShigella dysenteriae 1

Only one species of Shigella, Shigella dysenteriae 1, has been demonstrated to produce Shiga toxin (Stx). Stx is closely related Po the toxins produce d by Shiga toxin-producing Escherichia coli (STEC). In STEC, these toxins a re often encoded on lambdoid bacteriophages and are major virulence factors for these organisms. Although the bacteriophage-encoded stx genes of STEC are highly mobile, the stx genes in S. dysenteriae 1 have been believed to be chromosomally encoded and not transmissible. We have located the toxin g enes of S. dysenteriae 1 to a region homologous to minute 30 of the E. coli chromosome, within a 22.4 kbp putative composite transposon bracketed by I S600 insertion sequences. This region is present In all the a dysenteriae 1 strains examined. Tandem amplification occurs via the flanking insertion s equences, leading to increased toxin production. The global regulatory gene , fnr, is located within the stx region, allowing deletions of the toxin ge nes to be created by anaerobic growth on chlorate-containing medium. Deleti ons occur by recombination between the flanking IS600 elements. Lambdoid ba cteriophage genes are found both upstream and within the region, and we dem onstrate the lysogeny of Shigella species with STEC bacteriophages. These o bservations suggest that S. dysenteriae 1 originally carried a Stx-encoding lambdoid prophage, which became defective due to loss of bacteriophage seq uences after IS element insertions and rearrangements. These insertion sequ ences have subsequently allowed the amplification and deletion of the stx r egion.