Oxidative Stress in Shiga Toxin Production by Enterohemorrhagic Escherichia coli

Figure 1

Schematic map of a Shiga toxin-converting phage genome. At the top of (a), regions bearing genes for particular phage functions are shown (as they appear in a prophage). The region containing genes involved in regulation of phage development, DNA replication, Shiga toxin production, and cell lysis is enlarged and shown in more detail. Major transcripts are shown by arrows, with arrowheads demonstrating directionality of transcription, and promoters marked by short vertical lines at the beginning of transcripts. Terminators are marked by vertical lines crossing the transcript lines. The cI repressor binds to , , and operator sites, repressing and promoters and stimulating its own promoter . When DNA is damaged, single stranded DNA (ssDNA) fragments appear which are recognized by RecA protein. This activates RecA to switch to the form, able to stimulate self-cleavage by the cI repressor. Inactivated cI can no longer repress and and is not activated. This leads to effective transcription from and , prophage excision, and expression of vast majority of phage genes, including those coding for Shiga toxin. (b) represents a similar mechanism leading expression of the SOS regulon which under normal growth conditions is repressed by the LexA protein. Phage cI repressor resembles LexA; thus under conditions of the SOS response, induction of the prophage occurs.

(a)

(b)