1.c.135. The ShoB Pore-forming Toxin (ShoB) Family
Type I toxin-antitoxin loci consist of two genes: one encodes a small, toxic protein and the second encodes a small RNA antitoxin that represses toxin gene expression (Fozo 2012). These pairs were first described on plasmids where they regulate plasmid maintenance. However, type I loci, with no homology to plasmid sequences, are present in the chromosome of Escherichia coli and closely related species. The Ibs-Sib, ShoB-OhsC and Zor-Orz loci are examples of these loci. For these toxic proteins, much more is known about how their expression is regulated than their biological function. Although all are found in E. coli and closely related bacteria, there is great variation among species as to which loci they possess. Fozo 2012 discussed how these sRNA antitoxins prevent toxin production and how the distribution of these loci across species may be providing insights into their true function.