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. These pairs were first described on plasmids where they regulate plasmid maintenance. However, recent discoveries have found novel type I loci, with no homology to plasmid sequences, in the chromosome of Escherichia coli and closely related species. The Ibs-Sib, ShoB-OhsC and Zor-Orz loci are examples of these new 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. Herein, I discuss how these sRNA antitoxins prevent toxin production and how the distribution of these loci across species may be providing insights into their true function.
Keywords: antisense RNA; sRNA; small protein; toxin; toxin-antitoxin.