Ribosome-inactivating proteins in bacteria

Ribosome-inactivating proteins (RIPs), a group of biological toxins that depurinate a critical residue within the 28S rRNA, exhibit one of the most potent and elegant mechanisms of cellular toxicity. Examples of RIPs include ricin and Shiga toxin, which are amongst the most notorious toxins produced by plants and bacteria, respectively. Here, we discuss RIPs in the Bacteria domain. We review the biology of Shiga toxin and its role as a transformational virulence factor for some of the most virulent Escherichia coli and Shigella pathotypes, as well as recent work identifying an assortment of RIP toxins deployed by Spiroplasma endosymbionts to protect their host from parasitic attack. We further shed light on the expansive but poorly understood arsenal of proteins with the conserved RIP functional domain that can be found in the genomes of diverse bacterial species, including proteins with complex domain architectures thought to occur only in eukaryotic RIP toxins. Based on the impressive arsenal of RIPs found in Streptomyces, as well as the ubiquitous presence of this genus in the niches where many of the other organisms that produce RIPs are found, we speculate that Actinomycetes such as Streptomyces might have been the evolutionary origin of the RIP domain and hypothesize that Streptomyces has played a central role in the spread of RIPs throughout the biosphere.