Ribosome-targeting toxins typically recognize and attack their substrates directly. Here, we provide the functional characterization of SRLase1, a widespread member of a recently identified RNase superfamily, and show that it cleaves the sarcin-ricin loop (SRL) of 23S rRNA. Unlike previously described ribosome-targeting toxins, SRLase1 requires the host elongation factor Tu (EF-Tu) for activity. EF-Tu normally engages the ribosome as part of the aminoacyl-tRNA delivery cycle, but SRLase1 instead exploits this factor through a mechanism incompatible with canonical ternary-complex formation. This host-factor-assisted mechanism reveals a distinct strategy for translation inhibition, in which a toxin repurposes a conserved component of the elongation machinery to access a vulnerable ribosomal target. Homologs of SRLase1 are widespread across bacterial phyla and are frequently encoded in conflict-associated genomic loci. Together, these findings define a widespread family of SRL-targeting toxins, expand the mechanistic repertoire of ribosomal antibacterial toxins, and demonstrate how conserved components of the host translation machinery can be co-opted to promote toxin activity.
Hespanhol, J. T., Blackburn, T. M., Pham, K., Varshney, A., Nicastro, G. G., de Souza, R. F., Aravind, L., Johnson, A., Keiler, K. C., Dunham, C. M., Bayer-Santos, E.
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