# Ribonuclease toxin RelE1 inhibits growth of Mycobacterium tuberculosis through specific cleavage of the ribosomal anti-Shine–Dalgarno region

## TL;DR

The study reveals how the RelE1 toxin from tuberculosis bacteria inhibits growth by cleaving ribosomal RNA to block protein translation.

## Contribution

The novel finding is that RelE1 specifically cleaves the 16S rRNA in ribosomes, releasing the anti-Shine–Dalgarno region to inhibit translation.

## Key findings

- RelE1 cleaves 16S rRNA from free and assembled ribosomes in Mycobacterium tuberculosis.
- This cleavage releases the anti-Shine–Dalgarno region, preventing translation and inhibiting bacterial growth.
- RelE1's mechanism differs from other RelE toxins like RelE3, which target mRNA instead.

## Abstract

Toxin–antitoxin (TA) systems are central to bacterial immunity, genome maintenance, and pathogenicity. Toxins of TA systems use diverse strategies to control bacterial growth and represent attractive therapeutic targets to fight pathogens. In this work, we have investigated the toxic mechanism of the three RelE toxins of Mycobacterium tuberculosis, the bacterium responsible for tuberculosis in humans. Structural studies showed that RelBE1, RelBE2, and RelBE3 TA complexes share conserved structural motifs distinct from the RelBE complex of Escherichia coli. Although RelE homologs have previously been reported to perform ribosome-dependent messenger RNA (mRNA) cleavage, detection of cleavage products by nEMOTE demonstrated that only RelE3 targets mRNA. In contrast, in vitro and in vivo analyses using Mycobacterium smegmatis and M. tuberculosis revealed that RelE1 is a site-specific RNase, able to cleave 16S rRNA from free 30S and formed 70S ribosomes, to release the anti-Shine–Dalgarno region and prevent translation. This stunning mode of action, which is likely shared with RelE2, demonstrates that there is broader diversity for toxic mechanisms within the widespread RelE family.

Graphical Abstract