RelE

RelE is the toxin component of the RelBE toxin–antitoxin (TA) system found in many bacteria, including Escherichia coli and other Gram-negative species. It is a small, basic protein that functions as a ribosome-dependent endoribonuclease, cleaving mRNA within the ribosomal A site and thereby inhibiting protein synthesis. RelE activity is regulated by its cognate antitoxin RelB, with RelB binding RelE to form a neutralized complex under normal conditions.

Under stress or nutrient limitation, cellular proteases such as Lon degrade RelB more rapidly than RelE, releasing active RelE. The liberated toxin associates with translating ribosomes and cleaves mRNA positioned in the A site, leading to global translation arrest and growth inhibition. The cleavage occurs in the context of the ribosome’s decoding center and depends on the ribosome-bound state of the mRNA; the resulting fragments are typically unstable and degraded.

RelBE systems vary in genomic context, being found on both plasmids and chromosomes across diverse bacterial species. They are implicated in bacterial stress responses and persistence, contributing to survival under adverse conditions and to plasmid maintenance through post-segregational killing mechanisms. In research, RelBE systems serve as models for understanding toxin–antitoxin regulation and are explored as potential targets for antimicrobial strategies that disrupt toxin activation or antitoxin protection.