relBE

RelBE is a bacterial toxin–antitoxin module, most notably identified in Escherichia coli and related bacteria. It consists of two proteins, the toxin RelE and the antitoxin RelB, encoded in an operon and categorized as a type II toxin–antitoxin system, where a protein toxin is neutralized by a protein antitoxin.

The RelB–RelE interaction is central to the system’s function. In normal conditions, RelB binds RelE to form a neutral complex, preventing toxin activity. The RelB–RelE complex can also autorepress the relBE operon by binding to its promoter, reducing transcription. Under stress, RelB is more rapidly degraded by cellular proteases such as Lon, reducing antitoxin levels and freeing RelE. Active RelE is a ribosome-dependent endoribonuclease that cleaves mRNA at the ribosome A site, thereby inhibiting translation and causing growth arrest. This mechanism contributes to the formation of persister cells, a subpopulation of dormant cells that survive adverse conditions.

RelBE is distributed across many Gram-negative bacteria and can be found on chromosomes or plasmids, with the exact arrangement and regulation varying among species. The module is part of a broader network of toxin–antitoxin systems that are studied as models of bacterial stress responses and persistence. Its biological significance appears to be context-dependent, influencing survival under nutrient limitation or antibiotic exposure in some strains.

In research contexts, RelBE serves as a reference example of Type II TA systems, illustrating how toxin activation via antitoxin degradation links environmental stress to translational control and bacterial dormancy.