RELB

RelB is a protein component of the RelBE toxin–antitoxin (TA) system found in many bacteria. It functions as the antitoxin that binds and neutralizes the RelE toxin, a ribosome-dependent endoribonuclease that inhibits translation. In many species, the RelBE operon encodes RelE and RelB, and RelB can form complexes with RelE to prevent RelE from cleaving mRNA.

RelB also serves as a transcriptional repressor of the relBE operon. Its N-terminal region participates in RelE binding, while the C-terminal region often engages DNA to autorepress expression of the operon. Under normal conditions, RelB is relatively unstable due to proteolytic degradation by cellular proteases, maintaining low RelB levels and allowing conditional activation of RelE under stress.

Under nutritional or other stress, RelB is degraded by proteases such as Lon and ClpP, freeing RelE to cleave mRNA at ribosome-associated sites. This results in translation inhibition, growth arrest, and the formation of persister cells, contributing to bacterial stress adaptation and, in some contexts, antibiotic tolerance.

RelB homologs are widespread among bacteria, particularly in Enterobacteriaceae, and are frequently located adjacent to relE toxin genes. While variant stoichiometries and DNA-binding properties exist, RelB–RelE TA systems share the core mechanism of antitoxin neutralization paired with conditional toxin activation that modulates bacterial growth under adverse conditions.