Twin Small RNAs and Divergent Fates: The Expansive Regulatory Networks of OmrA and OmrB.

Small RNAs (sRNAs) have emerged as key regulators of bacterial physiology, enabling rapid adaptation to environmental changes through post-transcriptional control. The homologous sRNAs OmrA and OmrB, conserved in Escherichia coli and other Enterobacterales, share sequence similarity yet differ in expression dynamics and functional outputs. Both act by base-pairing with target mRNAs, a process facilitated by the RNA chaperone Hfq, which promotes duplex formation and influences RNA stability. In some contexts, regulation also depends on RelA, which stabilizes Hfq-RNA complexes and links OmrA/B activity to the stringent response. Together, OmrA and OmrB modulate outer membrane protein synthesis, motility, biofilm formation, iron uptake, and virulence. OmrA is preferentially induced under nutrient limitation and integrates into the σS stress regulon, whereas OmrB is more broadly expressed and can engage in context-dependent mechanisms such as target sequestration. This review highlights the molecular mechanisms of OmrA/B regulation and their contribution to global sRNA-mediated regulatory networks that coordinate bacterial adaptation and pathogenicity.