RhlIRhlR

RhlIRhlR refers to the RhlI/RhlR quorum-sensing system in Pseudomonas aeruginosa, a regulatory module that coordinates gene expression in response to population density. The system comprises RhlI, an acyl-homoserine lactone synthase that produces the signaling molecule N-butanoyl-L-homoserine lactone (C4-HSL), and RhlR, a LuxR-type transcriptional regulator that binds C4-HSL and modulates target gene transcription.

Mechanism and regulation: As cell density increases, C4-HSL accumulates and binds RhlR, promoting its activation and DNA binding. The RhlR-C4-HSL complex regulates a set of genes, including the rhlAB operon involved in rhamnolipid biosynthesis, and other factors related to virulence and metabolic adaptation. The system often features positive feedback, with RhlR-C4-HSL influencing rhlI expression to sustain signal production. The RhlI/RhlR circuit interacts with the Las quorum-sensing system, forming a hierarchical network that coordinates collective behaviors.

Biological role and significance: The Rhl system influences biofilm maturation, swarming motility, and the production of virulence-associated factors such as rhamnolipids and proteases. Through these effects, RhlI/RhlR contributes to pathogenicity, environmental adaptation, and community behavior in P. aeruginosa. Its activity is a subject of interest for anti-virulence research, as disrupting quorum sensing can attenuate pathogenic traits and biofilm resilience without exerting strong selective pressure for resistance. The RhlIRhlR module is fundamental to understanding how P. aeruginosa integrates cell-density signals to orchestrate complex social behaviors.