Qrrs

Qrrs, short for quorum regulatory RNAs, are a family of small non-coding RNAs that regulate quorum sensing in Vibrio species. They are typically 80–120 nucleotides long and exist as multiple paralogs (for example qrr1, qrr2, qrr3, qrr4) within Vibrio genomes. Expression of Qrrs is driven by the quorum-sensing pathway: at low cell density, the LuxO response regulator remains phosphorylated and activates transcription of qrrs with the help of the sigma-54 factor. Qrr RNAs function with the RNA chaperone Hfq to regulate target mRNAs by base-pairing, often leading to decreased translation and/or accelerated mRNA decay.

Key targets of Qrrs include master regulators such as hapR in Vibrio cholerae and luxR in other Vibrio species, as well as aphA. Through these interactions, Qrrs promote a low-cell-density gene expression program, influencing phenotypes such as virulence factor production, biofilm formation, and bioluminescence. In general, Qrrs act to suppress certain high-cell-density regulators while enabling downstream responses appropriate to early-stage population growth.

At high cell density, autoinducers trigger a phosphorelay that deactivates LuxO, leading to reduced qrr transcription and lower Qrr RNA levels. This relief from repression shifts the regulatory network toward a high-cell-density state, altering the expression of HapR/LuxR-dependent genes and associated behaviors.

Evolution and diversity: the number and exact targets of qrr genes vary among Vibrio species, but the core mechanism—post-transcriptional control by small RNAs integrated into the quorum-sensing circuit—remains a common feature. Qrrs are widely cited as a model system for studying sRNA-mediated regulation in bacteria.