Qrr is a non-coding RNA

Non-coding RNA

A non-coding RNA is a functional RNA molecule that is not translated into a protein. Less-frequently used synonyms are non-protein-coding RNA , non-messenger RNA and functional RNA . The term small RNA is often used for short bacterial ncRNAs...

 that is thought to be involved in the regulation of quorum sensing

Quorum sensing

Quorum sensing is a system of stimulus and response correlated to population density. Many species of bacteria use quorum sensing to coordinate gene expression according to the density of their local population. In similar fashion, some social insects use quorum sensing to determine where to nest...

 in Vibrio

Vibrio

Vibrio is a genus of Gram-negative bacteria possessing a curved rod shape, several species of which can cause foodborne infection, usually associated with eating undercooked seafood. Typically found in saltwater, Vibrio are facultative anaerobes that test positive for oxidase and do not form...

species. It is believed that these RNAs, guided by a protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

, Hfq

Hfq protein

The Hfq protein encoded by the hfq gene was discovered in 1968 as an Escherichia. coli host factor that was essential for replication of the bacteriophage Qβ. It is now clear that Hfq is an abundant bacterial RNA binding protein which has many important physiological roles. Usually mediated by...

, can mediate the destabilisation of the quorum-sensing master regulators LuxR/HapR/VanT mRNAs.

Qrr operates as part of a negative feedback

Negative feedback

Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...

 loop which regulates the shift in cell state from that of low density populations to that in high density populations. In Vibrio harveyi

Vibrio harveyi

Vibrio harveyi is a species of Gram-negative, bioluminescent, marine bacteria in the genus Vibrio. V. harveyi are rod-shaped, motile , facultatively anaerobic, halophilic, and competent for both fermentative and respiratory metabolism. They do not grow at 4°C or above 35°C. V...

, Qrr binds and represses expression of the luxR mRNA transcript, a master regulator of quorum-sensing genes. At higher population densities, increased autoinducer

Autoinducer

Autoinducers are chemical signaling molecules that are produced and used by bacteria participating in quorum sensing. Quorum sensing is a phenomenon that allows both Gram-negative and Gram-positive bacteria to sense one another and to regulate a wide variety of physiological activities. Such...

concentration leads to termination of qrr transcription hence derepression of luxR.

There are 5 different qrr genes (Qrr1-5) in V. harveyi; of these, qrr2, 3 and 4 are activated by LuxR.