## 5 Role of sRNA in Quorum Sensing
### Qrr 1-5  
Quorum sensing (QS) in _V. harveyi_ and _V. cholerae_ is controlled by sRNAs called quorum regulatory RNAs (Qrrs) and the RNA chaperone Hfq. This system enables bacteria to synchronize group behaviors, including bioluminescence, virulence factors production, type VI secretion, and biofilm formation as a function of cell density. _V. harveyi_ produces three autoinducers--an acyl homoserine lactone-HAI-1 (also known as harveyi auto inducer 1), a furanosyl borate diester-AI-2 (also known as **auto inducer-2**), and (5)-3-hydroxytridecan-4-one-CAI-1 (also known as cholerae autoinducer 1)--synthesized by LuxM, LuxS, and CqsA, respectively [232, 233, 234]. These autoinducers are detected by membrane-bound two-component sensors, LuxN, LuxQ, and CqsS (Figure 4). The autoinducers regulate QS through a phosphorelay system. At low cell density, when autoinducer concentrations are minimal, these sensors function as kinases, phosphorylating LuxU, which subsequently phosphorylates LuxO. Phosphorylated LuxO (LuxO-P), in conjunction with \(\sigma^{54}\), activatesthe transcription of five Qrr sRNAs (Qrr1-5) [233]. These Qrr sRNAs bind to Hfq and interact with target mRNAs through specific base pairing to regulate gene expression via catalytic degradation, coupled degradation, sequestration, or activation-induced degradation. Each Qrr sRNA contains four stem-loops, with the first and second being critical for base pairing and stability. This regulatory network represses the master QS regulator LuxR, preventing QS-induced gene expression at low cell density. At high cell density, accumulated autoinducers switch the sensors from kinases to phosphatases, resulting in LuxO dephosphorylation and the subsequent cessation of Qrr transcription. This permits LuxR translation and the activation of QS-regulated genes, leading to behaviors such as bioluminescence and biofilm formation (Figure 4). In _V. cholerae_, the QS system operates similarly but lacks the HAI-1 pathway, relying on AI-2 and CAI-1 autoinducers [235; 236]. The phosphorelay system involving LuxU and LuxO regulates QS gene expression. However, unlike _V. harveyi, V. cholerae_ produces four Qrr sRNAs (Qrr1-4), which redundantly repress HapR, the homolog of LuxR. Additionally, the VarS-VarA two-component sensory system modulates QS by regulating the activity of the CsrA protein, which indirectly influences LuxO activity [237]. At low cell density, CsrA enhances LuxO-P activity, promoting Qrr-mediated repression of HapR. At high cell density, VarS-VarA inhibits CsrA, allowing HapR expression, which suppresses virulence factor production and biofilm formation. A key player of _Vibrio cholerae_ virulence is the type VI secretion system (T6 SS). T6 SS is a QS-regulated, contact-dependent protein delivery apparatus that can eliminate competing cells [238; 239]. Structurally, T6 SS assembles into a membrane-spanning, spear-like device loaded with toxic effector proteins [240]. Upon deployment, the effectors penetrate competitor cell walls. To prevent self-toxicity, T6 SS-active cells produce immunity proteins that neutralize the toxic effectors [241]. Additional protective mechanisms, such as expolysaccharide and capsular polysaccharide production, also safeguard against incoming T6 SS-mediated attacks [242]. The Qrr sRNAs repress _t6 ss_ expression via two mechanisms: direct repression of the primary _t6 ss_ gene cluster by interacting with their mRNA, and indirect repression by inhibiting HapR, the HCD (high cell density) activator of auxiliary _t6 ss_ gene clusters [238]. Since T6 SS toxin deployment requires direct cell-to-cell contact, limiting maximal T6 SS machinery production to HCD may enhance killing efficiency and reduce energy expenditure. Interestingly, [PERSON] et al. identified an RNA sponge, QrrX, which base pairs with and inactivates the Qrr1-4 sRNAs [243]. The transcription of _qrrX_ is driven by QrrT, a previously uncharacterized LysR-type transcriptional regulator.