Many bacteria use quorum sensing (QS) to regulate virulence factor production in response to changes in population density. QS is mediated through the production, secretion, and detection of signaling molecules called autoinducers (AIs) to modulate population-wide behavioral changes. Four histidine kinases, LuxPQ, CqsS, CqsR and VpsS, have been identified in Vibrio cholerae as QS receptors to activate virulence gene expression at low cell density. Detection of AIs by these receptors leads to virulence gene repression at high cell density. The redundancy among these receptors is puzzling since any one of the four receptors is sufficient to support colonization of V. cholerae in the host small intestine. It is believed that one of the functions of such circuit architecture is to prevent interference on any single QS receptor. However, it is unclear what natural molecules can interfere V. cholerae QS and in what environment interference is detrimental. We show here mutants expressing only CqsR without the other three QS receptors are defective in colonizing the host large intestine. We identified ethanolamine, a common intestinal metabolite that can function as a chemical source of QS interference. Ethanolamine specifically interacts with the ligand-binding CACHE domain of CqsR and induces a premature QS response in V. cholerae mutants expressing only CqsR without the other three QS receptors. The effect of ethanolamine on QS gene expression and host colonization is abolished by mutations that disrupt CqsR signal sensing. V. cholerae defective in producing ethanolamine is still proficient in QS, therefore, ethanolamine functions only as an external cue for CqsR. Our findings suggest the inhibitory effect of ethanolamine on CqsR could be a possible source of QS interference but is masked by the presence of the other parallel QS pathways, allowing V. cholerae to robustly colonize the host.

No abstract

Sporulation in Bacillus subtilis is governed by a cascade of alternative RNA polymerase sigma factors. We previously identified a small protein Fin that is produced under the control of the sporulation sigma factor σF to create a negative feedback loop that inhibits σF-directed gene transcription. Cells deleted for fin are defective for spore formation and exhibit increased levels of σF-directed gene transcription. Based on pull-down experiments, chemical crosslinking, bacterial two-hybrid experiments, and nuclear magnetic resonance chemical shift analysis, we now report that Fin binds to RNA polymerase and specifically to the coiled-coil region of the β’ subunit. The coiled-coil is a docking site for sigma factors on RNA polymerase, and evidence is presented that the binding of Fin and σF to RNA polymerase is mutually exclusive. We propose that Fin functions by a mechanism distinct from that of classic sigma factor antagonists (anti-σ factors), which bind directly to a target sigma factor to prevent its association with RNA polymerase, and instead functions to inhibit σF by competing for binding to the β’ coiled-coil.

17The pathogen that causes cholera, Vibrio cholerae, uses the cell-cell communication process 18 known as quorum sensing (QS) to regulate virulence factor production and biofilm formation in 19 response to changes in population density and complexity. QS is mediated through the detection 20 of extracellular chemical signals called autoinducers. Four histidine kinases, LuxPQ, CqsS, CqsR 21 and VpsS, have been identified as receptors to activate the key QS regulator LuxO at low cell 22 density. At high cell density, detection of autoinducers by these receptors leads to deactivation of 23 LuxO, resulting in population-wide gene expression changes. While the cognate autoinducers that 24 regulate the activity of CqsS and LuxQ are known, the signals that regulate CqsR have not been 25 determined. Here we show that the common metabolite ethanolamine specifically interacts with 26 the ligand-binding CACHE domain of CqsR in vitro and induces the high cell-density QS response 27through CqsR kinase inhibition in V. cholerae cells. We also identified residues in the CqsR 28 CACHE domain important for ethanolamine detection and signal transduction. Moreover, 29 mutations disrupting endogenous ethanolamine production in V. cholerae delay the onset of, but 30 do not abolish, the high cell-density QS gene expression. Finally, we demonstrate that modulation 31 of CqsR QS response by ethanolamine occurs inside animal hosts. Our findings suggest that V. 32 cholerae uses CqsR as a dual-function receptor to integrate information from the self-made signals 33 as well as exogenous ethanolamine as an environmental cue to modulate QS response. 34 IMPORTANCE 35Many bacteria use quorum sensing to regulate cellular processes that are important for their 36 survival and adaptation to different environments. Quorum sensing usually depends on the 37 detection on chemical signals called autoinducers made endogenously by the bacteria. We show 38 here ethanolamine, a common metabolite made by various bacteria and eukaryotes, can modulate 39 the activity of one of the quorum-sensing receptors in Vibrio cholerae, the etiological agent of the 40 disease cholera. Our results raise the possibility that V. cholerae or other quorum-sensing bacteria 41 can combine environmental sensing and quorum sensing to control group behaviors.42 43 44 Quorum sensing (QS) is used by a wide variety of bacteria to coordinate population-wide 45 changes in behaviors in response to cell density (1). Vibrio cholerae, which causes the diarrheal 46 disease cholera in the human host, uses QS to regulate virulence factor production, biofilm 47 formation, Type VI secretion, metabolic regulation, and natural competence to maintain 48 competitive fitness in various environments (2-10). Four parallel QS signaling systems pathways 49 have been identified in V. cholerae that rely on a phosphorelay to regulate downstream gene 50 expression (11) (Figure 1). At low cell-density (LCD), four histidine kinases CqsS, LuxPQ, CqsR, 51 and VpsS function in parallel to phosphorylate LuxO...