Quorum-sensing regulators control virulence gene expression in Vibrio cholerae. Jun Zhu, Melissa B. Miller, Russell E. Vance, Michelle Dziejman, Bonnie L. Bassler, and John J. Mekalanos. The production of virulence factors, including cholera toxin and the toxin-coregulated pilus, in the human pathogen Vibrio cholerae is strongly influenced by environmental conditions. The ToxR signal transduction cascade is responsible for sensing and integrating environmental information and controlling the virulence regulon. This study shows that quorum-sensing regulators, such as LuxO and HapR, are involved in regulating V. cholerae virulence. Using an infant mouse model, it was found that a luxO mutant is severely defective in colonization of the small intestine. Gene arrays revealed that the ToxR regulon is repressed in the luxO mutant, mediated by HapR. LuxO represses hapR expression early in log-phase growth, and constitutive expression of hapR blocks ToxR-regulon expression. LuxO and HapR regulate various cellular processes, including motility, protease production, and biofilm formation. These findings suggest a role for quorum sensing in modulating the expression of virulence genes in a reciprocal manner in vivo. Vibrio cholerae, a Gram-negative bacterium, usually inhabits aquatic environments but is the causative agent of cholera. Two factors critical to its virulence are cholera enterotoxin (CT) and the toxin-coregulated pilus (TCP). Environmental cues influence CT and TCP expression. ToxR and TcpP likely detect environmental signals and initiate a signal transduction cascade that promotes ToxT expression, which activates genes involved in CT and TCP expression. Quorum sensing, the exchange of chemical signaling molecules, regulates processes like bioluminescence, virulence, biofilm formation, and sporulation in various bacteria. Gram-negative bacteria use acyl-homoserine lactones, while Gram-positive bacteria use modified oligopeptides. Quorum sensing is linked to virulence in some bacterial pathogens, such as Pseudomonas aeruginosa and Staphylococcus aureus. Vibrio harveyi uses a complex quorum-sensing system to regulate bioluminescence and other phenotypes. It produces two autoinducers, AI-1 and AI-2. LuxO integrates signals from two circuits and negatively regulates luminescence. LuxR is a homologue of HapR in V. cholerae. Analysis of the V. cholerae genome shows it lacks genes for light production but possesses genes for AI-2. V. cholerae produces AI-2, but the targets of this quorum-sensing circuit remain unknown. This study shows that V. harveyi-like quorum-sensing regulators control V. cholerae virulenceQuorum-sensing regulators control virulence gene expression in Vibrio cholerae. Jun Zhu, Melissa B. Miller, Russell E. Vance, Michelle Dziejman, Bonnie L. Bassler, and John J. Mekalanos. The production of virulence factors, including cholera toxin and the toxin-coregulated pilus, in the human pathogen Vibrio cholerae is strongly influenced by environmental conditions. The ToxR signal transduction cascade is responsible for sensing and integrating environmental information and controlling the virulence regulon. This study shows that quorum-sensing regulators, such as LuxO and HapR, are involved in regulating V. cholerae virulence. Using an infant mouse model, it was found that a luxO mutant is severely defective in colonization of the small intestine. Gene arrays revealed that the ToxR regulon is repressed in the luxO mutant, mediated by HapR. LuxO represses hapR expression early in log-phase growth, and constitutive expression of hapR blocks ToxR-regulon expression. LuxO and HapR regulate various cellular processes, including motility, protease production, and biofilm formation. These findings suggest a role for quorum sensing in modulating the expression of virulence genes in a reciprocal manner in vivo. Vibrio cholerae, a Gram-negative bacterium, usually inhabits aquatic environments but is the causative agent of cholera. Two factors critical to its virulence are cholera enterotoxin (CT) and the toxin-coregulated pilus (TCP). Environmental cues influence CT and TCP expression. ToxR and TcpP likely detect environmental signals and initiate a signal transduction cascade that promotes ToxT expression, which activates genes involved in CT and TCP expression. Quorum sensing, the exchange of chemical signaling molecules, regulates processes like bioluminescence, virulence, biofilm formation, and sporulation in various bacteria. Gram-negative bacteria use acyl-homoserine lactones, while Gram-positive bacteria use modified oligopeptides. Quorum sensing is linked to virulence in some bacterial pathogens, such as Pseudomonas aeruginosa and Staphylococcus aureus. Vibrio harveyi uses a complex quorum-sensing system to regulate bioluminescence and other phenotypes. It produces two autoinducers, AI-1 and AI-2. LuxO integrates signals from two circuits and negatively regulates luminescence. LuxR is a homologue of HapR in V. cholerae. Analysis of the V. cholerae genome shows it lacks genes for light production but possesses genes for AI-2. V. cholerae produces AI-2, but the targets of this quorum-sensing circuit remain unknown. This study shows that V. harveyi-like quorum-sensing regulators control V. cholerae virulence