A genetic analysis of the function of LuxO, a two-component response regulator involved in quorum sensing in Vibrio harveyi

Two independent quorum-sensing systems control the expression of bioluminescence (lux) in the marine bacterium Vibrio harveyi. Each system is composed of an autoinducer (AI-1 or AI-2) and its cognate sensor (LuxN or LuxQ). The sensors are two-component hybrid kinases, containing both sensor kinase domains and response regulator domains. Sensory information from the two systems is relayed by a phosphotransfer mechanism to a shared integrator protein called LuxO. LuxO is a member of the response regulator class of the two-component family of signal transduction proteins, and LuxO acts negatively to control luminescence. In this report, missense and in frame deletion mutations were constructed in luxO that encoded proteins mimicking either the phosphorylated or the unphosphorylated form, and these mutations were introduced into the V. harveyi chromosome at the luxO locus. Phenotypical analyses of the resulting mutant V. harveyi strains indicate that the phosphorylated form of LuxO is the repressor, and that the unphosphorylated form of the protein is inactive. Analysis of the lux phenotypes of V. harveyi strains containing single and double luxN and luxQ mutations indicate that LuxN and LuxQ have two activities on LuxO. They act as LuxO protein kinases at low cell density in the absence of autoinducers, and they switch to LuxO protein phosphatases at high cell density in the presence of autoinducers. Furthermore, the timing and potency of inputs from the two systems into regulation of quorum sensing are different.     

Regulation of quorum sensing in Vibrio harveyi by LuxO and sigma-54

The bioluminescent marine bacterium Vibrio harveyi controls light production (lux) by an elaborate quorum-sensing circuit. V. harveyi produces and responds to two different autoinducer signals (AI-1 and AI-2) to modulate the luciferase structural operon (luxCDABEGH) in response to changes in cell-population density. Unlike all other Gram-negative quorum-sensing organisms, V. harveyi regulates quorum sensing using a two-component phosphorylation-dephosphorylation cascade. Each autoinducer is recognized by a cognate hybrid sensor kinase (called LuxN and LuxQ). Both sensors transduce information to a shared phosphorelay protein called LuxU, which in turn conveys the signal to the response regulator protein LuxO. Phospho-LuxO is responsible for repression of luxCDABEGH expression at low cell density. In the present study, we demonstrate that LuxO functions as an activator protein via interaction with the alternative sigma factor, sigma54 (encoded by rpoN). Our results suggest that LuxO, together with sigma54, activates the expression of a negative regulator of luminescence. We also show that phenotypes other than lux are regulated by LuxO and sigma54, demonstrating that in Vibrio harveyi, quorum sensing controls multiple processes.     

Sequence and Function of LuxU: a Two-Component Phosphorelay Protein That Regulates Quorum Sensing in Vibrio harveyi

Vibrio harveyi regulates the expression of bioluminescence (lux) in response to cell density, a phenomenon known as quorum sensing. In V. harveyi, two independent quorum-sensing systems exist, and each produces, detects, and responds to a specific cell density-dependent autoinducer signal. The autoinducers are recognized by two-component hybrid sensor kinases called LuxN and LuxQ, and sensory information from both systems is transduced by a phosphorelay mechanism to the response regulator protein LuxO. Genetic evidence suggests that LuxO-phosphate negatively regulates the expression of luminescence at low cell density in the absence of autoinducers. At high cell density, interaction of the sensors with their cognate autoinducers results in dephosphorylation and inactivation of the LuxO repressor. In the present report, we show that LuxN and LuxQ channel sensory information to LuxO via a newly identified phosphorelay protein that we have named LuxU. LuxU shows sequence similarity to other described phosphorelay proteins, including BvgS, ArcB, and Ypd1. A critical His residue (His 58) of LuxU is required for phosphorelay function.     

AI-1 influences the kinase activity but not the phosphatase activity of LuxN of Vibrio harveyi

The Gram-negative bacterium Vibrio harveyi produces and responds to three autoinducers, AI-1, AI-2, and CAI-1 to regulate cell density dependent gene expression by a process referred to as quorum sensing. The concentration of the autoinducers is sensed by three cognate hybrid sensor kinases, and information is channeled via the HPt protein LuxU to the response regulator LuxO. Here, a detailed biochemical study on the enzymatic activities of the membrane-integrated hybrid sensor kinase LuxN, the sensor for N-(d-3-hydroxybutanoyl)homoserine lactone (AI-1), is provided. LuxN was heterologously overproduced as the full-length protein in Escherichia coli. LuxN activities were characterized in vitro and are an autophosphorylation activity with an unusually high ATP turnover rate, stable LuxU phosphorylation, and a slow phosphatase activity with LuxU approximately P as substrate. The presence of AI-1 affected the kinase but not the phosphatase activity of LuxN. The influence of AI-1 on the LuxN--> LuxU signaling step was monitored, and in the presence of AI-1, the kinase activity of LuxN, and hence the amount of LuxU approximately P produced, were significantly reduced. Half-maximal inhibition of kinase activity by AI-1 occurred at 20 mum. Together, these results indicate that AI-1 directly interacts with LuxN to down-regulate its autokinase activity and suggest that the key regulatory step of the AI-1 quorum sensing system of Vibrio harveyi is AI-1-mediated repression of the LuxN kinase activity.     

Solution structure and dynamics of LuxU from Vibrio harveyi, a phosphotransferase protein involved in bacterial quorum sensing

The marine bacterium Vibrio harveyi controls its bioluminescence by a process known as quorum sensing. In this process, autoinducer molecules are detected by membrane-bound sensor kinase/response regulator proteins (LuxN and LuxQ) that relay a signal via a series of protein phosphorylation reactions to another response regulator protein, LuxO. Phosphorylated LuxO indirectly represses the expression of the proteins responsible for bioluminescence. Integral to this quorum sensing process is the function of the phosphotransferase protein, LuxU. LuxU acts to shuttle the phosphate from the membrane-bound proteins, LuxN and LuxQ, to LuxO. LuxU is a 114 amino acid residue monomeric protein. Solution NMR was used to determine the three-dimensional structure of LuxU. LuxU contains a four-helix bundle topology with the active-site histidine residue (His58) located on alpha-helix C and exposed to solution. The active site represents a cluster of positively charged residues located on an otherwise hydrophobic protein face. NMR spin-relaxation experiments identify a collection of flexible residues localized on the same region of LuxU as His58. The studies described here represent the first structural characterization of an isolated, monomeric bacterial phosphotransferase protein.     

Ligand-induced asymmetry in histidine sensor kinase complex regulates quorum sensing

Bacteria sense their environment using receptors of the histidine sensor kinase family, but how kinase activity is regulated by ligand binding is not well understood. Autoinducer-2 (AI-2), a secreted signaling molecule originally identified in studies of the marine bacterium Vibrio harveyi, regulates quorum-sensing responses and allows communication between different bacterial species. AI-2 signal transduction in V. harveyi requires the integral membrane receptor LuxPQ, comprised of periplasmic binding protein (LuxP) and histidine sensor kinase (LuxQ) subunits. Combined X-ray crystallographic and functional studies show that AI-2 binding causes a major conformational change within LuxP, which in turn stabilizes a quaternary arrangement in which two LuxPQ monomers are asymmetrically associated. We propose that formation of this asymmetric quaternary structure is responsible for repressing the kinase activity of both LuxQ subunits and triggering the transition of V. harveyi into quorum-sensing mode.     

Three parallel quorum-sensing systems regulate gene expression in Vibrio harveyi

In a process called quorum sensing, bacteria communicate using extracellular signal molecules termed autoinducers. Two parallel quorum-sensing systems have been identified in the marine bacterium Vibrio harveyi. System 1 consists of the LuxM-dependent autoinducer HAI-1 and the HAI-1 sensor, LuxN. System 2 consists of the LuxS-dependent autoinducer AI-2 and the AI-2 detector, LuxPQ. The related bacterium, Vibrio cholerae, a human pathogen, possesses System 2 (LuxS, AI-2, and LuxPQ) but does not have obvious homologues of V. harveyi System 1. Rather, System 1 of V. cholerae is made up of the CqsA-dependent autoinducer CAI-1 and a sensor called CqsS. Using a V. cholerae CAI-1 reporter strain we show that many other marine bacteria, including V. harveyi, produce CAI-1 activity. Genetic analysis of V. harveyi reveals cqsA and cqsS, and phenotypic analysis of V. harveyi cqsA and cqsS mutants shows that these functions comprise a third V. harveyi quorum-sensing system that acts in parallel to Systems 1 and 2. Together these communication systems act as a three-way coincidence detector in the regulation of a variety of genes, including those responsible for bioluminescence, type III secretion, and metalloprotease production.     

Regulation of LuxPQ receptor activity by the quorum-sensing signal autoinducer-2

The extracellular signaling molecule autoinducer-2 (AI-2) mediates quorum-sensing communication in diverse bacterial species. In marine vibrios, binding of AI-2 to the periplasmic receptor LuxP modulates the activity of the inner membrane sensor kinase LuxQ, transducing the AI-2 information into the cytoplasm. Here, we show that Vibrio harveyi LuxP associates with LuxQ in both the presence and absence of AI-2. The 1.9 A X-ray crystal structure of apoLuxP, complexed with the periplasmic domain of LuxQ, reveals that the latter contains two tandem Per/ARNT/Simple-minded (PAS) folds. Thus, although many prokaryotic PAS folds themselves bind ligands, the LuxQ periplasmic PAS folds instead bind LuxP, monitoring its AI-2 occupancy. Mutations that disrupt the apoLuxP:LuxQ interface sensitize V. harveyi to AI-2, implying that AI-2 binding causes the replacement of one set of LuxP:LuxQ contacts with another. These conformational changes switch LuxQ between two opposing enzymatic activities, each of which conveys information to the cytoplasm about the cell density of the surrounding environment.     

Presence of LuxS/AI-2 based quorum-sensing system in Vibrio mimicus : luxO controls protease activity

Presence of the quorum-sensing regulation system in Vibrio mimicus was investigated. The culture supernatants of V. mimicus strains were found to possess AI-2 autoinducer like activity, and the strains were found to harbor the genes which are homologous to luxS, luxO, and luxR of V. harveyi. These genes of V. harveyi have been shown to be important components of V. harveyi-like quorum-sensing system. The luxO gene homologue known to encode LuxO, the central component of the regulation system, was disrupted, and effects on protease and hemolysin activity were studied. Disruption of luxO gene resulted in the increased protease activity, but the hemolysin activity did not vary considerably.     

Vibrio harveyi quorum sensing: a coincidence detector for two autoinducers controls gene expression

In a process called quorum sensing, bacteria communicate with one another by exchanging chemical signals called autoinducers. In the bioluminescent marine bacterium Vibrio harveyi, two different auto inducers (AI-1 and AI-2) regulate light emission. Detection of and response to the V.harveyi autoinducers are accomplished through two two-component sensory relay systems: AI-1 is detected by the sensor LuxN and AI-2 by LuxPQ. Here we further define the V.harveyi quorum-sensing regulon by identifying 10 new quorum-sensing-controlled target genes. Our examination of signal processing and integration in the V.harveyi quorum-sensing circuit suggests that AI-1 and AI-2 act synergistically, and that the V.harveyi quorum-sensing circuit may function exclusively as a 'coincidence detector' that discriminates between conditions in which both autoinducers are present and all other conditions.     

The quorum-sensing systems of Vibrio campbellii DS40M4 and BB120 are genetically and functionally distinct

Vibrio campbellii BB120 (previously classified as Vibrio harveyi) is a fundamental model strain for studying quorum sensing in vibrios. A phylogenetic evaluation of sequenced Vibrio strains in Genbank revealed that BB120 is closely related to the environmental isolate V. campbellii DS40M4. We exploited DS40M4's competence for exogenous DNA uptake to rapidly generate greater than 30 isogenic strains with deletions of genes encoding BB120 quorum-sensing system homologues. Our results show that the quorum-sensing circuit of DS40M4 is distinct from BB120 in three ways: (i) DS40M4 does not produce an acyl homoserine lactone autoinducer but encodes an active orphan LuxN receptor, (ii) the quorum regulatory small RNAs (Qrrs) are not solely regulated by autoinducer signalling through the response regulator LuxO and (iii) the DS40M4 quorum-sensing regulon is much smaller than BB120 (~100 genes vs. ~400 genes, respectively). Using comparative genomics to expand our understanding of quorum-sensing circuit diversity, we observe that conservation of LuxM/LuxN proteins differs widely both between and within Vibrio species. These strains are also phenotypically distinct: DS40M4 exhibits stronger interbacterial cell killing, whereas BB120 forms more robust biofilms and is bioluminescent. These results underscore the need to examine wild isolates for a broader view of bacterial diversity in the marine ecosystem.     

Multiple signalling systems controlling expression of luminescence in Vibrio harveyi: sequence and function of genes encoding a second sensory pathway

Density-dependent expression of luminescence in Vibrio harveyl is regulated by the concentration of extracellular signal molecules (autoinducers) in the culture medium. One signal-response system is encoded by the luxL,M,N locus. The luxL and luxM genes are required for the production of an autoinducer (probably β-hydroxybutryl homoserine lactone), and the luxN gene is required for the response to that autoinducer. Analysis of the phenotypes of LuxL,M and N mutants indicated that an additional signal-response system also controls density sensing. We report here the identification, cloning and analysis of luxP and luxQ, which encode functions required for a second density-sensing system. Mutants with defects in luxP and luxQ are defective in response to a second autoinducer substance. LuxQ, like LuxN, is similar to members of the family of two-component, signal transduction proteins and contains both a histidine protein kinase and a response regulator domain. Analysis of signalling mutant phenotypes indicates that there are at least two separate signal-response pathways which converge to regulate expression of luminescence in V. harveyl.     

Evidence for a signaling system in Helicobacter pylori: detection of a luxS-encoded autoinducer

Helicobacter pylori possesses a homolog of the luxS gene, initially identified by its role in autoinducer production for the quorum-sensing system 2 in Vibrio harveyi. The genomes of several other species of bacteria, notably Escherichia coli, Salmonella enterica serovar Typhimurium, and Vibrio cholerae, also include luxS homologs. All of these bacteria have been shown to produce active autoinducers capable of stimulating the expression of the luciferase operon in V. harveyi. In this report, we demonstrate that H. pylori also synthesizes a functional autoinducer (AI-2) that can specifically activate signaling system 2 in V. harveyi. Maximal activity is produced during early log phase, and the activity is diminished when cells enter stationary phase. We show that AI-2 is not involved in modulating any of the known or putative virulence factors in H. pylori and that a luxS null mutant has a two-dimensional protein profile identical to that of its isogenic parent strain. We discuss the implications of having an AI-2-like quorum-sensing system in H. pylori and suggest possible roles that it may play in H. pylori infection.     

Oxazaborolidine derivatives inducing autoinducer-2 signal transduction in Vibrio harveyi

The bioluminescence of the marine bacterium Vibrio harveyi is controlled by quorum sensing. This effect is mediated by production, accumulation, and auto-detection of the species-specific autoinducer 1 (AI-1), autoinducer 2 (AI-2), and the V. cholerae autoinducer 1 (CAI-1). The V. harveyi AI-2 was recently identified as furanosyl borate diester. We synthesized several oxazaborolidine derivatives that chemically resemble the structure of AI-2. Five oxazaborolidine derivatives (BNO-1 to BNO-5) were tested, however only BNO-1 (3,4-dimethyl-2,5-diphenyl-1,3,2-oxazaborolidine), and BNO-5 (2-butyl-3,4-dimethyl-5-phenyl-1,3,2-oxazaborolidine) strongly induced V. harveyi bioluminescence in V. harveyi mutant (BB170) lacking sensor 1. A dose-dependent relationship between those oxazaborolidine derivatives and bioluminescence induction was observed with this V. harveyi strain (BB170). BNO-1 and BNO-5 did not affect V. harveyi BB886 lacking sensor 2. Using a mutant strain which produces neither AI-1 nor AI-2 (V. harveyi MM77) we showed that the presence of spent medium containing AI-2 is essential for BNO-1 and BNO-5 activity. This effect was similar when introducing the spent medium and the BNOs together or at a 3-h interval. A comparable induction of bioluminescence was observed when using synthetic DPD (pre-AI-2) in the presence of BNO-1 or BNO-5. The mode of action of BNO-1 and BNO-5 on bioluminescence of V. harveyi is of a co-agonist category. BNO-1 and BNO-5 enhanced AI-2 signal transduction only in the presence of AI-2 and only via sensor 2 cascade. BNO-1 and BNO-5 are the first oxazaborolidines reported to affect AI-2 activity. Those derivatives represent a new class of borates which may become prototypes of novel agonists of quorum sensing mediated by AI-2 in V. harveyi.     

N-acyl homoserine lactone-degrading microbial enrichment cultures isolated from Penaeus vannamei shrimp gut and their probiotic properties in Brachionus plicatilis cultures

Three bacterial enrichment cultures (ECs) were isolated from the digestive tract of Pacific white shrimp Penaeus vannamei, by growing the shrimp microbial communities in a mixture of N-acyl homoserine lactone (AHL) molecules. The ECs, characterized by denaturing gradient gel electrophoresis analysis and subsequent rRNA sequencing, degraded AHL molecules in the degradation assays. Apparently, the resting cells of the ECs also degraded one of the three types of quorum-sensing signal molecules produced by Vibrio harveyi in vitro [i.e. harveyi autoinducer 1 (HAI-1)]. The most efficient AHL-degrading ECs, EC5, was tested in Brachionus experiments. EC5 degraded the V. harveyi HAI-1 autoinducer in vivo, neutralizing the negative effect of V. harveyi autoinducer 2 (AI-2) mutant, in which only the HAI-1- and CAI-1-mediated components of the quorum-sensing system are functional on the growth of Brachionus. This suggests that EC5 interferes with HAI-1-regulated metabolism in V. harveyi. These AHL-degrading ECs need to be tested in other aquatic systems for their probiotic properties, preferably in combination with specific AI-2-degrading bacteria.     

Deducing receptor signaling parameters from in vivo analysis: LuxN/AI-1 quorum sensing in Vibrio harveyi

Quorum sensing, a process of bacterial cell-cell communication, relies on production, detection, and response to autoinducer signaling molecules. LuxN, a nine-transmembrane domain protein from Vibrio harveyi, is the founding example of membrane-bound receptors for acyl-homoserine lactone (AHL) autoinducers. We used mutagenesis and suppressor analyses to identify the AHL-binding domain of LuxN and discovered LuxN mutants that confer both decreased and increased AHL sensitivity. Our analysis of dose-response curves of multiple LuxN mutants pins these inverse phenotypes on quantifiable opposing shifts in the free-energy bias of LuxN for occupying its kinase and phosphatase states. To understand receptor activation and to characterize the pathway signaling parameters, we exploited a strong LuxN antagonist, one of fifteen small-molecule antagonists we identified. We find that quorum-sensing-mediated communication can be manipulated positively and negatively to control bacterial behavior and, more broadly, that signaling parameters can be deduced from in vivo data.     

The ycf27 genes from cyanobacteria and eukaryotic algae: distribution and implications for chloroplast evolution

The bioluminescence assay system using Vibrio harveyi reporter strains were used to examine quorum-sensing autoinducer (AI) activity from Mannheimia haemolytica A1 cell-free culture supernatant. We showed that M. haemolytica A1 cell-free culture supernatant contains molecules that can stimulate the quorum-sensing system that regulates the expression of the luciferase operon in V. harveyi. Specifically, M. haemolytica A1 can stimulate only the quorum system 2 but not system 1, suggesting that the culture supernatant only contains molecules similar to AI-2 of V. harveyi. The bioluminescence assay was also used to show that culture supernatants from related Pasteurellaceae organisms, Pasteurella multocida, Pasteurella trehalosi, Actinobacillus suis and Actinobacillus pleuropneumoniae, also contain AI-2-like molecules. This is consistent with the presence of a luxS homolog in the genomes of P. multocida and A. pleuropneumoniae. A luxS homolog was cloned by PCR from M. haemolytica A1 using sequencing data from the ongoing genome sequencing project. The cloned luxS(M.h.) was able to complement AI-2 production in the Escherichia coli DH5alpha luxS mutant. This is the first report of a quorum-sensing activity in M. haemolytica A1 and suggests that this bacterium utilizes this mechanism to regulate expression of genes under specific conditions.