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1.  Autoinducers Act as Biological Timers in Vibrio harveyi 
PLoS ONE  2012;7(10):e48310.
Quorum sensing regulates cell density-dependent phenotypes and involves the synthesis, excretion and detection of so-called autoinducers. Vibrio harveyi strain ATCC BAA-1116 (recently reclassified as Vibrio campbellii), one of the best-characterized model organisms for the study of quorum sensing, produces and responds to three autoinducers. HAI-1, AI-2 and CAI-1 are recognized by different receptors, but all information is channeled into the same signaling cascade, which controls a specific set of genes. Here we examine temporal variations of availability and concentration of the three autoinducers in V. harveyi, and monitor the phenotypes they regulate, from the early exponential to the stationary growth phase in liquid culture. Specifically, the exponential growth phase is characterized by an increase in AI-2 and the induction of bioluminescence, while HAI-1 and CAI-1 are undetectable prior to the late exponential growth phase. CAI-1 activity reaches its maximum upon entry into stationary phase, while molar concentrations of AI-2 and HAI-1 become approximately equal. Similarly, autoinducer-dependent exoproteolytic activity increases at the transition into stationary phase. These findings are reflected in temporal alterations in expression of the luxR gene that encodes the master regulator LuxR, and of four autoinducer-regulated genes during growth. Moreover, in vitro phosphorylation assays reveal a tight correlation between the HAI-1/AI-2 ratio as input and levels of receptor-mediated phosphorylation of LuxU as output. Our study supports a model in which the combinations of autoinducers available, rather than cell density per se, determine the timing of various processes in V. harveyi populations.
PMCID: PMC3482212  PMID: 23110227
2.  AinS and a new family of autoinducer synthesis proteins. 
Journal of Bacteriology  1995;177(23):6946-6951.
In Vibrio fischeri, the autoinducer N-3-oxohexanoyl-L-homoserine lactone (AI-1) governs the cell density-dependent induction of the luminescence operon via the LuxR transcriptional activator. The synthesis of AI-1 from bacterial metabolic intermediates is dependent on luxI. Recently, we found a second V. fischeri autoinducer molecule, N-octanoyl-L-homoserine lactone (AI-2), that in E. coli also activates the luminescence operon via LuxR. A locus independent of luxI was identified as being required for AI-2 synthesis. This 2.7-kb ain (autoinducer) locus was characterized by transposon insertion mutagenesis, deletion and complementation analysis, and DNA sequencing. A single 1,185-bp gene, ainS, was found to be the sole exogenous gene necessary for the synthesis of AI-2 in Escherichia coli. In addition, a V. fischeri ainS mutant produced AI-1 but not AI-2, confirming that in its native species ainS is specific for the synthesis of AI-2. ainS is predicted to encode a 45,580-Da protein which exhibits no similarity to LuxI or to any of the LuxI homologs responsible for the synthesis of N-acyl-L-homoserine lactones in a variety of other bacteria. The existence of two different and unrelated autoinducer synthesis genes suggests the occurrence of convergent evolution in the synthesis of homoserine lactone signaling molecules. The C-terminal half of AinS shows homology to a putative protein in Vibrio harveyi, LuxM, which is required for the synthesis of a V. harveyi bioluminescence autoinducer. Together, AinS and LuxM define a new family of autoinducer synthesis proteins. Furthermore, the predicted product of another gene, ainR, encoded immediately downstream of ainS, shows homology to LuxN, which is similarly encoded downstream of luxM in V. harveyi and proposed to have sensor/regulator functions in the bioluminescence response to the V. harveyi auto inducer. This similarity presents the possibility that AI-2, besides interacting with LuxR, also interacts with AinR under presently unknown conditions.
PMCID: PMC177564  PMID: 7592489
3.  The Vibrio harveyi master quorum-sensing regulator, LuxR, a TetR-type protein is both an activator and a repressor: DNA recognition and binding specificity at target promoters 
Molecular Microbiology  2008;70(1):76-88.
Quorum sensing is the process of cell-to-cell communication by which bacteria communicate via secreted signal molecules called autoinducers. As cell population density increases, the accumulation of autoinducers leads to co-ordinated changes in gene expression across the bacterial community. The marine bacterium, Vibrio harveyi, uses three autoinducers to achieve intra-species, intra-genera and inter-species cell–cell communication. The detection of these autoinducers ultimately leads to the production of LuxR, the quorum-sensing master regulator that controls expression of the genes in the quorum-sensing regulon. LuxR is a member of the TetR protein superfamily; however, unlike other TetR repressors that typically repress their own gene expression and that of an adjacent operon, LuxR is capable of activating and repressing a large number of genes. Here, we used protein binding microarrays and a two-layered bioinformatics approach to show that LuxR binds a 21 bp consensus operator with dyad symmetry. In vitro and in vivo analyses of two promoters directly regulated by LuxR allowed us to identify those bases that are critical for LuxR binding. Together, the in silico and biochemical results enabled us to scan the genome and identify novel targets of LuxR in V. harveyi and thus expand the understanding of the quorum-sensing regulon.
PMCID: PMC2628434  PMID: 18681939
4.  The Vibrio fischeri LuxR protein is capable of bidirectional stimulation of transcription and both positive and negative regulation of the luxR gene. 
Journal of Bacteriology  1991;173(2):568-574.
Regulation of the genes required for bioluminescence in the marine bacterium Vibrio fischeri (the lux regulon) is a complex process requiring coordination of several systems. The primary level of regulation is mediated by a positive regulatory protein, LuxR, and a small diffusible molecule, N-(3-oxo-hexanoyl)-homoserine lactone, termed autoinducer. Transcription of the luxR gene, which encodes the regulatory protein, is positively regulated by the cyclic AMP-CAP system. The lux regulon of V. fischeri consists of two divergently transcribed operons designated operonL and operonR. Transcription of the rightward operon (operonR; luxICDABE), consisting of the genes required for autoinducer synthesis (luxI) and light production (luxCDABE), is activated by LuxR in an autoinducer-dependent fashion. The leftward operon (operonL) consists of a single known gene, luxR. The LuxR protein has also been shown to decrease transcription of operonL through an autoinducer-dependent mechanism, thereby negatively regulating its own synthesis. In this paper we demonstrate that the autoinducer-dependent repression of operonL transcription requires not only LuxR but also DNA sequences within operonR which occur upstream of the promoter for operonL. In the absence of these DNA sequences, the LuxR protein causes an autoinducer-dependent activation of transcription of operonL. The lux operator, located in the control region between the two operons, was required for both the positive and negative autoinducer-dependent responses. By titration of high levels of LuxR supplied in trans with synthetic autoinducer, we found that low levels of autoinducer could elicit a positive response even in the presence of the negative-acting DNA sequences, while higher levels of autoinducer resulted in a negative response. Without these DNA sequences in operonR, LuxR and autoinducer stimulated transcription regardless of the level of autoinducer. These results suggest that a switch between stimulation and repression of operonL transcription is mediated by the levels of the LuxR-autoinducer complex, which in these experiments reflects the level of autoinducer in the growth medium.
PMCID: PMC207047  PMID: 1987152
5.  Transcriptional regulation of lux genes transferred into Vibrio harveyi. 
Journal of Bacteriology  1990;172(4):2046-2054.
Past work has shown that transformed Escherichia coli is not a suitable vehicle for studying the expression and regulation of the cloned luminescence (lux) genes of Vibrio harveyi. Therefore, we have used a conjugative system to transfer lux genes cloned into E. coli back into V. harveyi, where they can be studied in the parental organism. To do this, lux DNA was inserted into a broad-spectrum vector, pKT230, cloned in E. coli, and then mobilized into V. harveyi by mating aided by the conjugative plasmid pRK2013, also contained in E. coli. Transfer of the wild-type luxD gene into the V. harveyi M17 mutant by this means resulted in complementation of the luxD mutation and full restoration of luminescence in the mutant; expression of transferase activity was induced if DNA upstream of luxC preceded the luxD gene on the plasmid, indicating the presence of a strong inducible promoter. To extend the usefulness of the transfer system, the gene for chloramphenicol acetyltransferase was inserted into the pKT230 vector as a reporter. The promoter upstream of luxC was verified to be cell density regulated and, in addition, glucose repressible. It is suggested that this promoter may be the primary autoregulated promoter of the V. harveyi luminescence system. Strong termination signals on both DNA strands were recognized and are located downstream from luxE at a point complementary to the longest mRNA from the lux operon. Structural lux genes transferred back into V. harveyi under control of the luxC promoter are expressed at very high levels in V. harveyi as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis: the gene transfer system is thus useful for expression of proteins as well as for studying the regulation of lux genes in their native environment.
PMCID: PMC208703  PMID: 2180915
6.  Vibrio fischeri LuxS and AinS: Comparative Study of Two Signal Synthases 
Journal of Bacteriology  2004;186(12):3873-3881.
Vibrio fischeri possesses two acyl-homoserine lactone quorum-sensing systems, ain and lux, both of which are involved in the regulation of luminescence gene expression and are required for persistent colonization of the squid host, Euprymna scolopes. We have previously demonstrated that the ain system induces luminescence at cell densities that precede lux system activation. Our data suggested that the ain system both relieves repression and initially induces the lux system, thereby achieving sequential induction of gene expression by these two systems. Analysis of the V. fischeri genome revealed the presence of a putative third system based on the enzyme LuxS, which catalyzes the synthesis of the Vibrio harveyi autoinducer 2 (AI-2). In this study, we investigated the impact of V. fischeri LuxS on luminescence and colonization competence in comparison to that of the ain system. Similar to the ain system, inactivation of the AI-2 system decreased light production in culture, but not in the squid host. However, while an ainS mutant produces no detectable light in culture, a luxS mutant expressed approximately 70% of wild-type luminescence levels. A mutation in luxS alone did not compromise symbiotic competence of V. fischeri; however, levels of colonization of an ainS luxS double mutant were reduced to 50% of the already diminished level of ainS mutant colonization, suggesting that these two systems regulate colonization gene expression synergistically through a common pathway. Introduction of a luxO mutation into the luxS and ainS luxS background could relieve both luminescence and colonization defects, consistent with a model in which LuxS, like AinS, regulates gene expression through LuxO. Furthermore, while luxS transcription appeared to be constitutive and the AI-2 signal concentration did not change dramatically, our data suggest that ainS transcription is autoregulated, resulting in an over 2,000-fold increase in signal concentration as culture density increased. Taken together, these data indicate that V. fischeri LuxS affects both luminescence regulation and colonization competence; however, its quantitative contribution is small when compared to that of the AinS signal.
PMCID: PMC419941  PMID: 15175301
7.  Quorum Sensing Regulates Type III Secretion in Vibrio harveyi and Vibrio parahaemolyticus 
Journal of Bacteriology  2004;186(12):3794-3805.
In a process known as quorum sensing, bacteria communicate with one another by producing, releasing, detecting, and responding to signal molecules called autoinducers. Vibrio harveyi, a marine pathogen, uses two parallel quorum-sensing circuits, each consisting of an autoinducer-sensor pair, to control the expression of genes required for bioluminescence and a number of other target genes. Genetic screens designed to discover autoinducer-regulated targets in V. harveyi have revealed genes encoding components of a putative type III secretion (TTS) system. Using transcriptional reporter fusions and TTS protein localization studies, we show that the TTS system is indeed functional in V. harveyi and that expression of the genes encoding the secretion machinery requires an intact quorum-sensing signal transduction cascade. The newly completed genome of the closely related marine bacterium Vibrio parahaemolyticus, which is a human pathogen, shows that it possesses the genes encoding both of the V. harveyi-like quorum-sensing signaling circuits and that it also has a TTS system similar to that of V. harveyi. We show that quorum sensing regulates TTS in V. parahaemolyticus. Previous reports connecting quorum sensing to TTS in enterohemorrhagic and enteropathogenic Escherichia coli show that quorum sensing activates TTS at high cell density. Surprisingly, we find that at high cell density (in the presence of autoinducers), quorum sensing represses TTS in V. harveyi and V. parahaemolyticus.
PMCID: PMC419960  PMID: 15175293
8.  Three Parallel Quorum-Sensing Systems Regulate Gene Expression in Vibrio harveyi†  
Journal of Bacteriology  2004;186(20):6902-6914.
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.
PMCID: PMC522208  PMID: 15466044
9.  Cloning and nucleotide sequence of luxR, a regulatory gene controlling bioluminescence in Vibrio harveyi. 
Journal of Bacteriology  1990;172(6):2946-2954.
Mutagenesis with transposon mini-Mulac was used previously to identify a regulatory locus necessary for expression of bioluminescence genes, lux, in Vibrio harveyi (M. Martin, R. Showalter, and M. Silverman, J. Bacteriol. 171:2406-2414, 1989). Mutants with transposon insertions in this regulatory locus were used to construct a hybridization probe which was used in this study to detect recombinants in a cosmid library containing the homologous DNA. Recombinant cosmids with this DNA stimulated expression of the genes encoding enzymes for luminescence, i.e., the luxCDABE operon, which were positioned in trans on a compatible replicon in Escherichia coli. Transposon mutagenesis and analysis of the DNA sequence of the cloned DNA indicated that regulatory function resided in a single gene of about 0.6-kilobases named luxR. Expression of bioluminescence in V. harveyi and in the fish light-organ symbiont Vibrio fischeri is controlled by density-sensing mechanisms involving the accumulation of small signal molecules called autoinducers, but similarity of the two luminescence systems at the molecular level was not apparent in this study. The amino acid sequence of the LuxR product of V. harveyi, which indicates a structural relationship to some DNA-binding proteins, is not similar to the sequence of the protein that regulates expression of luminescence in V. fischeri. In addition, reconstitution of autoinducer-controlled luminescence in recombinant E. coli, already achieved with lux genes cloned from V. fischeri, was not accomplished with the isolation of luxR from V. harveyi, suggesting a requirement for an additional regulatory component.
PMCID: PMC209093  PMID: 2160932
10.  Active regulation of receptor ratios controls integration of quorum-sensing signals in Vibrio harveyi 
Single-cell quantification of the input–output relation of the quorum-sensing circuit reveals how Vibrio harveyi employs multiple feedback loops to simultaneously control quorum-sensing signal integration and to ensure signal transmission fidelity.
We identify the role of multiple feedback loops in the quorum-sensing circuit of the model bacterium, Vibrio harveyi. Single-cell microscopy and genetic analysis demonstrate that a novel feedback loop regulates receptor ratios to control the integration of multiple signals.Quantitative investigation of cells with all feedback loops present as well as mutants with specific feedback loops disrupted reveals that the multiple feedback loops expand the input dynamic range and compress the output dynamic range of signal transmission, and also control the noise level of the output.Our experimental observations can be interpreted in terms of a simple model of the quorum-sensing network. Plotting output after reparameterizing the input variables directly reveals how feedback controls receptors ratios.
Organisms detect multiple environmental cues simultaneously and use the information to coordinate their behaviors. Correctly integrating signals generally requires complex signal transduction pathways (Pawson and Scott, 2010). In addition to accurately integrating signals, regulatory circuits must ensure signal transmission fidelity. Information can be lost or corrupted by internal or external perturbations, so circuits must be designed to function robustly in the presence of such fluctuations. For example, the circadian clock in Neurospora (Virshup and Forger, 2009) and the chemotaxis network in Escherichia coli (Oleksiuk et al, 2011) accurately compensate for temperature variation. However, while signal integration and signal transmission have been addressed separately, little is known about mechanisms cells use to solve both tasks simultaneously. In this study, we report how the model bacterium Vibrio harveyi simultaneously integrates and faithfully transmits multiple chemical signals.
In a process called quorum sensing, bacteria communicate by synthesizing, releasing, and detecting signal molecules called autoinducers (AIs). To study the integration of such signals, we studied a strain of V. harveyi that integrates two AI signals into its quorum-sensing circuit: AI-1, an intra-species signal, and AI-2, a ‘universal' inter-species signal. Each signal is detected by a cognate receptor AI-1 by LuxN, and AI-2 by LuxPQ (Figure 4A). The information encoded in the two AIs is transduced through a shared signaling pathway into the master quorum-sensing regulator LuxR. In this study, the AIs serve as inputs and LuxR serves as the output of the quorum-sensing circuit. Interestingly, there are five distinct feedback loops in the V. harveyi quorum-sensing circuit (Figure 4A). How does the circuit use shared components to distinguish between the two AI inputs and what role does each feedback loop have in signal integration and transmission?
Using single-cell microscopy, we assayed the activity of the quorum-sensing circuit with a focus on defining the functions of the feedback loops. We quantitatively investigated the signaling input–output relation both in cells with all feedback loops present (Figure 4A) as well as in mutants with specific feedback loops disrupted (Figure 4E, I, M, and Q). We compared the mean LuxR level (Figure 4B, F, J, N, and R) and noise level (Figure 4C, G, K, O, and S) for the input–output relation of five strains. We discovered that the LuxN feedback loop regulates receptor ratios (LuxN to LuxPQ) to control the integration of two signals. We also found that the multiple feedback loops expand the input dynamic range and compress the output dynamic range of signal transmission, and also control the noise in the output.
In summary, we used single-cell microscopy to quantify the integration of quorum-sensing signals in V. harveyi. Multiple feedback loops in the quorum-sensing circuit actively regulate receptor ratios to control signal integration, sculpt the input–output dynamic range, and regulate the noise level. This system presents a paradigm for how complex circuitry allows cells to appropriately detect and respond to multiple signals in a dynamically changing environment.
Quorum sensing is a chemical signaling mechanism used by bacteria to communicate and orchestrate group behaviors. Multiple feedback loops exist in the quorum-sensing circuit of the model bacterium Vibrio harveyi. Using fluorescence microscopy of individual cells, we assayed the activity of the quorum-sensing circuit, with a focus on defining the functions of the feedback loops. We quantitatively investigated the signaling input–output relation both in cells with all feedback loops present as well as in mutants with specific feedback loops disrupted. We found that one of the feedback loops regulates receptor ratios to control the integration of multiple signals. Together, the feedback loops affect the input–output dynamic range of signal transmission and the noise in the output. We conclude that V. harveyi employs multiple feedback loops to simultaneously control quorum-sensing signal integration and to ensure signal transmission fidelity.
PMCID: PMC3130561  PMID: 21613980
feedback loops; quorum sensing; signal integration; single-cell fluorescence microscopy
11.  Sequence and Function of LuxU: a Two-Component Phosphorelay Protein That Regulates Quorum Sensing in Vibrio harveyi 
Journal of Bacteriology  1999;181(3):899-906.
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.
PMCID: PMC93457  PMID: 9922254
12.  luxS and arcB Control Aerobic Growth of Actinobacillus actinomycetemcomitans under Iron Limitation  
Infection and Immunity  2003;71(1):298-308.
LuxS is responsible for the production of autoinducer 2 (AI-2), which functions in Vibrio harveyi as a quorum-sensing signal that controls the cell density-dependent expression of the lux operon. In nonluminescent organisms, the physiologic role of AI-2 is not clear. We report that inactivation of luxS in Actinobacillus actinomycetemcomitans JP2 results in reduced growth of the mutant, but not the wild-type organism, under aerobic, iron-limited conditions. Stunted cultures of the luxS mutant A. actinomycetemcomitans JP2-12 grew to high cell density when subcultured under iron-replete conditions. In addition, the mutant strain grew to high cell density under iron limitation after transformation with a plasmid containing a functional copy of luxS. Results of real-time PCR showed that A. actinomycetemcomitans JP2-12 exhibited significantly reduced expression of afuA (eightfold), fecBCDE (10-fold), and ftnAB (>50-fold), which encode a periplasmic ferric transport protein, a putative ferric citrate transporter, and ferritin, respectively. The expressions of putative receptors for transferrin, hemoglobin, and hemophore binding protein were also reduced at more modest levels (two- to threefold). In contrast, expressions of sidD and frpB (encoding putative siderophore receptors) were increased 10- and 3-fold, respectively, in the luxS mutant. To better understand the mechanism of the AI-2 response, the A. actinomycetemcomitans genome was searched for homologs of the V. harveyi signal transduction proteins, LuxP, LuxQ, LuxU, and LuxO. Interestingly, ArcB was found to be most similar to LuxQ sensor/kinase. To determine whether arcB plays a role in the response of A. actinomycetemcomitans to AI-2, an arcB-deficient mutant was constructed. The isogenic arcB mutant grew poorly under anaerobic conditions but grew normally under aerobic iron-replete conditions. However, the arcB mutant failed to grow aerobically under iron limitation, and reverse transcriptase PCR showed that inactivation of arcB resulted in decreased expression of afuA and ftnAB. Thus, isogenic luxS and arcB mutants of A. actinomycetemcomitans exhibit similar phenotypes when cultured aerobically under iron limitation, and both mutants exhibit reduced expression of a common set of genes involved in the transport and storage of iron. These results suggest that LuxS and ArcB may act in concert to control the adaptation of A. actinomycetemcomitans to iron-limiting conditions and its growth under such conditions.
PMCID: PMC143191  PMID: 12496179
13.  Characterization of Type 2 Quorum Sensing in Klebsiella pneumoniae and Relationship with Biofilm Formation 
Journal of Bacteriology  2005;187(8):2870-2880.
Quorum sensing is a process by which bacteria communicate by using secreted chemical signaling molecules called autoinducers. Many bacterial species modulate the expression of a wide variety of physiological functions in response to changes in population density by this mechanism. In this study, the opportunistic pathogen Klebsiella pneumoniae was observed to secrete type 2 signaling molecules. A homologue of luxS, the gene required for AI-2 synthesis in Vibrio harveyi, was isolated from the K. pneumoniae genome. A V. harveyi bioassay showed the luxS functionality in K. pneumoniae and its ability to complement the luxS-negative phenotype of Escherichia coli DH5α. Autoinducer activity was detected in the supernatant, and maximum expression of specific messengers detected by quantitative reverse transcription-PCR analysis occurred during the late exponential phase. The highest levels of AI-2 were observed in minimal medium supplemented with glycerol. To determine the potential role of luxS in colonization processes, a K. pneumoniae luxS isogenic mutant was constructed and tested for its capacity to form biofilms in vitro on an abiotic surface and to colonize the intestinal tract in a murine model. No difference was observed in the level of intestinal colonization between the wild-type strain and the luxS mutant. Microscopic analysis of biofilm structures revealed that the luxS mutant was able to form a mature biofilm but with reduced capacities in the development of microcolonies, mostly in the early steps of biofilm formation. These data suggest that a LuxS-dependent signal plays a role in the early stages of biofilm formation by K. pneumoniae.
PMCID: PMC1070389  PMID: 15805533
14.  Cell density-dependent modulation of the Vibrio fischeri luminescence system in the absence of autoinducer and LuxR protein. 
Journal of Bacteriology  1992;174(8):2440-2448.
Expression of the Vibrio fischeri luminescence genes (luxR and luxICDABEG) in Escherichia coli requires autoinducer (N-3-oxohexanoyl homoserine lactone) and LuxR protein, which activate transcription of luxICDABEG (genes for autoinducer synthase and the luminescence enzymes), and cyclic AMP (cAMP) and cAMP receptor protein (CRP), which activate transcription of the divergently expressed luxR gene. In E. coli and in V. fischeri, the autoinducer-LuxR protein-dependent induction of luxICDABEG transcription (called autoinduction) is delayed by glucose, whereas it is promoted by iron restriction, but the mechanisms for these effects are not clear. To examine in V. fischeri control of lux gene expression by autoinducer, cAMP, glucose, and iron, lux::Mu dI(lacZ) and lux deletion mutants of V. fischeri were constructed by conjugation and gene replacement procedures. beta-Galactosidase synthesis in a luxC::lacZ mutant exhibited autoinduction. In a luxR::lacZ mutant, complementation by the luxR gene was necessary for luminescence, and addition of cAMP increased beta-galactosidase activity four- to sixfold. Furthermore, a luxI::lacZ mutant produced no detectable autoinducer but responded to its addition with induced synthesis of beta-galactosidase. These results confirm in V. fischeri key features of lux gene regulation derived from studies with E. coli. However, beta-galactosidase specific activity in the luxI::lacZ mutant, without added autoinducer, exhibited an eight- to tenfold decrease and rise back during growth, as did beta-galactosidase and luciferase specific activities in the luxR::lacZ mutant and luciferase specific activity in a delta(luxR luxICD) mutant. The presence of glucose delayed the rise back in beta-galactosidase and luciferase specific activities in these strains, whereas iron restriction promoted it. Thus, in addition to transcriptional control by autoinducer and LuxR protein, the V. fischeri lux system exhibits a cell density-dependent modulation of expression that does not require autoinducer, LuxR protein, or known lux regulatory sites. The response of autoinducer-LuxR protein-independent modulation to glucose and iron may account for how these environmental factors control lux gene expressions.
PMCID: PMC205879  PMID: 1313412
15.  A small-RNA-mediated negative feedback loop controls quorum-sensing dynamics in Vibrio harveyi 
Molecular Microbiology  2008;70(4):896-907.
The bioluminescent marine bacterium Vibrio harveyi uses a cell-to-cell communication process called quorum sensing (QS) to co-ordinate behaviours in response to changes in population density. QS is accomplished through the secretion and detection of extracellular signalling molecules called autoinducers. At the centre of the V. harveyi QS circuit are five small regulatory RNAs called Qrr1–5 which destabilize the mRNA of luxR, encoding LuxR, the master transcriptional regulator of QS target genes. Here we show that LuxR directly activates transcription of qrr2, qrr3 and qrr4, leading to the rapid downregulation of luxR. The LuxR-binding sites in the promoters of qrr2, qrr3 and qrr4 were identified and mutated to determine the consequences of this regulatory loop on QS dynamics. Disruption of the loop delays the transition from high to low cell density, and more significantly, decreases the cell density at which the population reaches a quorum. Our results suggest that feedback is essential for optimizing the dynamics of the transitions between individual and group behaviours.
PMCID: PMC2680268  PMID: 18808382
16.  Control of Vibrio fischeri lux gene transcription by a cyclic AMP receptor protein-luxR protein regulatory circuit. 
Journal of Bacteriology  1988;170(9):4040-4046.
Expression of the Vibrio fischeri luminescence genes (lux genes) requires two transcriptional activators: the V. fischeri luxR gene product with autoinducer and the cyclic AMP (cAMP) receptor protein (CRP) with cAMP. It has been established that autoinducer and the luxR gene product are required for transcriptional activation of the luxICDABE operon, which contains a gene required for autoinducer synthesis and genes required for light emission. However, the role of cAMP-CRP in the induction of luminescence is not clear. We examined transcriptional control of the lux genes in Escherichia coli, using catabolite repression mutants carrying lux DNA-containing plasmids. Transcriptional fusions between the lacZ gene on Mu dI and luxR were used to assess luxR promoter activity, and the luxAB genes (which encode the two luciferase subunits) were used as a natural reporter of luxICDABE promoter activity. A plasmid containing luxR under control of the cAMP-CRP-independent tac promoter was constructed to direct the synthesis of the luxR gene product in cells containing compatible luxR::Mu dI insertion mutant plasmids. In E. coli, cAMP-CRP activated transcription of luxR and concurrently decreased luxICDABE transcription. In the presence of relatively high levels of the luxR gene product, cAMP and CRP were not required for induction of the luxICDABE operon. The luxR gene product in the presence of autoinducer activated transcription of the luxICDABE operon, as has been shown previously, and we demonstrate that it also decreased luxR transcription. Apparently, control of the V. fischeri luminescence genes involves a regulatory circuit in which cAMP and CRP activate luxR transcription and in turn the luxR gene product activates transcription of the operon responsible for light emission (uxICDABE). Furthermore, in lux gene regulation cAMP-CRP and autoinducer-LuxR protein appear to function as transcriptional antagonists.
PMCID: PMC211407  PMID: 3410823
17.  Signaling System in Porphyromonas gingivalis Based on a LuxS Protein 
Journal of Bacteriology  2001;183(13):3903-3909.
The luxS gene of quorum-sensing Vibrio harveyi is required for type 2 autoinducer production. We identified a Porphyromonas gingivalis open reading frame encoding a predicted peptide of 161 aa that shares 29% identity with the amino acid sequence of the LuxS protein of V. harveyi. Conditioned medium from a late-log-phase P. gingivalis culture induced the luciferase operon of V. harveyi, but that from a luxS insertional mutant did not. In P. gingivalis, the expression of luxS mRNA was environmentally controlled and varied according to the cell density and the osmolarity of the culture medium. In addition, differential display PCR showed that the inactivation of P. gingivalis luxS resulted in up-regulation of a hemin acquisition protein and an arginine-specific protease and reduced expression of a hemin-regulated protein, a TonB homologue, and an excinuclease. The data suggest that the luxS gene in P. gingivalis may function to control the expression of genes involved in the acquisition of hemin.
PMCID: PMC95272  PMID: 11395453
18.  Quantifying the Integration of Quorum-Sensing Signals with Single-Cell Resolution 
PLoS Biology  2009;7(3):e1000068.
Cell-to-cell communication in bacteria is a process known as quorum sensing that relies on the production, detection, and response to the extracellular accumulation of signaling molecules called autoinducers. Often, bacteria use multiple autoinducers to obtain information about the vicinal cell density. However, how cells integrate and interpret the information contained within multiple autoinducers remains a mystery. Using single-cell fluorescence microscopy, we quantified the signaling responses to and analyzed the integration of multiple autoinducers by the model quorum-sensing bacterium Vibrio harveyi. Our results revealed that signals from two distinct autoinducers, AI-1 and AI-2, are combined strictly additively in a shared phosphorelay pathway, with each autoinducer contributing nearly equally to the total response. We found a coherent response across the population with little cell-to-cell variation, indicating that the entire population of cells can reliably distinguish several distinct conditions of external autoinducer concentration. We speculate that the use of multiple autoinducers allows a growing population of cells to synchronize gene expression during a series of distinct developmental stages.
Author Summary
Although bacteria are unicellular, the individual cells communicate with each other via small diffusible molecules. This communication process, known as quorum sensing, allows groups of bacteria to track the density of the population they are in, synchronize gene expression across the population, and thereby carry out collective activities similar to those of cells in multi-cellular organisms. Many bacterial species use multiple signaling molecules, but it remains a mystery why multiple signals are required and how the information encoded in them is integrated by bacteria. To explore these questions, we studied a model quorum-sensing bacterium Vibrio harveyi. Using single-cell fluorescence microscopy, we quantified quorum-sensing responses and analyzed the mechanism of integration of multiple signals. Surprisingly, we found that information from two distinct signals is combined strictly additively, with precisely equal weight from each signal. Our results revealed a coherent response across the population with little cell-to-cell variation, allowing the entire population of bacterial cells to reliably distinguish multiple environmental states. We argue that multiple signals and multiple response states could be used to distinguish distinct stages in the development of a bacterial community.
How do bacteria integrate multiple cell-to-cell communication signals? Information from two signaling molecules is combined in a strictly additive way with equal weight from each signal.
PMCID: PMC2661960  PMID: 19320539
19.  LuxR- and Acyl-Homoserine-Lactone-Controlled Non-lux Genes Define a Quorum-Sensing Regulon in Vibrio fischeri 
Journal of Bacteriology  2000;182(10):2811-2822.
The luminescence (lux) operon (luxICDABEG) of the symbiotic bacterium Vibrio fischeri is regulated by the transcriptional activator LuxR and two acyl-homoserine lactone (acyl-HSL) autoinducers (the luxI-dependent 3-oxo-hexanoyl-HSL [3-oxo-C6-HSL] and the ainS-dependent octanoyl-HSL [C8-HSL]) in a population density-responsive manner called quorum sensing. To identify quorum-sensing-regulated (QSR) proteins different from those encoded by lux genes, we examined the protein patterns of V. fischeri quorum-sensing mutants defective in luxI, ainS, and luxR by two-dimensional polyacrylamide gel electrophoresis. Five non-Lux QSR proteins, QsrP, RibB, AcfA, QsrV, and QSR 7, were identified; their production occurred preferentially at high population density, required both LuxR and 3-oxo-C6-HSL, and was inhibited by C8-HSL at low population density. The genes encoding two of the QSR proteins were characterized: qsrP directs cells to synthesize an apparently novel periplasmic protein, and ribB is a homolog of the Escherichia coli gene for 3,4-dihydroxy-2-butanone 4-phosphate synthase, a key enzyme for riboflavin synthesis. The qsrP and ribB promoter regions each contained a sequence similar to the lux operon lux box, a 20-bp region of dyad symmetry necessary for LuxR/3-oxo-C6-HSL-dependent activation of lux operon transcription. V. fischeri qsrP and ribB mutants exhibited no distinct phenotype in culture. However, a qsrP mutant, in competition with its parent strain, was less successful in colonizing Euprymna scolopes, the symbiotic host of V. fischeri. The newly identified QSR genes, together with the lux operon, define a LuxR/acyl-HSL-responsive quorum-sensing regulon in V. fischeri.
PMCID: PMC101990  PMID: 10781550
20.  pfs-Dependent Regulation of Autoinducer 2 Production in Salmonella enterica Serovar Typhimurium 
Journal of Bacteriology  2002;184(13):3450-3456.
Bacterial intercellular communication provides a mechanism for signal-dependent regulation of gene expression to promote coordinated population behavior. Salmonella enterica serovar Typhimurium produces a non-homoserine lactone autoinducer in exponential phase as detected by a Vibrio harveyi reporter assay for autoinducer 2 (AI-2) (M. G. Surette and B. L. Bassler, Proc. Natl. Acad. Sci. USA 95:7046-7050, 1998). The luxS gene product mediates the production of AI-2 (M. G. Surette, M. B. Miller, and B. L. Bassler, Proc. Natl. Acad. Sci. USA 96:1639-1644, 1999). Environmental cues such as rapid growth, the presence of preferred carbon sources, low pH, and/or high osmolarity were found to influence the production of AI-2 (M. G. Surette and B. L. Bassler, Mol. Microbiol. 31:585-595, 1999). In addition to LuxS, the pfs gene product (Pfs) is required for AI-2 production, as well as S-adenosylhomocysteine (SAH) (S. Schauder, K. Shokat, M. G. Surette, and B. L. Bassler, Mol. Microbiol. 41:463-476, 2001). In bacterial cells, Pfs exhibits both 5′-methylthioadenosine (MTA) and SAH nucleosidase functions. Pfs is involved in methionine metabolism, regulating intracellular MTA and SAH levels (elevated levels of MTA and SAH are potent inhibitors of polyamine synthetases and S-adenosylmethionine dependent methyltransferase reactions, respectively). To further investigate regulation of AI-2 production in Salmonella, we constructed pfs and luxS promoter fusions to a luxCDABE reporter in a low-copy-number vector, allowing an examination of transcription of the genes in the pathway for signal synthesis. Here we report that luxS expression is constitutive but that the transcription of pfs is tightly correlated to AI-2 production in Salmonella serovar Typhimurium 14028. Neither luxS nor pfs expression appears to be regulated by AI-2. These results suggest that AI-2 production is regulated at the level of LuxS substrate availability and not at the level of luxS expression. Our results indicate that AI-2-dependent signaling is a reflection of metabolic state of the cell and not cell density.
PMCID: PMC135139  PMID: 12057938
21.  LuxS-Based Quorum Sensing Does Not Affect the Ability of Salmonella enterica Serovar Typhimurium To Express the SPI-1 Type 3 Secretion System, Induce Membrane Ruffles, or Invade Epithelial Cells ▿  
Journal of Bacteriology  2009;191(23):7253-7259.
Bacterial species can communicate by producing and sensing small autoinducer molecules by a process known as quorum sensing. Salmonella enterica produces autoinducer 2 (AI-2) via the luxS synthase gene, which is used by some bacterial pathogens to coordinate virulence gene expression with population density. We investigated whether the luxS gene might affect the ability of Salmonella enterica serovar Typhimurium to invade epithelial cells. No differences were found between the wild-type strain of S. Typhimurium, SL1344, and its isogenic luxS mutant with respect to the number and morphology of the membrane ruffles induced or their ability to invade epithelial cells. The dynamics of the ruffling process were also similar in the wild-type strain (SL1344) and the luxS mutant. Furthermore, comparing the Salmonella pathogenicity island 1 (SPI-1) type 3 secretion profiles of wild-type SL1344 and the luxS mutant by Western blotting and measuring the expression of a single-copy green fluorescent protein fusion to the prgH (an essential SPI-1 gene) promoter indicated that SPI-1 expression and activity are similar in the wild-type SL1344 and luxS mutant. Genetic deletion of luxS did not alter the virulence of S. Typhimurium in the mouse model, and therefore, it appears that luxS does not play a significant role in regulating invasion of Salmonella in vitro or in vivo.
PMCID: PMC2786567  PMID: 19783624
22.  The luxR gene product of Vibrio harveyi is a transcriptional activator of the lux promoter. 
Journal of Bacteriology  1992;174(22):7490-7493.
Expression of the lux operon from the marine bacterium Vibrio harveyi is dependent on cell density and requires an unlinked regulatory gene, luxR, and other cofactors for autoregulation. Escherichia coli transformed with the lux operon emits very low levels of light, and this deficiency can be partially alleviated by coexpression of luxR in trans. The V. harveyi lux promoter was analyzed in vivo by primer extension mapping to examine the function of luxR. RNA isolated from E. coli transformed with the Vibrio harveyi lux operon was shown to have a start site at 123 bp upstream of the first ATG codon of luxC. This is in sharp contrast to the start site found for lux RNA isolated from V. harveyi, at 26 bp upstream of the luxC initiation codon. However, when E. coli was cotransformed with both the lux operon and luxR, the start site of the lux mRNA shifted from -123 to -26. Furthermore, expression of the luxR gene caused a 350-fold increase in lux mRNA levels. The results suggest that LuxR of V. harveyi is a transcriptional activator stimulating initiation at the -26 lux promoter.
PMCID: PMC207451  PMID: 1385389
23.  Role of the C-Terminal Domain of the Alpha Subunit of RNA Polymerase in LuxR-Dependent Transcriptional Activation of the lux Operon during Quorum Sensing 
Journal of Bacteriology  2002;184(16):4520-4528.
During quorum sensing in Vibrio fischeri, the luminescence, or lux, operon is regulated in a cell density-dependent manner by the activator LuxR in the presence of an acylated homoserine lactone autoinducer molecule [N-(3-oxohexanoyl) homoserine lactone]. LuxR, which binds to the lux operon promoter at a position centered at −42.5 relative to the transcription initiation site, is thought to function as an ambidextrous activator making multiple contacts with RNA polymerase (RNAP). The specific role of the α-subunit C-terminal domain (αCTD) of RNAP in LuxR-dependent transcriptional activation of the lux operon promoter has been investigated. The effects of 70 alanine substitution variants of the α subunit were determined in vivo by measuring the rate of transcription of the lux operon via luciferase assays in recombinant Escherichia coli. The mutant RNAPs from strains exhibiting at least twofold-increased or -decreased activity in comparison to the wild type were further examined by in vitro assays. Since full-length LuxR has not been purified, an autoinducer-independent N-terminally truncated form of LuxR, LuxRΔN, was used for in vitro studies. Single-round transcription assays were performed using reconstituted mutant RNAPs in the presence of LuxRΔN, and 14 alanine substitutions in the αCTD were identified as having negative effects on the rate of transcription from the lux operon promoter. Five of these 14 α variants were also involved in the mechanisms of both LuxR- and LuxRΔN-dependent activation in vivo. The positions of these residues lie roughly within the 265 and 287 determinants in α that have been identified through studies of the cyclic AMP receptor protein and its interactions with RNAP. This suggests a model where residues 262, 265, and 296 in α play roles in DNA recognition and residues 290 and 314 play roles in α-LuxR interactions at the lux operon promoter during quorum sensing.
PMCID: PMC135237  PMID: 12142422
24.  Evidence for a Signaling System in Helicobacter pylori: Detection of a luxS-Encoded Autoinducer 
Journal of Bacteriology  2000;182(13):3638-3643.
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.
PMCID: PMC94532  PMID: 10850976
25.  Exposure to Static Magnetic Field Stimulates Quorum Sensing Circuit in Luminescent Vibrio Strains of the Harveyi Clade 
PLoS ONE  2014;9(6):e100825.
In this study, the evidence of electron-dense magnetic inclusions with polyhedral shape in the cytoplasm of Harveyi clade Vibrio strain PS1, a bioluminescent bacterium living in symbiosis with marine organisms, led us to investigate the behavior of this bacterium under exposure to static magnetic fields ranging between 20 and 2000 Gauss. When compared to sham-exposed, the light emission of magnetic field-exposed bacteria growing on solid medium at 18°C ±0.1°C was increased up to two-fold as a function of dose and growth phase. Stimulation of bioluminescence by magnetic field was more pronounced during the post-exponential growth and stationary phase, and was lost when bacteria were grown in the presence of the iron chelator deferoxamine, which caused disassembly of the magnetic inclusions suggesting their involvement in magnetic response. As in luminescent Vibrio spp. bioluminescence is regulated by quorum sensing, possible effects of magnetic field exposure on quorum sensing were investigated. Measurement of mRNA levels by reverse transcriptase real time-PCR demonstrated that luxR regulatory gene and luxCDABE operon coding for luciferase and fatty acid reductase complex were significantly up-regulated in magnetic field-exposed bacteria. In contrast, genes coding for a type III secretion system, whose expression was negatively affected by LuxR, were down-regulated. Up-regulation of luxR paralleled with down-regulation of small RNAs that mediate destabilization of luxR mRNA in quorum sensing signaling pathways. The results of experiments with the well-studied Vibrio campbellii strain BB120 (originally classified as Vibrio harveyi) and derivative mutants unable to synthesize autoinducers suggest that the effects of magnetic fields on quorum sensing may be mediated by AI-2, the interspecies quorum sensing signal molecule.
PMCID: PMC4069165  PMID: 24960170

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