The languages of bacteria.
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4-Butyrolactone↗
Biomedical subjects
B L Bassler
Publications and source records attributed to B L Bassler.
The LuxS family of bacterial autoinducers: biosynthesis of a novel quorum-sensing signal molecule.
Many bacteria control gene expression in response to cell population density, and this phenomenon is called quorum sensing. In Gram-negative bacteria, quorum sensing typically involves the production, release and detection of acylated homoserine lactone signalling molecules called autoinducers. Vibrio harveyi, a Gram-negative bioluminescent marine bacterium, regulates light production in response to two distinct autoinducers (AI-1 and AI-2). AI-1 is a homoserine lactone. The structure of AI-2 is not known. We have suggested previously that V. harveyi uses AI-1 for intraspecies communication and AI-2 for interspecies communication. Consistent with this idea, we have shown that many species of Gram-negative and Gram-positive bacteria produce AI-2 and, in every case, production of AI-2 is dependent on the function encoded by the luxS gene. We show here that LuxS is the AI-2 synthase and that AI-2 is produced from S-adenosylmethionine in three enzymatic steps. The substrate for LuxS is S-ribosylhomocysteine, which is cleaved to form two products, one of which is homocysteine, and the other is AI-2. In this report, we also provide evidence that the biosynthetic pathway and biochemical intermediates in AI-2 biosynthesis are identical in Escherichia coli, Salmonella typhimurium, V. harveyi, Vibrio cholerae and Enterococcus faecalis. This result suggests that, unlike quorum sensing via the family of related homoserine lactone autoinducers, AI-2 is a unique, 'universal' signal that could be used by a variety of bacteria for communication among and between species.
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The LuxS-dependent autoinducer AI-2 controls the expression of an ABC transporter that functions in AI-2 uptake in Salmonella typhimurium.
In a process called quorum sensing, bacteria communicate with one another using secreted chemical signalling molecules termed autoinducers. A novel autoinducer called AI-2, originally discovered in the quorum-sensing bacterium Vibrio harveyi, is made by many species of Gram-negative and Gram-positive bacteria. In every case, production of AI-2 is dependent on the LuxS autoinducer synthase. The genes regulated by AI-2 in most of these luxS-containing species of bacteria are not known. Here, we describe the identification and characterization of AI-2-regulated genes in Salmonella typhimurium. We find that LuxS and AI-2 regulate the expression of a previously unidentified operon encoding an ATP binding cassette (ABC)-type transporter. We have named this operon the lsr (luxS regulated) operon. The Lsr transporter has homology to the ribose transporter of Escherichia coli and S. typhimurium. A gene encoding a DNA-binding protein that is located adjacent to the Lsr transporter structural operon is required to link AI-2 detection to operon expression. This gene, which we have named lsrR, encodes a protein that represses lsr operon expression in the absence of AI-2. Mutations in the lsr operon render S. typhimurium unable to eliminate AI-2 from the extracellular environment, suggesting that the role of the Lsr apparatus is to transport AI-2 into the cells. It is intriguing that an operon regulated by AI-2 encodes functions resembling the ribose transporter, given recent findings that AI-2 is derived from the ribosyl moiety of S-ribosylhomocysteine.
ATP-Binding Cassette Transporters↗
Quorum sensing in bacteria.
Quorum sensing is the regulation of gene expression in response to fluctuations in cell-population density. Quorum sensing bacteria produce and release chemical signal molecules called autoinducers that increase in concentration as a function of cell density. The detection of a minimal threshold stimulatory concentration of an autoinducer leads to an alteration in gene expression. Gram-positive and Gram-negative bacteria use quorum sensing communication circuits to regulate a diverse array of physiological activities. These processes include symbiosis, virulence, competence, conjugation, antibiotic production, motility, sporulation, and biofilm formation. In general, Gram-negative bacteria use acylated homoserine lactones as autoinducers, and Gram-positive bacteria use processed oligo-peptides to communicate. Recent advances in the field indicate that cell-cell communication via autoinducers occurs both within and between bacterial species. Furthermore, there is mounting data suggesting that bacterial autoinducers elicit specific responses from host organisms. Although the nature of the chemical signals, the signal relay mechanisms, and the target genes controlled by bacterial quorum sensing systems differ, in every case the ability to communicate with one another allows bacteria to coordinate the gene expression, and therefore the behavior, of the entire community. Presumably, this process bestows upon bacteria some of the qualities of higher organisms. The evolution of quorum sensing systems in bacteria could, therefore, have been one of the early steps in the development of multicellularity.
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A genetic analysis of the functions of LuxN: a two-component hybrid sensor kinase that regulates quorum sensing in Vibrio harveyi.
The bioluminescent marine bacterium Vibrio harveyi controls light production using two parallel quorum-sensing systems. V. harveyi produces two autoinducers (AI-1 and AI-2), which are recognized by cognate membrane-bound two-component hybrid sensor kinases called LuxN and LuxQ respectively. Under conditions of low cell density, in the absence of autoinducer, the hybrid sensors are kinases, and under conditions of high cell density, in the presence of autoinducer, the sensors are phosphatases. These activities allow LuxN and LuxQ to modulate the level of phosphorylation of the response regulator protein LuxO. LuxO, in turn, controls the transcription of the genes encoding luciferase. The phosphorelay protein LuxU is required for signalling to LuxO. In this report, we present a genetic analysis of the activities of the AI-1 sensor LuxN. Point mutations and in frame deletions were constructed in luxN and recombined onto the chromosome of V. harveyi for in vivo phenotypic analysis. We show that the conserved histidine (H471) in the sensor kinase domain of LuxN is required for kinase activity but not for phosphatase activity. In contrast, the conserved aspartate (D771) in the response regulator domain of LuxN is required for both activities. Furthermore, the LuxN phosphatase activity is localized to the response regulator domain. Our results indicate that the LuxN kinase activity is regulated by the presence of AI-1, whereas the LuxN phosphatase activity is constitutive. We also show that signalling from the two V. harveyi quorum-sensing systems is not equivalent. AI-1 and LuxN have a much greater effect on the level of LuxO phosphate and therefore Lux expression than do AI-2 and LuxQ.
Bacterial Proteins↗
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.
Amino Acid Sequence↗
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.
Amino Acid Sequence↗
Quorum sensing in Escherichia coli, Salmonella typhimurium, and Vibrio harveyi: a new family of genes responsible for autoinducer production.
In bacteria, the regulation of gene expression in response to changes in cell density is called quorum sensing. Quorum-sensing bacteria produce, release, and respond to hormone-like molecules (autoinducers) that accumulate in the external environment as the cell population grows. In the marine bacterium Vibrio harveyi two parallel quorum-sensing systems exist, and each is composed of a sensor-autoinducer pair. V. harveyi reporter strains capable of detecting only autoinducer 1 (AI-1) or autoinducer 2 (AI-2) have been constructed and used to show that many species of bacteria, including Escherichia coli MG1655, E. coli O157:H7, Salmonella typhimurium 14028, and S. typhimurium LT2 produce autoinducers similar or identical to the V. harveyi system 2 autoinducer AI-2. However, the domesticated laboratory strain E. coli DH5alpha does not produce this signal molecule. Here we report the identification and analysis of the gene responsible for AI-2 production in V. harveyi, S. typhimurium, and E. coli. The genes, which we have named luxSV.h., luxSS.t., and luxSE.c. respectively, are highly homologous to one another but not to any other identified gene. E. coli DH5alpha can be complemented to AI-2 production by the introduction of the luxS gene from V. harveyi or E. coli O157:H7. Analysis of the E. coli DH5alpha luxSE.c. gene shows that it contains a frameshift mutation resulting in premature truncation of the LuxSE.c. protein. Our results indicate that the luxS genes define a new family of autoinducer-production genes.
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How bacteria talk to each other: regulation of gene expression by quorum sensing.
Quorum sensing, or the control of gene expression in response to cell density, is used by both gram-negative and gram-positive bacteria to regulate a variety of physiological functions. In all cases, quorum sensing involves the production and detection of extracellular signalling molecules called autoinducers. While universal signalling themes exist, variations in the design of the extracellular signals, the signal detection apparatuses, and the biochemical mechanisms of signal relay have allowed quorum sensing systems to be exquisitely adapted for their varied uses. Recent studies show that quorum sensing modulates both intra- and inter-species cell-cell communication, and it plays a major role in enabling bacteria to architect complex community structures.
Bacterial Proteins↗
Regulation of autoinducer production in Salmonella typhimurium.
Salmonella typhimurium strain LT2 secretes an organic signalling molecule that can be assayed by its ability to activate one of two specific quorum-sensing systems in Vibrio harveyi. Maximal activity is produced during mid- to late exponential phase when S. typhimurium is grown in the presence of glucose or other preferred carbohydrates. The signal is degraded by the onset of stationary phase or when the carbohydrate is depleted from the medium. Presumably, quorum sensing in S. typhimurium is operational during periods of rapid, nutrient-rich growth. Protein synthesis is required for degradation of the activity, suggesting that a complex regulatory circuitry controls signal production and detection in S. typhimurium. Increased signalling activity is observed if, after growth in the presence of glucose, S. typhimurium is transferred to a high-osmolarity (0.4 M NaCl) or to a low-pH (pH 5.0) environment. Degradation of the signal is induced by conditions of low osmolarity (0.1 M NaCl). High osmolarity and low pH are two conditions encountered by S. typhimurium cells when they undergo the transition to a pathogenic existence inside a host organism, suggesting that quorum sensing may have a role in the regulation of virulence in S. typhimurium.
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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.
Bacterial Proteins↗
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.
Amino Acid Sequence↗
Quorum sensing in Escherichia coli and Salmonella typhimurium.
Escherichia coli and Salmonella typhimurium strains grown in Luria-Bertani medium containing glucose secrete a small soluble heat labile organic molecule that is involved in intercellular communication. The factor is not produced when the strains are grown in Luria-Bertani medium in the absence of glucose. Maximal secretion of the substance occurs in midexponential phase, and the extracellular activity is degraded as the glucose is depleted from the medium or by the onset of stationary phase. Destruction of the signaling molecule in stationary phase indicates that, in contrast to other quorum-sensing systems, quorum sensing in E. coli and S. typhimurium is critical for regulating behavior in the prestationary phase of growth. Our results further suggest that the signaling factor produced by E. coli and S. typhimurium is used to communicate both the cell density and the metabolic potential of the environment. Several laboratory and clinical strains of E. coli and S. typhimurium were screened for production of the signaling molecule, and most strains make it under conditions similar to those shown here for E. coli AB1157 and S. typhimurium LT2. However, we also show that E. coli strain DH5alpha does not make the soluble factor, indicating that this highly domesticated strain has lost the gene(s) or biosynthetic machinery necessary to produce the signaling substance. Implications for the involvement of quorum sensing in pathogenesis are discussed.
Biological Assay↗
Cross-species induction of luminescence in the quorum-sensing bacterium Vibrio harveyi.
Different species of bacteria were tested for production of extracellular autoinducer-like activities that could stimulate the expression of the luminescence genes in Vibrio harveyi. Several species of bacteria, including the pathogens Vibrio cholerae and Vibrio parahaemolyticus, were found to produce such activities. Possible physiological roles for the two V. harveyi detection-response systems and their joint regulation are discussed.
Luminescent Measurements↗
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 harveyi 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 beta-hydroxybutyl 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. harveyi.
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Sequence and function of LuxO, a negative regulator of luminescence in Vibrio harveyi.
Density-dependent expression of luminescence in Vibrio harveyi is regulated by the concentration of extracellular signal molecules (autoinducers) in the culture medium. A recombinant clone that restored function to one class of spontaneous dim mutants was found to encode a function required for the density-dependent response. Transposon Tn5 insertions in the recombinant clone were isolated, and the mutations were transferred to the genome of V. harveyi for examination of mutant phenotypes. Expression of luminescence in V. harveyi strains with transposon insertions in one locus, luxO, was independent of the density of the culture and was similar in intensity to the maximal level observed in wild-type bacteria. Sequence analysis of luxO revealed one open reading frame that encoded a protein, LuxO, similar in amino acid sequence to the response regulator domain of the family of two-component, signal transduction proteins. The constitutive phenotype of LuxO- mutants indicates that LuxO acts negatively to control expression of luminescence, and relief of repression by LuxO in the wild type could result from interactions with other components in the Lux signalling system.
Amino Acid Sequence↗
Chemotaxis of the marine bacterium Vibrio furnissii to sugars. A potential mechanism for initiating the chitin catabolic cascade.
Immense quantities of chitin are catabolized by marine bacteria, and this process involves at least three signal transduction systems in Vibrio furnissii. One system, chemotaxis to chitin oligosaccharides, is probably used to colonize chitin particles. But how do the first few cells find this highly insoluble polysaccharide? The following hypothesis is proposed to answer this question: the bacteria respond to soluble chemo-attractants in exudates from injured organisms. Virtually all chitin-producing organisms also contain glucose and/or trehalose, often at high concentrations such as trehalose in insect hemolymph. Chemotaxis of V. furnissii was therefore studied with a variety of sugars. Fructose, ribose, and glycerol are catabolites but not attractants. The cells exhibit weak constitutive taxis to Glc and GlcNAc. After induction, they show a weak response to galactose but are strongly attracted to the following substrates of the phosphoenolpyruvate:glycose phosphotransferase system (PTS): GlcNAc, trehalose, glucose, sucrose, mannose, and mannitol. There is a rough qualitative but no quantitative correlation between the rate of phosphorylation and the chemotactic response to PTS sugars. Trehalose is especially noteworthy because it is phosphorylated at a very rapid rate by uninduced cells but is not an attractant until the cells are induced. We suggest that unidentified inducible factors link the PTS to chemotaxis.
Carbohydrate Metabolism↗
Intercellular signalling in Vibrio harveyi: sequence and function of genes regulating expression of luminescence.
Density-dependent expression of luminescence in Vibrio harveyi is regulated by the concentration of an extracellular signal molecule (autoinducer) in the culture medium. A recombinant clone that restored function to one class of spontaneous dim mutants was found to encode functions necessary for the synthesis of, and response to, a signal molecule. Sequence analysis of the region encoding these functions revealed three open reading frames, two (luxL and luxM) that are required for production of an autoinducer substance and a third (luxN) that is required for response to this signal substance. The LuxL and LuxM proteins are not similar in amino acid sequence to other proteins in the database, but the LuxN protein contains regions of sequence resembling both the histidine protein kinase and the response regulator domains of the family of two-component, signal transduction proteins. The phenotypes of mutants with luxL, luxM and luxN defects indicated that an additional signal-response system controlling density-dependent expression of luminescence remains to be identified.
Aldehyde Oxidoreductases↗