A diffusible compound can enhance conjugal transfer of the Ti plasmid in Agrobacterium tumefaciens

Agrobacterium tumefaciens: a Transformative Agent for Fundamental Insights into Host-Microbe Interactions, Genome Biology, Chemical Signaling, and Cell Biology

Agrobacterium tumefaciens incites the formation of readily visible macroscopic structures known as crown galls on plant tissues that it infects. Records from biologists as early as the 17th century noted these unusual plant growths and began examining the basis for their formation. These studies eventually led to isolation of the infectious agent, A. tumefaciens, and decades of study revealed the remarkable mechanisms by which A. tumefaciens causes crown gall through stable horizontal genetic transfer to plants. This fundamental discovery generated a barrage of applications in the genetic manipulation of plants that is still under way. As a consequence of the intense study of A. tumefaciens and its role in plant disease, this pathogen was developed as a model for the study of critical processes that are shared by many bacteria, including host perception during pathogenesis, DNA transfer and toxin secretion, bacterial cell-cell communication, plasmid biology, and more recently, asymmetric cell biology and composite genome coordination and evolution. As such, studies of A. tumefaciens have had an outsized impact on diverse areas within microbiology and plant biology that extend far beyond its remarkable agricultural applications. In this review, we attempt to highlight the colorful history of A. tumefaciens as a study system, as well as current areas that are actively demonstrating its value and utility as a model microorganism.

Conjugative plasmids inhibit extracellular electron transfer in Geobacter sulfurreducens

Geobacter sulfurreducens is part of a specialized group of microbes with the unique ability to exchange electrons with insoluble materials, such as iron oxides and electrodes. Therefore, G. sulfurreducens plays an essential role in the biogeochemical iron cycle and microbial electrochemical systems. In G. sulfurreducens this ability is primarily dependent on electrically conductive nanowires that link internal electron flow from metabolism to solid electron acceptors in the extracellular environment. Here we show that when carrying conjugative plasmids, which are self-transmissible plasmids that are ubiquitous in environmental bacteria, G. sulfurreducens reduces insoluble iron oxides at much slower rates. This was the case for all three conjugative plasmids tested (pKJK5, RP4 and pB10). Growth with electron acceptors that do not require expression of nanowires was, on the other hand, unaffected. Furthermore, iron oxide reduction was also inhibited in Geobacter chapellei, but not in Shewanella oneidensis where electron export is nanowire-independent. As determined by transcriptomics, presence of pKJK5 reduces transcription of several genes that have been shown to be implicated in extracellular electron transfer in G. sulfurreducens, including pilA and omcE. These results suggest that conjugative plasmids can in fact be very disadvantageous for the bacterial host by imposing specific phenotypic changes, and that these plasmids may contribute to shaping the microbial composition in electrode-respiring biofilms in microbial electrochemical reactors.

Innovative microbial disease biocontrol strategies mediated by quorum quenching and their multifaceted applications: A review

With the increasing resistance exhibited by undesirable bacteria to traditional antibiotics, the need to discover alternative (or, at least, supplementary) treatments to combat chemically resistant bacteria is becoming urgent. Quorum sensing (QS) refers to a novel bacterial communication system for monitoring cell density and regulation of a network of gene expression that is mediated by a group of signaling molecules called autoinducers (AIs). QS-regulated multicellular behaviors include biofilm formation, horizontal gene transfer, and antibiotic synthesis, which are demonstrating increasing pathogenicity to plants and aquacultural animals as well as contamination of wastewater treatment devices. To inhibit QS-regulated microbial behaviors, the strategy of quorum quenching (QQ) has been developed. Different quorum quenchers interfere with QS through different mechanisms, such as competitively inhibiting AI perception (e.g., by QS inhibitors) and AI degradation (e.g., by QQ enzymes). In this review, we first introduce different signaling molecules, including diffusible signal factor (DSF) and acyl homoserine lactones (AHLs) for Gram-negative bacteria, AIPs for Gram-positive bacteria, and AI-2 for interspecies communication, thus demonstrating the mode of action of the QS system. We next exemplify the QQ mechanisms of various quorum quenchers, such as chemical QS inhibitors, and the physical/enzymatic degradation of QS signals. We devote special attention to AHL-degrading enzymes, which are categorized in detail according to their diverse catalytic mechanisms and enzymatic properties. In the final part, the applications and advantages of quorum quenchers (especially QQ enzymes and bacteria) are summarized in the context of agricultural/aquacultural pathogen biocontrol, membrane bioreactors for wastewater treatment, and the attenuation of human pathogenic bacteria. Taken together, we present the state-of-the-art in research considering QS and QQ, providing theoretical evidence and support for wider application of this promising environmentally friendly biocontrol strategy.

Scent of a Symbiont: The Personalized Genetic Relationships of Rhizobium-Plant Interaction

Many molecular signals are exchanged between rhizobia and host legume plants, some of which are crucial for symbiosis to take place, while others are modifiers of the interaction, which have great importance in the competition with the soil microbiota and in the genotype-specific perception of host plants. Here, we review recent findings on strain-specific and host genotype-specific interactions between rhizobia and legumes, discussing the molecular actors (genes, gene products and metabolites) which play a role in the establishment of symbiosis, and highlighting the need for research including the other components of the soil (micro)biota, which could be crucial in developing rational-based strategies for bioinoculants and synthetic communities' assemblage.

The Roles of Microbial Cell-Cell Chemical Communication Systems in the Modulation of Antimicrobial Resistance

Rapid emergence of antimicrobial resistance (AMR) has become a critical challenge worldwide. It is of great importance to understand how AMR is modulated genetically in order to explore new antimicrobial strategies. Recent studies have unveiled that microbial communication systems, which are known to play key roles in regulation of bacterial virulence, are also associated with the formation and regulation of AMR. These microbial cell-to-cell chemical communication systems, including quorum sensing (QS) and pathogen-host communication mechanisms, rely on detection and response of various chemical signal molecules, which are generated either by the microbe itself or host cells, to activate the expression of virulence and AMR genes. This article summarizes the generic signaling mechanisms of representative QS and pathogen-host communications systems, reviews the current knowledge regarding the roles of these chemical communication systems in regulation of AMR, and describes the strategies developed over the years for blocking bacterial chemical communication systems in disease control. The research progress in this field suggests that the bacterial cell-cell communication systems are a promising target not only for disease control but also for curbing the problem of microbial drug resistance.

Transcriptome analysis revealed that a quorum sensing system regulates the transfer of the pAt megaplasmid in Agrobacterium tumefaciens

Background

Agrobacterium tumefaciens strain P4 is atypical, as the strain is not pathogenic and produces a for this species unusual quorum sensing signal, identified as N-(3-hydroxy-octanoyl)-homoserine lactone (3OH,C8-HSL).

Results

By sequence analysis and cloning, a functional luxI-like gene, named cinI, has been identified on the At plasmid of A. tumefaciens strain P4. Insertion mutagenesis in the cinI gene and transcriptome analyses permitted the identification of 32 cinI-regulated genes in this strain, most of them encoding proteins responsible for the conjugative transfer of pAtP4. Among these genes were the avhB genes that encode a type 4 secretion system (T4SS) involved in the formation of the conjugation apparatus, the tra genes that encode the DNA transfer and replication (Dtr) machinery and cinI and two luxR orthologs. These last two genes, cinR and cinX, exhibit an unusual organization, with the cinI gene surrounded by the two luxR orthologs. Conjugation experiments confirmed that the conjugative transfer of pAtP4 is regulated by 3OH,C8-HSL. Root colonization experiments indicated that the quorum sensing regulation of the conjugation of the pAtP4 does not confer a gain or a loss of fitness to the bacterial host in the tomato plant rhizosphere.

Conclusion

This work is the first identification of the occurrence of a quorum sensing regulation of the pAt conjugation phenomenon in Agrobacterium.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3007-5) contains supplementary material, which is available to authorized users.

D101 is critical for the function of AttJ, a repressor of quorum quenching system in Agrobacterium tumefaciens

The quorum quenching system of Agrobacterium tumefaciens is specifically activated upon entering the stationary phase. Evidence has shown that this system includes two key components: the IclR-type transcriptional factor AttJ (also named as BlcR) and the AHL-lactonase AttM (also named as BlcC). At exponential phase, AttJ binds to the promoter region of attM and thus suppresses the expression of attM. At stationary phase, however, the small molecule SSA directly binds to AttJ and relieves its inhibition of AttJ and thereby triggers the expression of attM. While the regulation of AttM has been extensively investigated, little is known about the regulation of AttJ. In this study, we demonstrated the D101 amino acid of AttJ is essential for the AttJ function. In vitro, the variant protein of AttJD101H appeared to be readily aggregated. In vivo, the D101H mutation in AttJ entirely abolished the inhibitory activity of AttJ and overexpressed attM in A. tumefaciens A6. In addition, D101H mutation led to an overexpression of attJ, indicating an auto-regulatory mechanism for the attJ regulation. Put together, these findings demonstrate that D101 is an important amino acid for the transcription activity of AttJ and the transcription of attJ is regulated by a negative feedback loop. These results expand previous biochemical characterization of AttJ and provide new mechanistic insights into the regulation of quorum quenching in A. tumefaciens.

Attachment of Agrobacterium tumefaciens to leaf tissue in response to infiltration conditions

Transient expression of recombinant proteins in plant tissues following Agrobacterium-mediated gene transfer is a promising technique for rapid protein production. However, transformation rates and transient expression levels can be sub-optimal depending on process conditions. Attachment of Agrobacterium tumefaciens to plant cells is an early, critical step in the gene transfer pathway. Bacterial attachment levels and patterns may influence transformation and, by extension, transient expression. In this study, attachment of A. tumefaciens to lettuce leaf tissue was investigated in response to varying infiltration conditions, including bacterial density, surfactant concentration, and applied vacuum level. Bacterial density was found to most influence attachment levels for the levels tested (10(8) , 10(9) , and 10(10) CFU/mL), with the relationship between bacterial density and attachment levels following a saturation trend. Surfactant levels tested (Break-Thru S240: 1, 10, 100, and 1,000 µL/L) also had a significant positive effect on bacterial attachment while vacuum level (5, 25, and 45 kPa) did not significantly affect attachment in areas exposed to bacteria. In planta transgene transient expression levels were measured following infiltration with 10(8) , 10(9) , and 10(10) CFU/mL bacterial suspension. Notably, the highest attachment level tested led to a decrease in transient expression, suggesting a potential link between bacterial attachment levels and downstream phenomena that may induce gene silencing. These results illustrate that attachment can be controlled by adjusting infiltration conditions and that attachment levels can impact transgene transient expression in leaf tissue.

Identification and characterization of a second quorum-sensing system in Agrobacterium tumefaciens A6

Quorum sensing (QS) is a widespread mechanism of bacterial communication in which individual cells produce and respond to small chemical signals. In Agrobacterium tumefaciens, an acylhomoserine lactone-dependent QS mechanism is known to regulate the replication and conjugation of the tumor-inducing (Ti) plasmid. Most of the QS regulatory proteins are encoded within the Ti plasmid. Among them, TraI is the LuxI-type enzyme synthesizing the QS signal N-3-oxooctanoyl-L-homoserine lactone (3OC8HSL), TraR is the LuxR-type transcriptional factor that recognizes 3OC8HSL, and TraM is an antiactivator that antagonizes TraR. Recently, we identified a TraM homolog encoded by the traM2 gene in the chromosomal background of A. tumefaciens A6. In this study, we further identified additional homologs (TraI2 and TraR2) of TraI and TraR in this strain. We showed that similar to TraI, TraI2 could predominantly synthesize the QS signal 3OC8HSL. We also showed that TraR2 could recognize 3OC8HSL and activate the tra box-containing promoters as efficiently as TraR. Further analysis showed that traM2, traI2, and traR2 are physically linked on a mobile genetic element that is not related to the Ti plasmid. These findings indicate that A. tumefaciens A6 carries a second QS system that may play a redundant role in the regulation of the replication and conjugation of the Ti plasmid.

Functions and regulation of quorum-sensing in Agrobacterium tumefaciens

In Agrobacterium tumefaciens, horizontal transfer and vegetative replication of oncogenic Ti plasmids involve a cell-to-cell communication process called quorum-sensing (QS). The determinants of the QS-system belong to the LuxR/LuxI class. The LuxI-like protein TraI synthesizes N-acyl-homoserine lactone molecules which act as diffusible QS-signals. Beyond a threshold concentration, these molecules bind and activate the LuxR-like transcriptional regulator TraR, thereby initiating the QS-regulatory pathway. For the last 20 years, A. tumefaciens has stood as a prominent model in the understanding of the LuxR/LuxI type of QS systems. A number of studies also unveiled features which are unique to A. tumefaciens QS, some of them being directly related to the phytopathogenic lifestyle of the bacteria. In this review, we will present the current knowledge of QS in A. tumefaciens at both the genetic and molecular levels. We will also describe how interactions with plant host modulate the QS pathway of A. tumefaciens, and discuss what could be the advantages for the agrobacteria to use such a tightly regulated QS-system to disseminate the Ti plasmids.

Quorum-dependent mannopine-inducible conjugative transfer of an Agrobacterium opine-catabolic plasmid

The Ti plasmid in Agrobacterium tumefaciens strain 15955 carries two alleles of traR that regulate conjugative transfer. The first is a functional allele, called traR, that is transcriptionally induced by the opine octopine. The second, trlR, is a nonfunctional, dominant-negative mutant located in an operon that is inducible by the opine mannopine (MOP). Based on these findings, we predicted that there exist wild-type agrobacterial strains harboring plasmids in which MOP induces a functional traR and, hence, conjugation. We analyzed 11 MOP-utilizing field isolates and found five where MOP induced transfer of the MOP-catabolic element and increased production of the acyl-homoserine lactone (acyl-HSL) quormone. The transmissible elements in these five strains represent a set of highly related plasmids. Sequence analysis of one such plasmid, pAoF64/95, revealed that the 176-kb element is not a Ti plasmid but carries genes for catabolism of MOP, mannopinic acid (MOA), agropinic acid (AGA), and the agrocinopines. The plasmid additionally carries all of the genes required for conjugative transfer, including the regulatory genes traR, traI, and traM. The traR gene, however, is not located in the MOP catabolism region. The gene, instead, is monocistronic and located within the tra-trb-rep gene cluster. A traR mutant failed to transfer the plasmid and produced little to no quormone even when grown with MOP, indicating that TraRpAoF64/95 is the activator of the tra regulon. A traM mutant was constitutive for transfer and acyl-HSL production, indicating that the anti-activator function of TraM is conserved.

Horizontal gene exchange in environmental microbiota

Horizontal gene transfer (HGT) plays an important role in the evolution of life on the Earth. This view is supported by numerous occasions of HGT that are recorded in the genomes of all three domains of living organisms. HGT-mediated rapid evolution is especially noticeable among the Bacteria, which demonstrate formidable adaptability in the face of recent environmental changes imposed by human activities, such as the use of antibiotics, industrial contamination, and intensive agriculture. At the heart of the HGT-driven bacterial evolution and adaptation are highly sophisticated natural genetic engineering tools in the form of a variety of mobile genetic elements (MGEs). The main aim of this review is to give a brief account of the occurrence and diversity of MGEs in natural ecosystems and of the environmental factors that may affect MGE-mediated HGT.

Modulation of Pseudomonas aeruginosa biofilm dispersal by a cyclic-Di-GMP phosphodiesterase with a putative hypoxia-sensing domain

Pseudomonas aeruginosa encodes many enzymes that are potentially associated with the synthesis or degradation of the widely conserved second messenger cyclic-di-GMP (c-di-GMP). In this study, we show that mutation of rbdA, which encodes a fusion protein consisting of PAS-PAC-GGDEF-EAL multidomains, results in decreased biofilm dispersal. RbdA contains a highly conserved GGDEF domain and EAL domain, which are involved in the synthesis and degradation of c-di-GMP, respectively. However, in vivo and in vitro analyses show that the full-length RbdA protein only displays phosphodiesterase activity, causing c-di-GMP degradation. Further analysis reveals that the GGDEF domain of RbdA plays a role in activating the phosphodiesterase activity of the EAL domain in the presence of GTP. Moreover, we show that deletion of the PAS domain or substitution of the key residues implicated in sensing low-oxygen stress abrogates the functionality of RbdA. Subsequent study showed that RbdA is involved in positive regulation of bacterial motility and production of rhamnolipids, which are associated with biofilm dispersal, and in negative regulation of production of exopolysaccharides, which are required for biofilm formation. These data indicate that the c-di-GMP-degrading regulatory protein RbdA promotes biofilm dispersal through its two-pronged effects on biofilm development, i.e., downregulating biofilm formation and upregulating production of the factors associated with biofilm dispersal.

The BlcC (AttM) lactonase of Agrobacterium tumefaciens does not quench the quorum-sensing system that regulates Ti plasmid conjugative transfer

The conjugative transfer of Agrobacterium plasmids is controlled by a quorum-sensing system consisting of TraR and its acyl-homoserine lactone (HSL) ligand. The acyl-HSL is essential for the TraR-mediated activation of the Ti plasmid Tra genes. Strains A6 and C58 of Agrobacterium tumefaciens produce a lactonase, BlcC (AttM), that can degrade the quormone, leading some to conclude that the enzyme quenches the quorum-sensing system. We tested this hypothesis by examining the effects of the mutation, induction, or mutational derepression of blcC on the accumulation of acyl-HSL and on the conjugative competence of strain C58. The induction of blc resulted in an 8- to 10-fold decrease in levels of extracellular acyl-HSL but in only a twofold decrease in intracellular quormone levels, a measure of the amount of active intracellular TraR. The induction or mutational derepression of blc as well as a null mutation in blcC had no significant effect on the induction of or continued transfer of pTiC58 from donors in any stage of growth, including stationary phase. In matings performed in developing tumors, wild-type C58 transferred the Ti plasmid to recipients, yielding transconjugants by 14 to 21 days following infection. blcC-null donors yielded transconjugants 1 week earlier, but by the following week, transconjugants were recovered at numbers indistinguishable from those of the wild type. Donors mutationally derepressed for blcC yielded transconjugants in planta at numbers 10-fold lower than those for the wild type at weeks 2 and 3, but by week 4, the two donors showed no difference in recoverable transconjugants. We conclude that BlcC has no biologically significant effect on Ti plasmid transfer or its regulatory system.

The acyl-homoserine lactone-type quorum-sensing system modulates cell motility and virulence of Erwinia chrysanthemi pv. zeae

Erwinia chrysanthemi pv. zeae is one of the Erwinia chrysanthemi pathovars that infects on both dicotyledons and monocotyledons. However, little is known about the molecular basis and regulatory mechanisms of its virulence. By using a transposon mutagenesis approach, we cloned the genes coding for an E. chrysanthemi pv. zeae synthase of acyl-homoserine lactone (AHL) quorum-sensing signals (expI(Ecz)) and a cognate response regulator (expR(Ecz)). Chromatography analysis showed that expI(Ecz) encoded production of the AHL signal N-(3-oxo-hexanoyl)-homoserine lactone (OHHL). Null mutation of expI(Ecz) in the E. chrysanthemi pv. zeae strain EC1 abolished AHL production, increased bacterial swimming and swarming motility, disabled formation of multicell aggregates, and attenuated virulence of the pathogen on potato tubers. The mutation also marginally reduced the inhibitory activity of E. chrysanthemi pv. zeae on rice seed germination. The mutant phenotypes were rescued by either exogenous addition of AHL signal or in trans expression of expI(Ecz). These data demonstrate that the AHL-type QS signal plays an essential role in modulation of E. chrysanthemi pv. zeae cell motility and the ability to form multicell aggregates and is involved in regulation of bacterial virulence.

Modulation of bacterial Type III secretion system by a spermidine transporter dependent signaling pathway

Background

Many gram-negative bacterial pathogens employ Type III secretion systems (T3SS) to inject effector proteins into host cells in infectious processes.

Methodology/Principal Findings

By screening a transposon mutant library of P. aeruginosa, we found that mutation of spuDEFGH, which encode a major spermidine uptake system, abolished the expression of the exsCEBA operon that codes for key T3SS regulators under inducing conditions (low calcium). Whole genome microarray analysis revealed that inactivation of the spermidine uptake system significantly decreased the transcriptional expression of most, if not all, T3SS genes. Consistently, the spermidine uptake mutants showed decreased expression of the T3SS genes in responding to host cell extract and attenuated cytotoxicity. Furthermore, exogenous addition of spermidine to the wild type strain PAO1 enhanced the expression of exsCEBA and also the effector protein genes.

Conclusion/Significance

Cumulatively, these data have depicted a novel spermidine transporter-dependent signaling pathway, which appears to play an essential role in modulation of T3SS expression in P. aeruginosa.

Dual control of quorum sensing by two TraM-type antiactivators in Agrobacterium tumefaciens octopine strain A6

Agrobacterium tumefaciens wild-type strains have a unique quorum-sensing (QS)-dependent Ti plasmid conjugative transfer phenotype in which QS signaling is activated by corresponding conjugative opine inducers. Strain K588, with a nopaline-type chromosomal background harboring an octopine-type Ti plasmid, however, is a spontaneous mutant displaying a constitutive phenotype in QS. In this study, we show that a single amino acid mutation (L54P) in the QS antiactivator TraM encoded by the traM gene of Ti plasmid is responsible for the constitutive phenotype of strain K588. Introduction of the L54P point mutation to the TraM of wild-type strain A6 by allelic replacement, however, failed to generate the expected constitutive phenotype in this octopine-type strain. Intriguingly, the QS-constitutive phenotype appeared when the pTiA6 carrying the mutated traM was placed in the chromosomal background of the nopaline-type strain C58C1RS, suggesting an unknown inhibitory factor(s) encoded by the chromosomal background of strain A6 but not by C58C1RS. Low-stringency Southern blotting analysis showed that strain A6, but not strain C58 and its derivatives, contains a second traM homologue. The homologue, designated traM2, has 64% and 65% identities with traM at the DNA and peptide levels, respectively. Similar to TraM, TraM2 is a potent antiactivator that functions by blocking TraR, the QS activator, from specific binding to the tra gene promoters. Deletion of traM2 in strain A6 harboring the mutated traM confers a constitutive QS phenotype. The results demonstrate that the QS system in strain A6 is subjected to the dual control of TraM and TraM2.

Members of the IclR family of bacterial transcriptional regulators function as activators and/or repressors

Members of the IclR family of regulators are proteins with around 250 residues. The IclR family is best defined by a profile covering the effector binding domain. This is supported by structural data and by a number of mutants showing that effector specificity lies within a pocket in the C-terminal domain. These regulators have a helix-turn-helix DNA binding motif in the N-terminal domain and bind target promoters as dimers or as a dimer of dimers. This family comprises regulators acting as repressors, activators and proteins with a dual role. Members of the IclR family control genes whose products are involved in the glyoxylate shunt in Enterobacteriaceae, multidrug resistance, degradation of aromatics, inactivation of quorum-sensing signals, determinants of plant pathogenicity and sporulation. No clear consensus exists on the architecture of DNA binding sites for IclR activators: the MhpR binding site is formed by a 15-bp palindrome, but the binding sites of PcaU and PobR are three perfect 10-bp sequence repetitions forming an inverted and a direct repeat. IclR-type positive regulators bind their promoter DNA in the absence of effector. The mechanism of repression differs among IclR-type regulators. In most of them the binding sites of RNA polymerase and the repressor overlap, so that the repressor occludes RNA polymerase binding. In other cases the repressor binding site is distal to the RNA polymerase, so that the repressor destabilizes the open complex.

VqsM, a novel AraC-type global regulator of quorum-sensing signalling and virulence in Pseudomonas aeruginosa

Human pathogen Pseudomonas aeruginosa uses quorum-sensing (QS) signalling systems to synchronize the production of virulence factors. There are two interrelated QS systems, las and rhl, in P. aeruginosa. In addition to this complexity, a number of transcriptional regulators were shown to have complicated interplays with las and rhl central QS components. Here, we describe a novel virulence and QS modulator (VqsM) that positively regulates the QS systems in P. aeruginosa. Mutation in vqsM resulted in much reduced production of N-acylhomoserine lactones (AHLs) and extracellular enzymes. Sequence analysis revealed that vqsM encodes a transcriptional regulator with an AraC-type helix-turn-helix DNA binding domain at the C-terminal of the peptide. Global gene expression profile analysis showed at least a total of 302 genes to be influenced, directly or indirectly, by VqsM. Among the 203 VqsM-promoted genes, 52.2% were known to be QS upregulated. Several genes encoding the key regulators implicated in QS, such as rhlR, rsaL, vqsR, mvfR, pprB and rpoS, and two AHL synthesis genes, lasI and rhlI, were suppressed in the vqsM mutant. Similar to the 'AHL-blind' phenotype of vqsR and pprB mutants, vqsM mutant did not respond to external addition of N-3-oxo-dodecanoyl-homoserine lactone signals. Moreover, overexpression of vqsR in vqsM mutant more or less restored the production of both AHL and virulence factors. The results demonstrate that VqsM, largely through modulation of vqsR expression, plays a vital role in regulation of QS signalling in P. aeruginosa.

Detection of and response to signals involved in host-microbe interactions by plant-associated bacteria

Diverse interactions between hosts and microbes are initiated by the detection of host-released chemical signals. Detection of these signals leads to altered patterns of gene expression that culminate in specific and adaptive changes in bacterial physiology that are required for these associations. This concept was first demonstrated for the members of the family Rhizobiaceae and was later found to apply to many other plant-associated bacteria as well as to microbes that colonize human and animal hosts. The family Rhizobiaceae includes various genera of rhizobia as well as species of Agrobacterium. Rhizobia are symbionts of legumes, which fix nitrogen within root nodules, while Agrobacterium tumefaciens is a pathogen that causes crown gall tumors on a wide variety of plants. The plant-released signals that are recognized by these bacteria are low-molecular-weight, diffusible molecules and are detected by the bacteria through specific receptor proteins. Similar phenomena are observed with other plant pathogens, including Pseudomonas syringae, Ralstonia solanacearum, and Erwinia spp., although here the signals and signal receptors are not as well defined. In some cases, nutritional conditions such as iron limitation or the lack of nitrogen sources seem to provide a significant cue. While much has been learned about the process of host detection over the past 20 years, our knowledge is far from being complete. The complex nature of the plant-microbe interactions makes it extremely challenging to gain a comprehensive picture of host detection in natural environments, and thus many signals and signal recognition systems remain to be described.

The two-component response regulator PprB modulates quorum-sensing signal production and global gene expression in Pseudomonas aeruginosa

The response regulator PprB and its cognate sensor PprA were recently reported as a two-component regulatory system that controls membrane permeability and antibiotic sensitivity of Pseudomonas aeruginosa. We found that a Tn5 insertion mutation in pprB caused a drastic reduction in virulence factor production and cell motility. A transcriptome analysis revealed that 175 genes were regulated by PprB. Among the 113 PprB-activated genes, 85.5% are known to be activated by N-3-oxo-dodecanoyl-homoserine lactone (OdDHL) and N-butanoyl-homoserine lactone (BHL). In particular, the expression of lasI, rhlI and rhlR, which encode key components of the las and rhl quorum-sensing (QS) systems, were significantly decreased in the pprB mutant. These data suggest that PprB might regulate QS signal production. Measurement of OdDHL and BHL in cultures of the mutant sustained this hypothesis. By using various OdDHL- or BHL-responsive QS reporter systems, including lasB-lacZ, lasI-lacZ and rsaL-lacZ, we found that the mutation in pprB resulted in a large decrease in the sensitivity of P. aeruginosa to exogenous OdDHL. However, there was no difference in sensitivity to BHL. Further analysis showed that the OdDHL influx was significantly reduced in the pprB mutant. We conclude that PprB is a novel QS modulator that positively regulates N-acylhomoserine lactone production probably by affecting the OdDHL signal influx and thereby influences global expression of the QS-dependent genes.

The quormone degradation system of Agrobacterium tumefaciens is regulated by starvation signal and stress alarmone (p)ppGpp

A unique signal degradation system has recently been discovered in Agrobacterium tumefaciens. Upon entering stationary phase, A. tumefaciens terminates quorum sensing-dependent Ti-plasmid conjugation by degradation of acyl homoserine lactone (AHL) quormone via the enzyme AttM (AHL-lactonase). attM, together with attK and attL, constitute one transcriptional unit subjected to the control of a common promoter. AttJ, the other member of the signal degradation system, is an IclR-like negative transcriptional factor, which tightly represses the expression of AttM at the early stage of bacterial growth. In this study, we found that this quormone degradation system is activated by either carbon or nitrogen starvation. Quormone degradation was significantly delayed when bacterial culture was supplemented with extra carbon or nitrogen source in the nutrient-limited minimal medium before the onset of stationary phase. To identify the signalling pathway and regulatory mechanisms that mediate quormone degradation, we constructed a reporter strain A6(attKLM::lacZ) in which the promoterless lacZ was transcriptionally fused to the attKLM promoter. Transposon mutagenesis of strain A6(attKLM::lacZ) led to identification of the relA gene, which encodes the stress alarmone (p)ppGpp synthetase. Tn5 knock-out of relA abolished the stationary phase-dependent expression of attM. We concluded that the A. tumefaciens quormone degradation system is coupled to and regulated by the generic (p)ppGpp stress response machinery.

Lsr-mediated transport and processing of AI-2 in Salmonella typhimurium

The LuxS-dependent autoinducer AI-2 is proposed to function in interspecies cell-cell communication in bacteria. In Salmonella typhimurium, AI-2 is produced and released during exponential growth and is subsequently imported into the bacteria via the Lsr (luxS regulated) ATP binding cassette (ABC) transporter. AI-2 induces transcription of the lsrACDBFGE operon, the first four genes of which encode the Lsr transport apparatus. In this report, we identify and characterize LsrK, a new protein that is required for the regulation of the lsr operon and the AI-2 uptake process. LsrK is a kinase that phosphorylates AI-2 upon entry into the cell. Our data indicate that phosphorylation of AI-2 results in its sequestration in the cytoplasm. We suggest that phospho-AI-2 is the inducer responsible for inactivation of LsrR, the repressor of the lsr operon. We also show that two previously uncharacterized members of the lsr operon, LsrF and LsrG, are necessary for the further processing of phospho-AI-2. Transport and processing of AI-2 could be required for removing the quorum-sensing signal, conveying the signal to an internal detector and/or scavenging boron.

Genetic control of quorum-sensing signal turnover in Agrobacterium tumefaciens

A signal turnover system is an essential component of many genetic regulatory mechanisms. The best-known example is the ubiquitin-dependent protein degradation system that exists in many organisms. We found that Agrobacterium tumefaciens adopts a unique signal turnover system to control exiting from a quorum-sensing mode. A. tumefaciens regulates Ti plasmid conjugal transfer by a quorum-sensing signal, N-3-oxo-octanoyl homoserine lactone (3OC8HSL), also known as Agrobacterium autoinducer. By using Tn5 mutagenesis and a functional cloning approach, we identified two genes that are involved in switching from a conjugal quorum-sensing mode to a nonconjugal mode at the onset of stationary phase. First, we located attJ, which codes for an IclR-type suppressor that regulates the second gene attM. The latter encodes a homologue of N-acylhomoserine lactone (AHL)-lactonase. Mass spectrometry analysis shows that the enzyme encoded by attM is an AHL-lactonase that hydrolyzes the lactone ring of 3OC8HSL. In wild-type A. tumefaciens, attM expression is initially suppressed by AttJ but significantly elevated at the stationary phase accompanied a sharp decline in 3OC8HSL. DNA gel retardation analysis shows that AttJ specifically binds to the promoter that controls AHL-lactonase expression. Mutation of attJ resulted in constitutive production of AHL-lactonase that abolishes 3OC8HSL accumulation and Ti plasmid transfer. These data suggest that A. tumefaciens has a sophisticated multicomponent quorum-sensing signal turnover system, allowing the cell to sense a change in growth and adjust cellular activities accordingly.

Quorum sensing as a population-density-dependent determinant of bacterial physiology

The discovery that bacterial cells can communicate with each other has led to the realization that bacteria are capable of exhibiting much more complex patterns of co-operative behaviour than would be expected for simple unicellular microorganisms. Now generically termed 'quorum sensing', bacterial cell-to-cell communication enables a bacterial population to mount a unified response that is advantageous to its survival by improving access to complex nutrients or environmental niches, collective defence against other competitive microorganisms or eukaryotic host defence mechanisms and optimization of population survival by differentiation into morphological forms better adapted to combating environmental threats. The principle of quorum sensing encompasses the production and release of signal molecules by bacterial cells within a population. Such molecules are released into the environment and, as cell numbers increase, so does the extracellular level of signal molecule, until the bacteria sense that a threshold has been reached and gene activation, or in some cases depression or repression, occurs via the activity of sensor-regulator systems. In this review, we will describe the biochemistry and molecular biology of a number of well-characterized N-acylhomoserine lactone quorum sensing systems to illustrate how bacteria employ cell-to-cell signalling to adjust their physiology in accordance with the prevailing high-population-density environment.

Multiple N-acyl homoserine lactone signals of Rhizobium leguminosarum are synthesized in a distinct temporal pattern

A common form of bacterial quorum sensing involves the production and release of acyl homoserine lactone (AHL) signal metabolites. The nitrogen-fixing symbiont Rhizobium leguminosarum reportedly produces at least six different AHLs, but little is known about the regulation of biosynthesis of these molecules. We used a radiolabeling protocol to quantify the relative amounts of AHLs synthesized over time by R. leguminosarum cells with and without the symbiosis plasmid pRL1JI. Cells containing pRL1JI were found to produce three predominant signals. In decreasing order of abundance, these were N-(3-oxo)octanoyl homoserine lactone [(3-O)C(8)HSL], N-octanoyl homoserine lactone, and N-hexanoyl homoserine lactone. Cells without pRL1JI produced only two major signals, N-(3-hydroxy-7-cis)tetradecanoyl homoserine lactone [(3-OH)C(14:1)HSL] and (3-O)C(8)HSL. Each AHL exhibited a distinct temporal pattern of synthesis, suggesting that each AHL is subject to unique regulatory mechanisms. While (3-O)C(8)HSL was produced in both cultures, the patterns of synthesis were different in cells with and without pRL1JI, possibly as a result of redundant gene functions that are present on both the chromosome and the symbiosis plasmid. None of the AHLs appeared to regulate its own biosynthesis, although exogenous (3-OH)C(14:1)HSL did activate synthesis of the three AHLs made by cells containing pRL1JI. These results indicate that the synthesis of multiple AHLs in R. leguminosarum is regulated by complex mechanisms that operate independently of quorum sensing itself but that (3-OH)C(14:1)HSL can supersede these controls in pRL1JI-containing cells. This work provides an important global perspective for AHL regulation that both complements and contrasts with the results of previous studies performed with isolated gene systems.

The antiactivator TraM interferes with the autoinducer-dependent binding of TraR to DNA by interacting with the C-terminal region of the quorum-sensing activator

Conjugal transfer of Agrobacterium tumefaciens Ti plasmids is regulated by quorum sensing via the transcriptional activator TraR and the acyl-homoserine lactone Agrobacterium autoinducer (AAI). Unique to this system, the activity of TraR is negatively modulated by an antiactivator called TraM. Analyses from yeast two-hybrid studies suggest that TraM directly interacts with the activator, but the conditions under which these components interact and the region of TraR responsible for this interaction are not known. Induction of traM in a strain in which TraR was activating transcription of a reporter system led to rapid cessation of gene expression. As assessed by a genetic assay that measures AAI-dependent DNA binding, TraM inhibited TraR function before and after the transcription factor had bound to its DNA recognition site. Consistent with this observation, in gel retardation assays, purified TraM abolished the DNA binding activity of TraR in a concentration-dependent manner. Such inhibition occurred independent of the order of addition of the reactants. As assessed by far Western analyses TraM interacts with TraR by directly binding the activator. TraM in its native form interacted with native TraR and also with heat-treated TraR but only when SDS was included with the denatured protein. TraM interacted with TraR on blots prepared with total lysates of cells grown in the presence and absence of AAI. Far Western analysis of N- and C-terminal deletion mutants localized a domain of TraR contributing to TraM binding to the C-terminal portion of the activator protein. Random mutagenesis by hydroxylamine treatment and error-prone polymerase chain reaction identified several residues in this region of TraR important for interacting with TraM as well as for transcriptional activation or/and DNA binding. We conclude that TraM inhibits TraR by binding to the activator at a domain within or close to the helix-turn-helix motif located at the C terminus of the protein.

Modulating quorum sensing by antiactivation: TraM interacts with TraR to inhibit activation of Ti plasmid conjugal transfer genes

Conjugal transfer of the Ti plasmid pTiC58 is regulated by a quorum-sensing system involving the transcriptional activator TraR and the acyl homoserine lactone autoinducer N-(3-oxo-octanoyl)-L-homoserine lactone (AAI). Activation of tra gene expression by TraR and AAI is inhibited by TraM, an 11 kDa protein also coded for by the Ti plasmid. Previous studies suggested that TraM interferes with TraR activity by directly interacting with the activator protein. Using the yeast two-hybrid system, constructs of Saccharomyces cerevisiae containing a fusion of traR to the B42 domain of the prey plasmid pJG4.5 and a fusion of traM to the lexA gene of the bait plasmid pEG202 produced beta-galactosidase and grew on medium lacking leucine, both phenotypes indicative of an interaction between the two proteins. Early termination mutants and substitution mutants mapping to the C-terminus of TraM were isolated by screening for alleles unable to interfere with TraR activity in Agrobacterium tumefaciens. These mutants all failed to interact with the TraR fusion in the two-hybrid system. An N-terminal deletion mutant of TraM lacking the first 27 residues weakly interacted with TraR in the two-hybrid system whereas deletions of 48 amino acids or more abolished the interaction. As assessed by Western blot analysis, the mutant fusion proteins were produced at levels indistinguishable from that of the wild-type TraM in the yeast tester strain. Mutants of TraR that were not inhibited by TraM in A. tumefaciens were isolated and fell into two classes. In the first, the mutation resulted in increased expression of wild-type TraR. In the second, a proline residue at position 176 was changed to serine (P176 --> S) or to leucine (P176 --> L). The P176 --> S mutant interacted with wild-type TraM, but at a detectably lower level, in the two-hybrid assay. Mutants of TraR with N-terminal deletions as large as 105 amino acids interfered with the ability of TraM to inhibit wild-type TraR in A. tumefaciens. Two-hybrid assays indicated that these mutants, as well as a C-terminal 49 residue fragment of TraR, can interact with TraM. We conclude that TraM and TraR interact in vivo and that this interaction is responsible for inhibition of TraR-mediated activation. We also conclude that the two proteins interact with each other through domains located at their respective C-termini.

Hierarchical gene regulatory systems arising from fortuitous gene associations: controlling quorum sensing by the opine regulon in Agrobacterium

Conjugation of the Agrobacterium Ti plasmid pTiC58 is regulated by a hierarchy involving induction by the opines agrocinopines A and B and a quorum-sensing system. Regulation by the opines is mediated by the repressor AccR, while quorum sensing is effected by the transcriptional activator TraR and its ligand, the acyl-homoserine lactone signal molecule Agrobacterium autoinducer (AAI). These last two elements combine to activate expression of the tra system at high population densities. Sequence analysis indicated that traR is the fourth gene of an operon, which we named arc, that is transcribed divergently from accR. Complementation analysis of mutations in the genes 5' to traR showed that the other members of the arc operon are not required for conjugation. Analysis of lacZ reporter fusions demonstrated that traR expression is regulated directly by AccR. Deletion analysis showed that AccR-regulated expression of traR initiates from a promoter located in the intergenic region between accR and orfA, the first gene of the arc operon. Reverse transcriptase-polymerase chain reaction (RT-PCR) and primer extension analyses indicated that the arc transcript initiates upstream of orfA and proceeds uninterrupted through traR. These results are consistent with a model in which quorum sensing is subordinate to the opine regulon because traR has become associated with an operon controlled by the opine-responsive transcriptional regulator.

Conjugal transfer but not quorum-dependent tra gene induction of pTiC58 requires a solid surface

Donors of Agrobacterium tumefaciens harboring a transfer-constitutive derivative of the nopaline-type Ti plasmid pTiC58 transferred this element at frequencies 3 to 4 orders of magnitude higher in matings conducted on solid surfaces than in those conducted in liquid medium. However, as measured with a lacZ reporter fusion, the tra genes of the wild-type Ti plasmid were inducible by opines to indistinguishable levels on solid and in liquid medium. Donors induced in liquid transferred the Ti plasmid at high frequency when mated with recipients on solid medium. We conclude that while formation of stable mating pairs and subsequent transfer of the Ti plasmid is dependent on a solid stratum, the regulatory system can activate tra gene expression to equivalent levels in liquid and on solid surfaces.

Temperature affects the T-DNA transfer machinery of Agrobacterium tumefaciens

Early studies on Agrobacterium tumefaciens showed that development of tumors on plants following infection by A. tumefaciens was optimal at temperatures around 22 degrees C and did not occur at temperatures above 29 degrees C. To assess whether this inability to induce tumors is due to a defect in the T-DNA transfer machinery, mobilization of an incompatibility group Q (IncQ) plasmid by the T-DNA transfer machinery of A. tumefaciens was tested at various temperatures. Optimal transfer occurred when matings were performed at 19 degrees C, and transfer was not seen when matings were incubated above 28 degrees C. Transfer of the IncQ plasmid was dependent upon induction of the virB and virD operons by acetosyringone but was not dependent upon induction of the tra genes by octopine. However, alterations in the level of vir gene induction could not account for the decrease in transfer with increasing temperature. A. tumefaciens did successfully mobilize IncQ plasmids at higher temperatures when alternative transfer machineries were provided. Thus, the defect in transfer at high temperature is apparently in the T-DNA transfer machinery itself. As these data correlate with earlier tumorigenesis studies, we propose that tumor suppression at higher temperatures results from a T-DNA transfer machinery which does not function properly.

Conserved cis-acting promoter elements are required for density-dependent transcription of Agrobacterium tumefaciens conjugal transfer genes

Ti plasmids of Agrobacterium tumefaciens, in addition to transferring oncogenic DNA to the nuclei of infected plant cells, can conjugally transfer between agrobacteria. Conjugation of wide-host-range octopine-type Ti plasmids requires a tumor-released arginine derivative called octopine. Octopine stimulates expression of the traR gene, whose product directly activates other tra genes in the presence of an acylated homoserine lactone called Agrobacterium autoinducer (AAI). We have localized the transcription starts of three tra promoters and find conserved elements (tra boxes) at virtually identical positions upstream of each promoter. Disruption of these tra boxes abolished induction of each promoter. Deletion analysis of the traI promoter indicates that tra boxes are the only upstream elements required for transcriptional activation. Since Ti plasmid donor cells both produce and respond to AAI, we tested whether expression of tra promoters was enhanced by high concentrations of bacteria. Both tra gene expression and conjugation itself were strongly stimulated either by high donor densities or by exogenous AAI.

AinS and a new family of autoinducer synthesis proteins

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.

A new regulatory element modulates homoserine lactone-mediated autoinduction of Ti plasmid conjugal transfer

Conjugal transfer of the Agrobacterium tumefaciens nopaline-type Ti plasmid pTiC58 is induced by agrocinopines A and B, opines secreted by crown gall tumors induced by the bacterium. This regulation functions through the transcriptional repressor, AccR. However, actual transcription of the tra genes is regulated by autoinduction through the activator TraR and the substituted homoserine lactone second messenger, Agrobacterium autoinducer (AAI). We have identified a new regulatory element that modulates the response of TraR to AAI. The gene, called traM, suppresses TraR-AAI activation of transcription of tra genes carried on recombinant clones. The suppression could be relieved by increasing the expression of TraR but not by increasing AAI levels. traM is located between traR and traAF on pTiC58 and is transcribed in the clockwise direction. The 306-bp gene encodes an 11.2-kDa protein showing no significant relatedness to other proteins in the databases. Mutations in traM in pTiC58 conferred a transfer-constitutive phenotype, and strains harboring the Ti plasmid produced easily detectable amounts of AAI. These same mutations engineered into the transfer-constitutive Ti plasmid pTiC58 delta accR conferred a hyperconjugal phenotype and very high levels of AAI production. Expression of traM required TraR, indicating that transcription of the gene is regulated by the autoinduction system. TraM had no effect on the expression of traR, demonstrating that the suppressive effect is not due to repression of the gene encoding the activator. These results suggest that TraM is not a direct transcriptional regulator. Since the suppressive effect is demonstrable only when traM is overexpressed with respect to traR, we suggest that TraM functions to sequester TraR from the very small amounts of AAI produced under conditions when the agrocinopines are not present.

TraI, a LuxI homologue, is responsible for production of conjugation factor, the Ti plasmid N-acylhomoserine lactone autoinducer

Conjugal transfer of the nopaline-type Agrobacterium Ti plasmid pTiC58 is regulated by a transcriptional activator, TraR, and a diffusible signal molecule, conjugation factor (CF). CF is a member of a family of substituted homoserine lactones (HSLs) that act as coinducers for regulating gene expression in diverse Gram-negative bacteria by a mechanism called autoinduction. In Vibrio fischeri HSL production is conferred by the luxI gene. Homologues of this gene are responsible for HSL production by other Gram-negative bacteria. A gene that we call traI, conferring production of material with CF activity, was localized to a 1-kb region at the upstream end of tra3 of pTiC58. Spectroscopy showed that the activity was authentic CF. Sequence analysis showed that traI could encode a protein of 211 amino acids, TraI, that is related to the proteins responsible for HSL production by other bacteria. A second, partial open reading frame immediately downstream of traI could encode a protein related to TrbB of plasmid RP4, which is required for conjugal transfer. Transcription of traI and of the downstream tra3 genes requires TraR and CF and initiates from the traI promoter. The results show that traI is responsible for CF production, that it is the first gene of the tra3 operon, and that expression of this operon is regulated by autoinduction.

A LuxR-LuxI type regulatory system activates Agrobacterium Ti plasmid conjugal transfer in the presence of a plant tumor metabolite

Conjugal transfer of Agrobacterium octopine-type Ti plasmids is activated by octopine, a metabolite released from plant tumors. Octopine causes conjugal donors to secrete a pheromone, Agrobacterium autoinducer (AAI), and exogenous AAI further stimulates conjugation. The putative AAI synthase and an AAI-responsive transcriptional regulator were found to be encoded by the Ti plasmid traI and traR genes, respectively, and the expression of traR was induced by octopine. The octopine-type traR gene product is highly homologous to the TraR protein recently characterized from a nopaline-type Ti plasmid. TraR and TraI are homologous to the LuxR and LuxI regulatory proteins of Vibrio fischeri, and AAI is similar in structure to the diffusable V. fischeri autoinducer, the inducing ligand of LuxR. TraR activated target genes in the presence of AAI and also activated traR and traI themselves, creating two positive-feedback loops. TraR-AAI-mediated activation in wild-type Agrobacterium strains was dramatically enhanced by culturing on solid media, suggesting a possible role in cell density sensing.

Agrobacterium conjugation and gene regulation by N-acyl-L-homoserine lactones

Conjugal opines secreted by crown gall tumours induce strains of Agrobacterium tumefaciens that are donors of Ti plasmids to produce a diffusible conjugation factor. This enhances the conjugal transfer efficiency of the Ti plasmid in other strains of A. tumefaciens. This factor behaves as a secondary messenger, transmitting the environmental information to tra genes. Here we report the use of spectrometry to show that this factor is identical to synthetic N-(beta-oxo-octan-1-oyl)-L-homoserine lactone and confirm that the synthetic compound is biologically active. N-(Hexan-1-oyl)-L-homoserine lactone has also been detected. A closely related molecule, N-(beta-oxo-hexan-1-oyl)-L-homoserine lactone, autoinduces bioluminescence in the distantly related bacterium, Vibrio fischeri. N-Acyl-homoserine lactones thus seem to be conserved molecules in which the length and nature of the lipophilic acyl chain determines the biological function to be regulated. Mutants that do not produce the factor fail to conjugate unless supplied with it in the induction medium (our unpublished data). These data indicate that the conjugation factor is an autoinducer and a key signal molecule in the conjugation system of A. tumefaciens. It is, to our knowledge, the first example of a second messenger molecule in a bacterial conjugation system.

Conjugation factor of Agrobacterium tumefaciens regulates Ti plasmid transfer by autoinduction

Conjugal transfer of Ti plasmids from Agrobacterium donors to bacterial recipients is controlled by two types of diffusible signal molecules. Induction is mediated by novel compounds, called opines, that are secreted by crown gall tumours. These neoplasias result from infection of susceptible plants by virulent agrobacteria. The second diffusible signal, called conjugation factor, is synthesized by the donor bacteria themselves. Production of this factor is induced by the opine. Here we show that conjugation is regulated directly by a transcriptional activator, TraR, which requires conjugation factor as a coinducer to activate tra gene expression. TraR is a homologue of LuxR, the lux gene activator from Vibrio fischeri which also requires an endogenously synthesized diffusible coinducer. The two regulatory systems are related; the two activator proteins show amino-acid sequence similarities and the lux system cofactor, autoinducer, will substitute for conjugation factor in the TraR-dependent activation of Ti plasmid tra genes.

A general role for the lux autoinducer in bacterial cell signalling: control of antibiotic biosynthesis in Erwinia

Micro-organisms have evolved complex and diverse mechanisms to sense environmental changes. Activation of a sensory mechanism typically leads to alterations in gene expression facilitating an adaptive response. This may take several forms, but many are mediated by response-regulator proteins. The luxR-encoded protein (LuxR) has previously been characterised as a member of the response-regulator superfamily and is known to respond to the small diffusible autoinducer signal molecule N-(beta-ketocaproyl) homoserine lactone (KHL). Observed previously in only a few marine bacteria, we now report that KHL is in fact produced by a diverse group of terrestrial bacteria. In one of these (Erwinia carotovora), we show that it acts as a molecular control signal for the expression of genes controlling carbapenem antibiotic biosynthesis. This represents the first substantive evidence to support the previous postulate that the lux autoinducer, KHL, is widely involved in bacterial signalling.