Bacterial Signal Transduction Research Articles

A compilation of histidine kinase genes in Enterobacter hormaechei strain HCF3 isolated from the North West Province of South Africa

This study investigates the histidine kinase (HK) gene repertoire of Enterobacter hormaechei strain HCF3, isolated from fresh cow dung in Mogosane Village, Northwest Province, South Africa. Histidine kinases are critical components of bacterial two-component signal transduction systems, enabling bacteria to sense and adapt to diverse environmental conditions. Given the growing concern over antimicrobial resistance (AMR) associated with E. hormaechei, this research elucidates the genetic components that facilitate its environmental adaptability. After isolating the strain, genomic sequencing using Illumina technology, resulting in high-quality sequence data, was conducted. The assembled genome was meticulously annotated and deposited in the National Center for Biotechnology Information (NCBI) under BioProject number PRJNA991313, with additional accession numbers for raw reads (JAUOLV000000000.1) and BioSample (SAMN36292742). Histidine kinase genes were identified based on conserved domains, particularly HisKA and HATPase. This led to compiling a comprehensive HK gene catalogue with locus tags, protein accession numbers, and functional annotations. To validate the HK gene set of E. hormaechei HCF3, we conducted a rigorous comparative analysis with other strains. This revealed that strain HCF1 contains 21 histidine kinase genes, HCF2 has 25, while HCF4 has 19. These findings underscore the diversity and conservation of HK genes across different Enterobacter species, providing a new perspective on their evolutionary significance. The assembled dataset provides valuable insights into the signalling pathways of E. hormaechei, highlighting the potential roles of HKs in environmental sensing, adaptation, and pathogenicity. Furthermore, this research lays the groundwork for future studies on the applications of these genes in agriculture and biotechnology, offering new avenues for understanding and managing E. hormaechei in various ecological contexts.

Unveiling the Coordinated Action of DesK/DesR and YvfT/YvfU to Control the Expression of an ABC Transporter in Bacillus subtilis.

Two-component systems (TCSs) are vital signal transduction pathways ubiquitous among bacteria, facilitating their responses to diverse environmental stimuli. In Bacillus subtilis, the DesK histidine kinase thermosensor, together with the response regulator DesR, constitute a TCS dedicated to membrane lipid homeostasis maintenance. This TCS orchestrates the transcriptional regulation of the des gene, encoding the sole desaturase in these bacteria, Δ5-Des. Additionally, B. subtilis possesses a paralog TCS, YvfT/YvfU, with unknown target gene(s). In this work, we show that YvfT/YvfU controls the expression of the yvfRS operon that codes for an ABC transporter. Interestingly, we found that this regulation also involves the action of DesK/DesR. Notably, opposite to des, yvfRS transcription is induced at 37°C and not at 25°C. Our invivo and invitro experiments demonstrate that both YvfU and DesR directly bind to the operon promoter region, with DesR exerting its control over yvfRS expression in its unphosphorylated state. Our study uncovers an intriguing case of cross-regulation where two homologous TCSs interact closely to finely tune gene expression in response to environmental cues. These findings shed light on the complexity of bacterial signal transduction systems and their critical role in bacterial adaptability.

Evaluation of expanded 2-aminobenzothiazole library as inhibitors of a model histidine kinase and virulence suppressors in Pseudomonas aeruginosa

Bacterial resistance to antibiotics is a rapidly increasing threat to human health. New strategies to combat resistant organisms are desperately needed. One potential avenue is targeting two-component systems, which are the main bacterial signal transduction pathways used to regulate development, metabolism, virulence, and antibiotic resistance. These systems consist of a homodimeric membrane-bound sensor histidine kinase, and a cognate effector, the response regulator. Histidine kinases play an essential role in the regulation of multiple virulence mechanisms including toxin production, immune evasion, and antibiotic resistance. Targeting virulence, as opposed to development of bactericidal compounds, could reduce evolutionary pressure for acquired resistance. Additionally, compounds targeting the highly conserved catalytic and adenosine triphosphate-binding (CA) domain have the potential to impair multiple two-component systems that regulate virulence in one or more pathogens. We conducted in vitro structure–activity relationship studies of 2-aminobenzothiazole-based inhibitors designed to target the CA domain. We found that these compounds, which inhibit the model histidine kinase, HK853 from Thermotoga maritima, have anti-virulence activities inPseudomonas aeruginosa, reducing motility phenotypes and toxin production associated with the pathogenic functions of this bacterium.

RNA-seq reveals the important role of transcriptional regulator DeoR1 in regulating Brucella abortus various pathways

Brucella spp. is an intracellular bacterium that uses its transcriptional regulator DeoR1 to promote intracellular transport and survival, but the molecular mechanism remains unknown. To analyze the role of DeoR1 in the virulence of B. abortus and the genes regulated by DeoR1, we created a A19ΔdeoR1 mutant of B. abortus A19 (A19). Virulence assay was performed using a murine macrophage cell line (RAW264.7) and mice. We observed that A19ΔdeoR1 mutant is attenuated in RAW264.7 cells and mice. We performed RNA-seq whole transcriptome analysis of A19ΔdeoR1 and A19 from infected RAW264.7 cells. A total of 135 differentially expressed genes were identified, including 100 up-regulated and 35 down-regulated genes. These differentially expressed genes were involved in amino acid synthesis and metabolism, energy production and conversion, stress proteins, chaperonin, hypothetical proteins and protein of unknown function, cell wall/membrane/envelope, intracellular transporting and secretion, and transcriptional regulator. Interestingly, genes involved in the intracellular trafficking and secretion were significantly down-regulated in A19ΔdeoR1. Furthermore, selected RNA-seq results were experimentally confirmed by qRT-PCR. Overall, these results deciphered differential phenomena associated with virulence in A19ΔdeoR1 and A19 from infected RAW264.7 cells, which provided important information for understanding the detailed role of DeoR1 in Brucella pathogenesis. SignificanceTranscriptional regulators are predominant bacterial signal transduction factors. The pathogenicity of Brucella is due to its ability to regulate the expression of virulence related genes. Transcriptional regulators are designed to regulate gene expression and enact an appropriate adaptive physiological response. Here, a total of 135 differentially expressed genes were identified in transcriptional regulator deoR1 mutant.

A stage-dependent seed defense response to explain efficient seed transmission of Xanthomonas citri pv. fuscans to common bean.

Although seed represents an important means of plant pathogen dispersion, the seed-pathogen dialogue remains largely unexplored. A multiomic approach was performed at different seed developmental stages of common bean (Phaseolus vulgaris L.) during asymptomatic colonization by Xanthomonas citri pv. fuscans (Xcf), At the early seed developmental stages, we observed high transcriptional changes both in seeds with bacterial recognition and defense signal transduction genes, and in bacteria with up-regulation of the bacterial type 3 secretion system. This high transcriptional activity of defense genes in Xcf-colonized seeds during maturation refutes the widely diffused assumption considering seeds as passive carriers of microbes. At later seed maturation stages, few transcriptome changes indicated a less intense molecular dialogue between the host and the pathogen, but marked by changes in DNA methylation of plant defense genes, in response to Xcf colonization. We showed examples of pathogen-specific DNA methylations in colonized seeds acting as plant defense silencing to repress plant immune response during the germination process. Finally, we propose a novel plant-pathogen interaction model, specific to the seed tissues, highlighting the existence of distinct phases during seed-pathogen interaction with seeds being actively interacting with colonizing pathogens, then both belligerents switching to more passive mode at later stages.

MSphere of Influence: The complex world of bacterial biogeography.

Drew Bridges works in the field of bacterial signal transduction and studies the formation and disassembly of bacterial biofilms. In this mSphere of Influence article, he reflects on how the paper "Biogeography of a human oral microbiome at the micron scale" by Mark Welch et al. (J. L. Mark Welch, B. J. Rossetti, C. W. Rieken, F. E. Dewhirst, et al., Proc Natl Acad Sci U S A 113:E791-E800, 2016, https://doi.org/10.1073/pnas.1522149113) inspired him to change his research trajectory.

Type I Photosensitizer Targeting Glycans: Overcoming Biofilm Resistance by Inhibiting the Two-Component System, Quorum Sensing, and Multidrug Efflux.

Stubborn biofilm infections pose serious threats to human health due to the persistence, recurrence, and dramatically magnified antibiotic resistance. Photodynamic therapy has emerged as a promising approach to combat biofilm. Nevertheless, how to inhibit the bacterial signal transduction system and the efflux pump to conquer biofilm recurrence and resistance remains a challenging and unaddressed issue. Herein, a boric acid-functionalized lipophilic cationic type I photosensitizer, ACR-DMP, is developed, which efficiently generates •OH to overcome the hypoxic microenvironment and photodynamically eradicates methicillin-resistant Staphylococcus aureus (MRSA) and biofilms. Furthermore, it not only alters membrane potential homeostasis and osmotic pressure balance due to its strong binding ability with plasma membrane but also inhibits quorum sensing and the two-component system, reduces virulence factors, and regulates the activity of the drug efflux pump attributed to the glycan-targeting ability, helping to prevent biofilm recurrence and conquer biofilm resistance. In vivo, ACR-DMP successfully obliterates MRSA biofilms attached to implanted medical catheters, alleviates inflammation, and promotes vascularization, thereby combating infections and accelerating wound healing. This work not only provides an efficient strategy to combat stubborn biofilm infections and bacterial multidrug resistance but also offers systematic guidance for the rational design of next-generation advanced antimicrobial materials.

Identification of two-component system ArcAB and the universal stress protein E in Pasteurella multocida and their effects on bacterial fitness and pathogenesis

Two-component regulatory system (TCS) is a widespread bacterial signal transduction mechanism and plays a critical role in bacterial adaptation to environments as well as regulating bacterial virulence. However, few studies have reported the actions of TCS in Pasteurella multocida, a zoonotic bacterial pathogen. In this study, genes encoding proteins homologous to the ArcAB TCS were identified in genome sequences of P. multocida belonging to different serogroups, and the transcription of both arcA and arcB was up-regulated in anaerobic and superoxygen environment. Compared to wild type strains, P. multocida arcA-deletion mutants (ΔarcA) displayed a decrease in growing under anaerobic conditions, biofilm formation, as well as the capacities of anti-serum bactericidal effect, cell adherence and invasion, anti-phagocytosis, and virulence in different in vivo models (Galleria mellonella and mice). RNA-Seq identified 70 significantly downregulated genes in ΔarcA compared to the wild type strain, and several of them are associated with P. multocida virulence. Among them, a universal stress protein E encoding gene uspE was characterized in P. multocida for the first time. Electrophoretic mobility shift assay (EMSA) demonstrated that the ArcAB TCS could regulate uspE directly. Deletion of uspE also led to a decrease of P. multocida in growing under anaerobic conditions, biofilm formation, anti-serum bactericidal effect, cell adherence and invasion, anti-phagocytosis, and virulence in mice. The data provided from this study will help further understanding the fitness and pathogenesis of P. multocida.

Novel and efficient synthesis of lithium crown ether metal–organic framework nanoparticles (15C5-Li MOF@Bi2O3@TiO2 NPs): Characterization and evaluation of its in vitro antibacterial studies

Novel lithium crown ether metal-organic framework nanoparticles ([email protected]2O3@TiO2 NPs) have been well documented for their biological activity as anti-bacterial entities. An efficient synthesis of novel lithium crown ether metal-organic framework nanoparticles has been reported in this study using Bi2O3, TiO2, and LiCl to investigate its antibacterial properties. The newly synthesized nanoparticles ([email protected]2O3@TiO2 NPs) found a significant antibacterial effect. A comparative study of gram-positive bacteria with Ofloxacin demonstrates good control for antibacterial properties. Similarly, gram-negative bacteria were studied to compare antibacterial properties using reference control Ofloxacin. The results against the gram-negative bacteria were found dose-dependent and more effective than against gram-positive bacteria, causing cell lysis with a change in phosphotyrosine profile affecting bacterial signal transduction and impeding the growth of organisms. To know the morphological study, FESEM analysis, energy dispersive X-ray (EDX) analysis, and elemental mapping of novel nanoparticles are performed, demonstrating more stability and antibacterial efficacy of prepared nanoparticles, which were spherical on mesoporous sheets. The crystalline pattern of NPs is studied using UV-visible, XRD, and FTIR spectroscopic analysis supports the structure of the novel ([email protected]@TiO2 NPs).

Past, Present, and Future of Extracytoplasmic Function σ Factors: Distribution and Regulatory Diversity of the Third Pillar of Bacterial Signal Transduction.

Responding to environmental cues is a prerequisite for survival in the microbial world. Extracytoplasmic function σ factors (ECFs) represent the third most abundant and by far the most diverse type of bacterial signal transduction. While archetypal ECFs are controlled by cognate anti-σ factors, comprehensive comparative genomics efforts have revealed a much higher abundance and regulatory diversity of ECF regulation than previously appreciated. They have also uncovered a diverse range of anti-σ factor-independent modes of controlling ECF activity, including fused regulatory domains and phosphorylation-dependent mechanisms. While our understanding of ECF diversity is comprehensive for well-represented and heavily studied bacterial phyla-such as Proteobacteria, Firmicutes, and Actinobacteria (phylum Actinomycetota)-our current knowledge about ECF-dependent signaling in the vast majority of underrepresented phyla is still far from complete. In particular, the dramatic extension of bacterial diversity in the course of metagenomic studies represents both a new challenge and an opportunity in expanding the world of ECF-dependent signal transduction.

The involvement of the T6SS vgrG gene in the pathogenicity of Pseudomonas plecoglossicida.

Pseudomonas plecoglossicida, the causative agent of white spot disease of large yellow croaker, has caused serious economic losses to the aquaculture industry. The type VI secretion system (T6SS) is a significant virulence system widely distributed among Gram-negative bacteria. VgrG, a structural and core component of T6SS, is crucial to the function of T6SS. To explore the biological profiles mediated by vgrG gene and its effects on the pathogenicity of P. plecoglossicida, the vgrG gene deletion (ΔvgrG) strain and complementary (C-ΔvgrG) strain were constructed and the differences in pathogenicity and virulence-related characteristics between different strains were analysed. The results showed that vgrG gene deletion significantly affected the virulence-related characteristics of P. plecoglossicida, including chemotaxis, adhesion, and biofilm formation. In addition, the LD50 of ΔvgrG strain was nearly 50-fold higher than that of the NZBD9 strain. Transcriptome data analysis suggested that the vgrG gene may affect the virulence of P. plecoglossicida by regulating the quorum sensing pathway to inhibit the secretion of virulence factors and affect biofilm formation. Besides, deletion of the vgrG gene may reduce bacterial pathogenicity by affecting bacterial signal transduction processes and the ability to adapt to chemotactic substances.

A peek inside the 2023 Bacterial Locomotion and Signal Transduction (BLAST) conference

A peek inside the 2023 Bacterial Locomotion and Signal Transduction (BLAST) conference

Deletion of Brucella transcriptional regulator GntR10 regulated the expression of quorum sensing system and type IV secretion system effectors, which affected the activation of NF-κB

GntR10 is a transcriptional regulator in Brucella. Nuclear factor-kappa B (NF-κB) is involved in many cellular activities, playing major roles in orchestrating the expression of inflammatory genes and regulating protein function that is essential for pathogenic bacteria during infection. GntR10 deletion was previously found to affect the growth and the virulence of Brucella and expression levels of target genes of GntR10 in mice. However, the mechanisms of affection of NF-κB regulated by Brucella GntR10 are still unclear. Here, GntR10 deletion could regulate the expression of LuxR-type transcriptional activators (VjbR and BlxR) of the quorum sensing system (QSS) and type IV secretion system (T4SS) effectors (BspE and BspF) of Brucella. It could further inhibit the activation of the regulator NF-κB and affect the virulence of Brucella. This research provides new insights into the designing of Brucella vaccines and the screening of drug targets. SignificanceTranscriptional regulators are predominant bacterial signal transduction factors. The pathogenicity of Brucella is due to its ability to regulate the expression of virulence related genes including quorum sensing system (QSS) and type IV secretion system (T4SS). Transcriptional regulators are designed to regulate gene expression and enact an appropriate adaptive physiological response. Here, we show that Brucella transcriptional regulator GntR10 regulated the expression of QSS and T4SS effectors, which affected the activation of NF-κB.

Two Permeases Associated with the Multifunctional CtaP Cysteine Transport System in Listeria monocytogenes Play Distinct Roles in Pathogenesis.

The soil-dwelling bacterium Listeria monocytogenes survives a multitude of conditions when residing in the outside environment and as a pathogen within host cells. Key to survival within the infected mammalian host is the expression of bacterial gene products necessary for nutrient acquisition. Similar to many bacteria, L. monocytogenes uses peptide import to acquire amino acids. Peptide transport systems play an important role in nutrient uptake as well as in additional functions that include bacterial quorum sensing and signal transduction, recycling of peptidoglycan fragments, adherence to eukaryotic cells, and alterations in antibiotic susceptibility. It has been previously described that CtaP, encoded by lmo0135, is a multifunctional protein associated with activities that include cysteine transport, resistance to acid, membrane integrity, and bacterial adherence to host cells. ctaP is located next to two genes predicted to encode membrane-bound permeases lmo0136 and lmo0137, termed CtpP1 and CtpP2, respectively. Here, we show that CtpP1 and CtpP2 are required for bacterial growth in the presence of low concentrations of cysteine and for virulence in mouse infection models. Taken together, the data identify distinct nonoverlapping roles for two related permeases that are important for the growth and survival of L. monocytogenes within host cells. IMPORTANCE Bacterial peptide transport systems are important for nutrient uptake and may additionally function in a variety of other roles, including bacterial communication, signal transduction, and bacterial adherence to eukaryotic cells. Peptide transport systems often consist of a substrate-binding protein associated with a membrane-spanning permease. The environmental bacterial pathogen Listeria monocytogenes uses the substrate-binding protein CtaP not only for cysteine transport but also for resistance to acid, maintenance of membrane integrity, and bacterial adherence to host cells. In this study, we demonstrate complementary yet distinct functional roles for two membrane permeases, CtpP1 and CtpP2, that are encoded by genes linked to ctaP and that contribute to bacterial growth, invasion, and pathogenicity.

Study of NIT domain-containing chemoreceptors from two global phytopathogens and identification of NIT domains in eukaryotes.

Bacterial signal transduction systems are typically activated by the binding of signal molecules to receptor ligand binding domains (LBDs), such as the NIT LBD. We report here the identification of the NIT domain in more than 15,000 receptors that were present in 30 bacterial phyla, but also in 19 eukaryotic phyla, expanding its known phylogenetic distribution. The NIT domain formed part of seven receptor families that either control transcription, mediate chemotaxis or regulate second messenger levels. We have produced the NIT domains from chemoreceptors of the bacterial phytopathogens Pectobacterium atrosepticum (PacN) and Pseudomonas savastanoi (PscN) as individual purified proteins. High-throughput ligand screening using compound libraries revealed a specificity for nitrate and nitrite binding. Isothermal titration calorimetry experiments showed that PacN-LBD bound preferentially nitrate ( K D = 1.9 μM), whereas the affinity of PscN-LBD for nitrite ( K D = 2.1 μM) was 22 times higher than that for nitrate. Analytical ultracentrifugation experiments indicated that PscN-LBD is monomeric in the presence and absence of ligands. The R182A mutant of PscN did not bind nitrate or nitrite. This residue is not conserved in the NIT domain of the Pseudomonas aeruginosa chemoreceptor PA4520, which may be related to its failure to bind nitrate/nitrite. The magnitude of P. atrosepticum chemotaxis towards nitrate was significantly greater than that of nitrite and pacN deletion almost abolished responses to both compounds. This study highlights the important role of nitrate and nitrite as signal molecules in life and advances our knowledge on the NIT domain as universal nitrate/nitrite sensor module.

All DACs in a Row: Domain Architectures of Bacterial and Archaeal Diadenylate Cyclases.

Cyclic dimeric AMP (c-di-AMP) is a widespread second messenger that controls such key functions as osmotic homeostasis, peptidoglycan biosynthesis, and response to various stresses. C-di-AMP is synthesized by diadenylate cyclases that contain the DAC (DisA_N) domain, which was originally characterized as the N-terminal domain in the DNA integrity scanning protein DisA. In other experimentally studied diadenylate cyclases, DAC domain is typically located at the protein C termini and its enzymatic activity is controlled by one or more N-terminal domains. As in other bacterial signal transduction proteins, these N-terminal modules appear to sense environmental or intracellular signals through ligand binding and/or protein-protein interactions. Studies of bacterial and archaeal diadenylate cyclases also revealed numerous sequences with uncharacterized N-terminal regions. This work provides a comprehensive review of the N-terminal domains of bacterial and archaeal diadenylate cyclases, including the description of five previously undefined domains and three PK_C-related domains of the DacZ_N superfamily. These data are used to classify diadenylate cyclases into 22 families, based on their conserved domain architectures and the phylogeny of their DAC domains. Although the nature of the regulatory signals remains obscure, the association of certain dac genes with anti-phage defense CBASS systems and other phage-resistance genes suggests that c-di-AMP might also be involved in the signaling of phage infection.

Molecular insights into Escherichia coli Cpx envelope stress response activation by the sensor lipoprotein NlpE.

Bacterial two-component signal transduction systems provide sensory inputs for appropriately adapting gene expression. These systems rely on a histidine kinase that phosphorylates a response regulator which alters gene expression. Several two-component systems include additional sensory components that can activate the histidine kinase. In Escherichia coli, the lipoprotein NlpE was identified as a sensor for the Cpx cell envelope stress response. It has remained unclear how NlpE signals to Cpx in the periplasm. In this study, we used a combination of genetics, biochemistry, and AlphaFold2 complex modeling to uncover the molecular details of how NlpE triggers the Cpx response through an interaction with the CpxA histidine kinase. Remarkably, only a short loop of NlpE is required to activate the Cpx response. A single substitution in this loop inactivates NlpE signaling to Cpx and abolishes an in vivo biochemical NlpE:CpxA interaction. An independent AlphaFold multimer prediction supported a role for the loop and predicted an interaction interface at CpxA. Mutations in this CpxA region specifically blind the histidine kinase to NlpE activation but preserve the ability to respond to other cell envelope stressors. Hence, our work additionally reveals a previously unrecognized complexity in signal integration by the CpxA periplasmic sensor domain.

The Cellular Abundance of Chemoreceptors, Chemosensory Signaling Proteins, Sensor Histidine Kinases, and Solute Binding Proteins of Pseudomonas aeruginosa Provides Insight into Sensory Preferences and Signaling Mechanisms.

Chemosensory pathways and two-component systems are important bacterial signal transduction systems. In the human pathogen Pseudomonas aeruginosa, these systems control many virulence traits. Previous studies showed that inorganic phosphate (Pi) deficiency induces virulence. We report here the abundance of chemosensory and two-component signaling proteins of P. aeruginosa grown in Pi deficient and sufficient media. The cellular abundance of chemoreceptors differed greatly, since a 2400-fold difference between the most and least abundant receptors was observed. For many chemoreceptors, their amount varied with the growth condition. The amount of chemoreceptors did not correlate with the magnitude of chemotaxis to their cognate chemoeffectors. Of the four chemosensory pathways, proteins of the Che chemotaxis pathway were most abundant and showed little variation in different growth conditions. The abundance of chemoreceptors and solute binding proteins indicates a sensing preference for amino acids and polyamines. There was an excess of response regulators over sensor histidine kinases in two-component systems. In contrast, ratios of the response regulators CheY and CheB to the histidine kinase CheA of the Che pathway were all below 1, indicative of different signaling mechanisms. This study will serve as a reference for exploring sensing preferences and signaling mechanisms of other bacteria.

Emergence of an Auxin Sensing Domain in Plant-Associated Bacteria.

Bacteria have evolved a sophisticated array of signal transduction systems that allow them to adapt their physiology and metabolism to changing environmental conditions. Typically, these systems recognize signals through dedicated ligand binding domains (LBDs) to ultimately trigger a diversity of physiological responses. Nonetheless, an increasing number of reports reveal that signal transduction receptors also bind antagonists to inhibit responses mediated by agonists. The mechanisms by which antagonists block the downstream signaling cascade remain largely unknown. To advance our knowledge in this field, we used the LysR-type transcriptional regulator AdmX as a model. AdmX activates the expression of an antibiotic biosynthetic cluster in the rhizobacterium Serratia plymuthica. AdmX specifically recognizes the auxin phytohormone indole-3-acetic acid (IAA) and its biosynthetic intermediate indole-3-pyruvic acid (IPA) as signals. However, only IAA, but not IPA, was shown to regulate antibiotic production in S. plymuthica. Here, we report the high-resolution structures of the LBD of AdmX in complex with IAA and IPA. We found that IAA and IPA compete for binding to AdmX. Although IAA and IPA binding does not alter the oligomeric state of AdmX, IPA binding causes a higher degree of compactness in the protein structure. Molecular dynamics simulations revealed significant differences in the binding modes of IAA and IPA by AdmX, and the inspection of the three-dimensional structures evidenced differential agonist- and antagonist-mediated structural changes. Key residues for auxin binding were identified and an auxin recognition motif defined. Phylogenetic clustering supports the recent evolutionary emergence of this motif specifically in plant-associated enterobacteria. IMPORTANCE Although antagonists were found to bind different bacterial signal transduction receptors, we are still at the early stages of understanding the molecular details by which these molecules exert their inhibitory effects. Here, we provide insight into the structural changes resulting from the binding of an agonist and an antagonist to a sensor protein. Our data indicate that agonist and antagonist recognition is characterized by small conformational differences in the LBDs that can be efficiently transmitted to the output domain to modulate the final response. LBDs are subject to strong selective pressures and are rapidly evolving domains. An increasing number of reports support the idea that environmental factors drive the evolution of sensor domains. Given the recent evolutionary history of AdmX homologs, as well as their narrow phyletic distribution within plant-associated bacteria, our results are in accordance with a plant-mediated evolutionary process that resulted in the emergence of receptor proteins that specifically sense auxin phytohormones.

QseBC regulates in vitro and in vivo virulence of Aeromonas hydrophila in response to norepinephrine.

The inter-kingdom communication between host and pathogenic bacteria mediated by the host hormones epinephrine (Epi)/norepinephrine (NE)/autoinducer-3 (AI-3) and transduced by the bacterial two-component signal transduction system QseBC has been well demonstrated in mammalian pathogens. Aeromonas hydrophila, a common opportunistic pathogen in freshwater aquaculture, responds to NE by increased bacterial growth and enhanced virulence. However, the underlying mechanisms remain poorly understood. Our study demonstrated that deletion of qseB and qseC significantly inhibited NE-promoted growth, biofilm formation, and hemolytic activity of A. hydrophila. The adhesion ability of ΔqseB and ΔqseC to J774a.1cells was significantly decreased compared with the wild-type strain in the presence and absence of NE, whereas NE still enhanced the adhesion ability of the mutant and wild-type strains with a similar effect, suggesting that NE-enhanced cell adhesion was independent of QseBC. Moreover, QseBC did not affect the swimming and swarming motility of A. hydrophila with or without NE. Quantitative real-time PCR analyses revealed the down-regulated expression of some virulence-related genes (hly, ast, act, aerA) in each mutant compared with the wild-type strain in the presence of NE. Tilapia infection experiments indicated that deletion of qseB or qseC weakened NE-promoted virulence of A. hydrophila. In conclusion, our study suggests that NE stimulates the growth, biofilm formation, and hemolytic activity of A. hydrophila and enhances the virulence of the pathogen in fish via the QseBC system.