Ligand specificity determined by differentially arranged common ligand-binding residues in bacterial amino acid chemoreceptors Tsr and Tar

Insights into Chemoreceptor MCP2201-Sensing D-Malate

Bacterial chemoreceptors sense extracellular stimuli and drive bacteria toward a beneficial environment or away from harm. Their ligand-binding domains (LBDs) are highly diverse in terms of sequence and structure, and their ligands cover various chemical molecules that could serve as nitrogen, carbon, and energy sources. The mechanism of how this diverse range of LBDs senses different ligands is essential to signal transduction. Previously, we reported that the chemoreceptor MCP2201 from Comamonas testosteroni CNB-1 sensed citrate and L-malate, altered the ligand-free monomer-dimer equilibrium of LBD to citrate-bound monomer (with limited monomer) and L-malate-bound dimer, and triggered positive and negative chemotactic responses. Here, we present our findings, showing that D-malate binds to MCP2201, induces LBD dimerization, and triggers the chemorepellent response exactly as L-malate did. A single site mutation, T105A, can alter the D-malate-bound LBD dimer into a monomer-dimer equilibrium and switch the negative chemotactic response to D-malate to a positive one. Differences in attractant-bound LBD oligomerization, such as citrate-bound wildtype LBD monomer and D-malate-bound T105A dimer, indicated that LBD oligomerization is a consequence of signal transduction instead of a trigger. Our study expands our knowledge of chemoreceptor-sensing ligands and provides insight into the evolution of bacterial chemoreceptors.

Chemotaxis and Related Signaling Systems in Vibrio cholerae

The motility and chemotaxis of Vibrio cholerae, the bacterial pathogen responsible for cholera, play crucial roles in both environmental survival and infection. Understanding their molecular mechanisms is therefore essential not only for fundamental biology but also for infection control and therapeutic development. The bacterium's sheathed, polar flagellum-its motility organelle-is powered by a sodium-driven motor. This motor's rotation is regulated by the chemotaxis (Che) signaling system, with a histidine kinase, CheA, and a response regulator, CheY, serving as the central processing unit. However, V. cholerae possesses two additional, parallel Che signaling systems whose physiological functions remain unclear. Furthermore, the bacterium harbors over 40 receptors/transducers that interact with CheA homologs, forming a complex regulatory network likely adapted to diverse environmental cues. Despite significant progress, many aspects of these systems remain to be elucidated. Here, we summarize the current understanding to facilitate future research.

Bacterial amino acid chemotaxis: a widespread strategy with multiple physiological and ecological roles

Chemotaxis is the directed, flagellum-based movement of bacteria in chemoeffector gradients. Bacteria respond chemotactically to a wide range of chemoeffectors, including amino, organic, and fatty acids, sugars, polyamines, quaternary amines, purines, pyrimidines, aromatic hydrocarbons, oxygen, inorganic ions, or polysaccharides. Most frequent are chemotactic responses to amino acids (AAs), which were observed in numerous bacteria regardless of their phylogeny and lifestyle. Mostly chemoattraction responses are observed, although a number of bacteria are repelled from certain AAs. Chemoattraction is associated with the important metabolic value of AAs as growth substrates or building blocks of proteins. However, additional studies revealed that AAs are also sensed as environmental cues. Many chemoreceptors are specific for AAs, and signaling is typically initiated by direct ligand binding to their four-helix bundle or dCache ligand-binding domains. Frequently, bacteria possess multiple AA-responsive chemoreceptors that at times possess complementary AA ligand spectra. The identification of sequence motifs in the binding sites at dCache_1 domains has permitted to define an AA-specific family of dCache_1AA chemoreceptors. In addition, AAs are among the ligands recognized by broad ligand range chemoreceptors, and evidence was obtained for chemoreceptor activation by the binding of AA-loaded solute-binding proteins. The biological significance of AA chemotaxis is very ample including in biofilm formation, root and seed colonization by beneficial bacteria, plant entry of phytopathogens, colonization of the intestine, or different virulence-related features in human/animal pathogens. This review provides insights that may be helpful for the study of AA chemotaxis in other uncharacterized bacteria.

Surface-programmed microbiome assembly in phycosphere to microplastics contamination

Recalcitrance in microplastics accounts for ubiquitous white pollution. Of special interest are the capabilities of microorganisms to accelerate their degradation sustainably. Compared to the well-studied pure cultures in degrading natural polymers, the algal-bacterial symbiotic system is considered as a promising candidate for microplastics removal, cascading bottom-up impacts on ecosystem-scale processes. This study selected and enriched the algae-associated microbial communities hosted by the indigenous isolation Desmodesmus sp. in wastewater treatment plants with micro-polyvinyl chloride, polyethylene terephthalate, polyethylene, and polystyrene contamination. Results elaborated that multiple settled and specific affiliates were recruited by the uniform algae protagonist from the biosphere under manifold microplastic stress. Alteration of distinct chemical functionalities and deformation of polymers provide direct evidence of degradation in phycosphere under illumination. Microplastic-induced phycosphere-derived DOM created spatial gradients of aromatic protein, fulvic and humic acid-like and tryptophan components to expanded niche-width. Surface thermodynamic analysis was conducted to simulate the reciprocal and reversible interaction on algal-bacterial and phycosphere-microplastic interface, revealing the enhancement of transition to stable and irreversible aggregation for functional microbiota colonization and microplastics capture. Furthermore, pangenomic analysis disclosed the genes related to the chemotaxis and the proposed microplastics biodegradation pathway in enriched algal-bacterial microbiome, orchestrating the evidence for common synthetic polymer particles and ultimately to confirm the effectiveness and potential. The present study emphasizes the necessity for future endeavors aimed at fully leveraging the potential of algal-bacterial mutualistic systems within sustainable bioremediation strategies targeting the eradication of microplastic waste.

Potassium-mediated bacterial chemotactic response

Bacteria in biofilms secrete potassium ions to attract free swimming cells. However, the basis of chemotaxis to potassium remains poorly understood. Here, using a microfluidic device, we found that Escherichia coli can rapidly accumulate in regions of high potassium concentration on the order of millimoles. Using a bead assay, we measured the dynamic response of individual flagellar motors to stepwise changes in potassium concentration, finding that the response resulted from the chemotaxis signaling pathway. To characterize the chemotactic response to potassium, we measured the dose-response curve and adaptation kinetics via an Förster resonance energy transfer (FRET) assay, finding that the chemotaxis pathway exhibited a sensitive response and fast adaptation to potassium. We further found that the two major chemoreceptors Tar and Tsr respond differently to potassium. Tar receptors exhibit a biphasic response, whereas Tsr receptors respond to potassium as an attractant. These different responses were consistent with the responses of the two receptors to intracellular pH changes. The sensitive response and fast adaptation allow bacteria to sense and localize small changes in potassium concentration. The differential responses of Tar and Tsr receptors to potassium suggest that cells at different growth stages respond differently to potassium and may have different requirements for potassium.

Bacterial vampirism mediated through taxis to serum

Bacteria of the family Enterobacteriaceae are associated with gastrointestinal (GI) bleeding and bacteremia and are a leading cause of death, from sepsis, for individuals with inflammatory bowel diseases. The bacterial behaviors and mechanisms underlying why these bacteria are prone to bloodstream entry remain poorly understood. Herein, we report that clinical isolates of non-typhoidal Salmonella enterica serovars, Escherichia coli, and Citrobacter koseri are rapidly attracted toward sources of human serum. To simulate GI bleeding, we utilized an injection-based microfluidics device and found that femtoliter volumes of human serum are sufficient to induce bacterial attraction to the serum source. This response is orchestrated through chemotaxis and the chemoattractant L-serine, an amino acid abundant in serum that is recognized through direct binding by the chemoreceptor Tsr. We report the first crystal structures of Salmonella Typhimurium Tsr in complex with L-serine and identify a conserved amino acid recognition motif for L-serine shared among Tsr orthologues. We find Tsr to be widely conserved among Enterobacteriaceae and numerous World Health Organization priority pathogens associated with bloodstream infections. Lastly, we find that Enterobacteriaceae use human serum as a source of nutrients for growth and that chemotaxis and the chemoreceptor Tsr provide a competitive advantage for migration into enterohemorrhagic lesions. We define this bacterial behavior of taxis toward serum, colonization of hemorrhagic lesions, and the consumption of serum nutrients as 'bacterial vampirism', which may relate to the proclivity of Enterobacteriaceae for bloodstream infections.

Bacterial sensor evolved by decreasing complexity

Bacterial receptors feed into multiple signal transduction pathways that regulate a variety of cellular processes including gene expression, second messenger levels and motility. Receptors are typically activated by signal binding to ligand binding domains (LBD). Cache domains are omnipresent LBDs found in bacteria, archaea, and eukaryotes, including humans. They form the predominant family of extracytosolic bacterial LBDs and were identified in all major receptor types. Cache domains are composed of either a single (sCache) or a double (dCache) structural module. The functional relevance of bimodular LBDs remains poorly understood. Here, we identify the PacF chemoreceptor in the phytopathogen Pectobacterium atrosepticum that recognizes formate at the membrane distal module of its dCache domain, triggering chemoattraction. We further demonstrate that a family of formate-specific sCache domains has evolved from a dCache domain, exemplified by PacF, by losing the membrane proximal module. By solving high-resolution structures of two family members in complex with formate, we show that the molecular basis for formate binding at sCache and dCache domains is highly similar, despite their low sequence identity. The apparent loss of the membrane proximal module may be related to the observation that dCache domains bind ligands typically at the membrane distal module, whereas the membrane proximal module is not involved in signal sensing. This work advances our understanding of signal sensing in bacterial receptors and suggests that evolution by reducing complexity may be a common trend shaping their diversity.

Significance

Many bacterial receptors contain multi-modular sensing domains indicative of complex sensory processes. The presence of more than one sensing module likely permits the integration of multiple signals, although, the molecular detail and functional relevance for these complex sensors remain poorly understood. Bimodular sensory domains are likely to have arisen from the fusion or duplication of monomodular domains. Evolution by increasing complexity is generally believed to be a dominant force. Here we reveal the opposite - how a monomodular sensing domain has evolved from a bimodular one. Our findings will thus motivate research to establish whether evolution by decreasing complexity is typical of other sensory domains.

Bacterial vampirism mediated through taxis to serum

Bacteria of the family Enterobacteriaceae are associated with gastrointestinal (GI) bleeding and bacteremia and are a leading cause of death, from sepsis, for individuals with inflammatory bowel diseases. The bacterial behaviors and mechanisms underlying why these bacteria are prone to bloodstream entry remains poorly understood. Herein, we report that clinical isolates of non-typhoidal Salmonella enterica serovars, Escherichia coli, and Citrobacter koseri are rapidly attracted toward sources of human serum. To simulate GI bleeding, we utilized a custom injection-based microfluidics device and found that femtoliter volumes of human serum are sufficient to induce the bacterial population to swim toward and aggregate at the serum source. This response is orchestrated through chemotaxis, and a major chemical cue driving chemoattraction is L-serine, an amino acid abundant in serum that is recognized through direct binding by the chemoreceptor Tsr. We report the first crystal structures of Salmonella Typhimurium Tsr in complex with L-serine and identify a conserved amino acid recognition motif for L-serine shared among Tsr orthologues. By mapping the phylogenetic distribution of this chemoreceptor we found Tsr to be widely conserved among Enterobacteriaceae and numerous World Health Organization priority pathogens associated with bloodstream infections. Lastly, we find that Enterobacteriaceae use human serum as a source of nutrients for growth and that chemotaxis and the chemoreceptor Tsr provides a competitive advantage for migration into enterohaemorrhagic lesions. We term this bacterial behavior of taxis toward serum, colonization of hemorrhagic lesions, and the consumption of serum nutrients, as 'bacterial vampirism' which may relate to the proclivity of Enterobacteriaceae for bloodstream infections.

Structure of the native chemotaxis core signaling unit from phage E-protein lysed E. coli cells

IMPORTANCE

Bacterial chemotaxis is a ubiquitous behavior that enables cell movement toward or away from specific chemicals. It serves as an important model for understanding cell sensory signal transduction and motility. Characterization of the molecular mechanisms underlying chemotaxis is of fundamental interest and requires a high-resolution structural picture of the sensing machinery, the chemosensory array. In this study, we combine cryo-electron tomography and molecular simulation to present the complete structure of the core signaling unit, the basic building block of chemosensory arrays, from Escherichia coli. Our results provide new insight into previously poorly-resolved regions of the complex and offer a structural basis for designing new experiments to test mechanistic hypotheses.

D-amino acids signal a stress-dependent run-away response in Vibrio cholerae

To explore favourable niches while avoiding threats, many bacteria use a chemotaxis navigation system. Despite decades of studies on chemotaxis, most signals and sensory proteins are still unknown. Many bacterial species release D-amino acids to the environment; however, their function remains largely unrecognized. Here we reveal that D-arginine and D-lysine are chemotactic repellent signals for the cholera pathogen Vibrio cholerae. These D-amino acids are sensed by a single chemoreceptor MCPDRK co-transcribed with the racemase enzyme that synthesizes them under the control of the stress-response sigma factor RpoS. Structural characterization of this chemoreceptor bound to either D-arginine or D-lysine allowed us to pinpoint the residues defining its specificity. Interestingly, the specificity for these D-amino acids appears to be restricted to those MCPDRK orthologues transcriptionally linked to the racemase. Our results suggest that D-amino acids can shape the biodiversity and structure of complex microbial communities under adverse conditions.

Three unrelated chemoreceptors provide Pectobacterium atrosepticum with a broad-spectrum amino acid sensing capability

Amino acids are important nutrients and also serve as signals for diverse signal transduction pathways. Bacteria use chemoreceptors to recognize amino acid attractants and to navigate their gradients. In Escherichia coli two likely paralogous chemoreceptors Tsr and Tar detect 9 amino acids, whereas in Pseudomonas aeruginosa the paralogous chemoreceptors PctA, PctB and PctC detect 18 amino acids. Here, we show that the phytobacterium Pectobacterium atrosepticum uses the three non-homologous chemoreceptors PacA, PacB and PacC to detect 19 proteinogenic and several non-proteinogenic amino acids. PacB recognizes 18 proteinogenic amino acids as well as 8 non-proteinogenic amino acids. PacB has a ligand preference for the three branched chain amino acids L-leucine, L-valine and L-isoleucine. PacA detects L-proline next to several quaternary amines. The third chemoreceptor, PacC, is an ortholog of E. coli Tsr and the only one of the 36 P. atrosepticum chemoreceptors that is encoded in the cluster of chemosensory pathway genes. Surprisingly, in contrast to Tsr, which primarily senses serine, PacC recognizes aspartate as the major chemoeffector but not serine. Our results demonstrate that bacteria use various strategies to sense a wide range of amino acids and that it takes more than one chemoreceptor to achieve this goal.

The pH Robustness of Bacterial Sensing

Bacteria have evolved many different signal transduction systems to sense and respond to changing environmental conditions. Signal integration is mainly achieved by signal recognition at extracytosolic ligand-binding domains (LBDs) of receptors. Hundreds of different LBDs have been reported, and our understanding of their sensing properties is growing. Receptors must function over a range of environmental pH values, but there is little information available on the robustness of sensing as a function of pH. Here, we have used isothermal titration calorimetry to determine the pH dependence of ligand recognition by nine LBDs that cover all major LBD superfamilies, of periplasmic solute-binding proteins, and cytosolic LBDs. We show that periplasmic LBDs recognize ligands over a very broad pH range, frequently stretching over eight pH units. This wide pH range contrasts with a much narrower pH response range of the cytosolic LBDs analyzed. Many LBDs must be dimeric to bind ligands, and analytical ultracentrifugation studies showed that the LBD of the Tar chemoreceptor forms dimers over the entire pH range tested. The pH dependences of Pseudomonas aeruginosa motility and chemotaxis were bell-shaped and centered at pH 7.0. Evidence for pH robustness of signaling in vivo was obtained by Förster Resonance Energy Transfer (FRET) measurements of the chemotaxis pathway responses in Escherichia coli. Bacteria have evolved several strategies to cope with extreme pH, such as periplasmic chaperones for protein refolding. The intrinsic pH resistance of periplasmic LBDs appears to be another strategy that permits bacteria to survive under adverse conditions.

Discovery of a New Chemoeffector for Escherichia coli Chemoreceptor Tsr and Identification of a Molecular Mechanism of Repellent Sensing

Motile bacteria use chemotaxis to search for nutrients and escape from harmful chemicals. While the sensing mechanisms for chemical attractants are well established, the molecular details of chemorepellent detection are poorly understood. Here, by using combined computational and experimental approaches to screen potential chemoeffectors for the Escherichia coli chemoreceptor Tsr, we identified a specific chemorepellent, 1-aminocyclohexanecarboxylic acid (ACHC). Our study strongly suggests that ACHC directly binds to the periplasmic sensory domain of Tsr and competes with l-serine, the amino acid attractant of Tsr. We further characterized the binding features of l-serine, ACHC, and l-leucine (a natural repellent that binds Tsr) and found that Asn68 plays a key role in mediating chemotactic response. Mutating Asn68 to Ala inverted the response to l-leucine from a repellent to an attractant. Our study provides important insights into the molecular mechanisms of ligand sensing via bacterial chemoreceptors.

Structural signatures of Escherichia coli chemoreceptor signaling states revealed by cellular crosslinking

The chemotaxis machinery of Escherichia coli has served as a model for exploring the molecular signaling mechanisms of transmembrane chemoreceptors known as methyl-accepting chemotaxis proteins (MCPs). Yet, fundamental questions about signal transmission through MCP molecules remain unanswered. Our work with the E. coli serine chemoreceptor Tsr has developed in vivo reporters that distinguish kinase-OFF and kinase-ON structures in the cytoplasmic methylation helix (MH) cap, which receives stimulus signals from an adjoining, membrane-proximal histidine kinase, adenylyl cyclases, MCPs, and phosphatases (HAMP) domain. The cytoplasmic helices of the Tsr homodimer interact mainly through packing interactions of hydrophobic residues at a and d heptad positions. We investigated the in vivo crosslinking properties of Tsr molecules bearing cysteine replacements at functionally tolerant g heptad positions in the N-terminal and C-terminal cap helices. Upon treatment of cells with bismaleimidoethane (BMOE), a bifunctional thiol-reagent, Tsr-G273C/Q504C readily formed a doubly crosslinked product in the presence of serine but not in its absence. Moreover, a serine stimulus combined with BMOE treatment during in vivo Förster resonance energy transfer-based kinase assays locked Tsr-G273C/Q504C in kinase-OFF output. An OFF-shifting lesion in MH1 (D269P) promoted the formation of the doubly crosslinked species in the absence of serine, whereas an ON-shifting lesion (G268P) suppressed the formation of the doubly crosslinked species. Tsr-G273C/Q504C also showed output-dependent crosslinking patterns in combination with ON-shifting and OFF-shifting adaptational modifications. Our results are consistent with a helix breathing-axial rotation-bundle repacking signaling mechanism and imply that in vivo crosslinking tools could serve to probe helix-packing transitions and their output consequences in other regions of the receptor molecule.

Chemotaxis of the Human Pathogen Pseudomonas aeruginosa to the Neurotransmitter Acetylcholine

Acetylcholine is a central biological signal molecule present in all kingdoms of life. In humans, acetylcholine is the primary neurotransmitter of the peripheral nervous system; it mediates signal transmission at neuromuscular junctions. Here, we show that the opportunistic human pathogen Pseudomonas aeruginosa exhibits chemoattraction toward acetylcholine over a concentration range of 1 μM to 100 mM. The maximal magnitude of the response was superior to that of many other P. aeruginosa chemoeffectors. We demonstrate that this chemoattraction is mediated by the PctD (PA4633) chemoreceptor. Using microcalorimetry, we show that the PctD ligand-binding domain (LBD) binds acetylcholine with a equilibrium dissociation constant (K_D) of 23 μM. It also binds choline and with lower affinity betaine. Highly sensitive responses to acetylcholine and choline, and less sensitive responses to betaine and l-carnitine, were observed in Escherichia coli expressing a chimeric receptor comprising the PctD-LBD fused to the Tar chemoreceptor signaling domain. We also identified the PacA (ECA_RS10935) chemoreceptor of the phytopathogen Pectobacterium atrosepticum, which binds choline and betaine but fails to recognize acetylcholine. To identify the molecular determinants for acetylcholine recognition, we report high-resolution structures of PctD-LBD (with bound acetylcholine and choline) and PacA-LBD (with bound betaine). We identified an amino acid motif in PctD-LBD that interacts with the acetylcholine tail. This motif is absent in PacA-LBD. Significant acetylcholine chemotaxis was also detected in the plant pathogens Agrobacterium tumefaciens and Dickeya solani. To the best of our knowledge, this is the first report of acetylcholine chemotaxis and extends the range of host signals perceived by bacterial chemoreceptors.

Diversity in Sensing and Signaling of Bacterial Sensor Histidine Kinases

Two-component signal transduction systems (TCSs) are widely conserved in bacteria to respond to and adapt to the changing environment. Since TCSs are also involved in controlling the expression of virulence, biofilm formation, quorum sensing, and antimicrobial resistance in pathogens, they serve as candidates for novel drug targets. TCSs consist of a sensor histidine kinase (HK) and its cognate response regulator (RR). Upon perception of a signal, HKs autophosphorylate their conserved histidine residues, followed by phosphotransfer to their partner RRs. The phosphorylated RRs mostly function as transcriptional regulators and control the expression of genes necessary for stress response. HKs sense their specific signals not only in their extracytoplasmic sensor domain but also in their cytoplasmic and transmembrane domains. The signals are sensed either directly or indirectly via cofactors and accessory proteins. Accumulating evidence shows that a single HK can sense and respond to multiple signals in different domains. The underlying molecular mechanisms of how HK activity is controlled by these signals have been extensively studied both biochemically and structurally. In this article, we introduce the wide diversity of signal perception in different domains of HKs, together with their recently clarified structures and molecular mechanisms.

A catalogue of signal molecules that interact with sensor kinases, chemoreceptors and transcriptional regulators

Bacteria have evolved many different signal transduction systems that sense signals and generate a variety of responses. Generally, most abundant are transcriptional regulators, sensor histidine kinases and chemoreceptors. Typically, these systems recognize their signal molecules with dedicated ligand-binding domains (LBDs), which, in turn, generate a molecular stimulus that modulates the activity of the output module. There are an enormous number of different LBDs that recognize a similarly diverse set of signals. To give a global perspective of the signals that interact with transcriptional regulators, sensor kinases and chemoreceptors, we manually retrieved information on the protein-ligand interaction from about 1,200 publications and 3D structures. The resulting 811 proteins were classified according to the Pfam family into 127 groups. These data permit a delineation of the signal profiles of individual LBD families as well as distinguishing between families that recognize signals in a promiscuous manner and those that possess a well-defined ligand range. A major bottleneck in the field is the fact that the signal input of many signaling systems is unknown. The signal repertoire reported here will help the scientific community design experimental strategies to identify the signaling molecules for uncharacterised sensor proteins.

The structural basis for signal promiscuity in a bacterial chemoreceptor

Signalling through chemosensory pathways is typically initiated by the binding of signal molecules to the chemoreceptor ligand binding domain (LBD). The PcaY_PP chemoreceptor from Pseudomonas putida KT2440 is characterized by an unusually broad signal range, and minimal requisites for signal binding are the presence of a C6-membered ring and that of a carboxyl group. Previous studies have shown that only some of the multiple signals recognized by this chemoreceptor are of apparent metabolic value. We report here high-resolution structures of PcaY_PP-LBD in the absence and presence of four cognate chemoeffectors and glycerol. The domain formed a four-helix bundle (4HB), and both ligand binding sites of the dimer were occupied with the high-affinity ligands protocatechuate and quinate, whereas the lower-affinity ligands benzoate and salicylate were present in only one site. Ligand binding was verified by microcalorimetric titration of site-directed mutants revealing important roles of an arginine and number of polar residues that establish an extensive hydrogen bonding network with bound ligands. The comparison of the apo and holo structures did not provide evidence for this receptor employing a transmembrane signalling mechanism that involves piston-like shifts of the final helix. Instead, ligand binding caused rigid-body scissoring movements of both monomers of the dimer. Comparisons with the 4HB domains of the Tar and Tsr chemoreceptors revealed significant structural differences. Importantly, the ligand binding site in PcaY_PP-LBD is approximately 8 Å removed from that of the Tar and Tsr receptors. Data indicate a significant amount of structural and functional diversity among 4HB domains. DATABASES: The coordinates and structure factors have been deposited in the protein data band with the following IDs: 6S1A (apo form), 6S18 (bound glycerol), 6S33 (bound protocatechuate), 6S38 (bound quinate), 6S3B (bound benzoate) and 6S37 (bound salicylate).

The use of isothermal titration calorimetry to unravel chemotactic signalling mechanisms

Chemotaxis is based on the action of chemosensory pathways and is typically initiated by the recognition of chemoeffectors at chemoreceptor ligand-binding domains (LBD). Chemosensory signalling is highly complex; aspect that is not only reflected in the intricate interaction between many signalling proteins but also in the fact that bacteria frequently possess multiple chemosensory pathways and often a large number of chemoreceptors, which are mostly of unknown function. We review here the usefulness of isothermal titration calorimetry (ITC) to study this complexity. ITC is the gold standard for studying binding processes due to its precision and sensitivity, as well as its capability to determine simultaneously the association equilibrium constant, enthalpy change and stoichiometry of binding. There is now evidence that members of all major LBD families can be produced as individual recombinant proteins that maintain their ligand-binding properties. High-throughput screening of these proteins using thermal shift assays offer interesting initial information on chemoreceptor ligands, providing the basis for microcalorimetric analyses and microbiological experimentation. ITC has permitted the identification and characterization of many chemoreceptors with novel specificities. This ITC-based approach can also be used to identify signal molecules that stimulate members of other families of sensor proteins.

Persister Cells Resuscitate Using Membrane Sensors that Activate Chemotaxis, Lower cAMP Levels, and Revive Ribosomes

Persistence, the stress-tolerant state, is arguably the most vital phenotype since nearly all cells experience nutrient stress, which causes a sub-population to become dormant. However, how persister cells wake to reconstitute infections is not understood well. Here, using single-cell observations, we determined that Escherichia coli persister cells resuscitate primarily when presented with specific carbon sources, rather than spontaneously. In addition, we found that the mechanism of persister cell waking is through sensing nutrients by chemotaxis and phosphotransferase membrane proteins. Furthermore, nutrient transport reduces the level of secondary messenger cAMP through enzyme IIA; this reduction in cAMP levels leads to ribosome resuscitation and rescue. Resuscitating cells also immediately commence chemotaxis toward nutrients, although flagellar motion is not required for waking. Hence, persister cells wake by perceiving nutrients via membrane receptors that relay the signal to ribosomes via the secondary messenger cAMP, and persisters wake and utilize chemotaxis to acquire nutrients.

Molecular Mechanism for Attractant Signaling to DHMA by E. coli Tsr

The attractant chemotaxis response of Escherichia coli to norepinephrine requires that it be converted to 3,4-dihydroxymandelic acid (DHMA) by the monoamine oxidase TynA and the aromatic aldehyde dehydrogenase FeaB. DHMA is sensed by the serine chemoreceptor Tsr, and the attractant response requires that at least one subunit of the periplasmic domain of the Tsr homodimer (pTsr) has an intact serine-binding site. DHMA that is generated in vivo by E. coli is expected to be a racemic mixture of the (R) and (S) enantiomers, so it has been unclear whether one or both chiral forms are active. Here, we used a combination of state-of-the-art tools in molecular docking and simulations, including an in-house simulation-based docking protocol, to investigate the binding properties of (R)-DHMA and (S)-DHMA to E. coli pTsr. Our studies computationally predicted that (R)-DHMA should promote a stronger attractant response than (S)-DHMA because of a consistently greater-magnitude piston-like pushdown of the pTsr α-helix 4 toward the membrane upon binding of (R)-DHMA than upon binding of (S)-DHMA. This displacement is caused primarily by interaction of DHMA with Tsr residue Thr156, which has been shown by genetic studies to be critical for the attractant response to L-serine and DHMA. These findings led us to separate the two chiral species and test their effectiveness as chemoattractants. Both the tethered cell and motility migration coefficient assays validated the prediction that (R)-DHMA is a stronger attractant than (S)-DHMA. Our study demonstrates that refined computational docking and simulation studies combined with experiments can be used to investigate situations in which subtle differences between ligands may lead to diverse chemotactic responses.

The ligand-binding domain of a chemoreceptor from Comamonas testosteroni has a previously unknown homotrimeric structure

Transmembrane chemoreceptors are widely present in Bacteria and Archaea. They play a critical role in sensing various signals outside and transmitting to the cell interior. Here, we report the structure of the periplasmic ligand-binding domain (LBD) of the transmembrane chemoreceptor MCP2201, which governs chemotaxis to citrate and other organic compounds in Comamonas testosteroni. The apo-form LBD crystal revealed a typical four-helix bundle homodimer, similar to previously well-studied chemoreceptors such as Tar and Tsr of Escherichia coli. However, the citrate-bound LBD revealed a four-helix bundle homotrimer that had not been observed in bacterial chemoreceptor LBDs. This homotrimer was further confirmed with size-exclusion chromatography, analytical ultracentrifugation and cross-linking experiments. The physiological importance of the homotrimer for chemotaxis was demonstrated with site-directed mutations of key amino acid residues in C. testosteroni mutants.

The Response of Cupriavidus metallidurans CH34 to Cadmium Involves Inhibition of the Initiation of Biofilm Formation, Decrease in Intracellular c-di-GMP Levels, and a Novel Metal Regulated Phosphodiesterase

Cadmium is a highly toxic heavy metal for biological systems. Cupriavidus metallidurans CH34 is a model strain to study heavy metal resistance and bioremediation as it is able to deal with high heavy metal concentrations. Biofilm formation by bacteria is mediated by the second messenger bis-(3′–5′)-cyclic dimeric guanosine monophosphate (c-di-GMP). The aim of this study was to characterize the response of C. metallidurans CH34 planktonic and biofilm cells to cadmium including their c-di-GMP regulatory pathway. Inhibition of the initiation of biofilm formation and EPS production by C. metallidurans CH34 correlates with increased concentration of cadmium. Planktonic and biofilm cells showed similar tolerance to cadmium. During exposure to cadmium an acute decrease of c-di-GMP levels in planktonic and biofilm cells was observed. Transcription analysis by RT-qPCR showed that cadmium exposure to planktonic and biofilm cells induced the expression of the urf2 gene and the mercuric reductase encoding merA gene, which belong to the Tn501/Tn21 mer operon. After exposure to cadmium, the cadA gene involved in cadmium resistance was equally upregulated in both lifestyles. Bioinformatic analysis and complementation assays indicated that the protein encoded by the urf2 gene is a functional phosphodiesterase (PDE) involved in the c-di-GMP metabolism. We propose to rename the urf2 gene as mrp gene for metal regulated PDE. An increase of the second messenger c-di-GMP content by the heterologous expression of the constitutively active diguanylate cyclase PleD correlated with an increase in biofilm formation and cadmium susceptibility. These results indicate that the response to cadmium in C. metallidurans CH34 inhibits the initiation of biofilm lifestyle and involves a decrease in c-di-GMP levels and a novel metal regulated PDE.

In Situ Conformational Changes of the Escherichia coli Serine Chemoreceptor in Different Signaling States

Tsr, the serine chemoreceptor in Escherichia coli, transduces signals from a periplasmic ligand-binding site to its cytoplasmic tip, where it controls the activity of the CheA kinase. To function, Tsr forms trimers of homodimers (TODs), which associate in vivo with the CheA kinase and CheW coupling protein. Together, these proteins assemble into extended hexagonal arrays. Here, we use cryo-electron tomography and molecular dynamics simulation to study Tsr in the context of a near-native array, characterizing its signaling-related conformational changes at both the individual dimer and the trimer level. In particular, we show that individual Tsr dimers within a trimer exhibit asymmetric flexibilities that are a function of the signaling state, highlighting the effect of their different protein interactions at the receptor tips. We further reveal that the dimer compactness of the Tsr trimer changes between signaling states, transitioning at the glycine hinge from a compact conformation in the kinase-OFF state to an expanded conformation in the kinase-ON state. Hence, our results support a crucial role for the glycine hinge: to allow the receptor flexibility necessary to achieve different signaling states while also maintaining structural constraints imposed by the membrane and extended array architecture.IMPORTANCE In Escherichia coli, membrane-bound chemoreceptors, the histidine kinase CheA, and coupling protein CheW form highly ordered chemosensory arrays. In core signaling complexes, chemoreceptor trimers of dimers undergo conformational changes, induced by ligand binding and sensory adaptation, which regulate kinase activation. Here, we characterize by cryo-electron tomography the kinase-ON and kinase-OFF conformations of the E. coli serine receptor in its native array context. We found distinctive structural differences between the members of a receptor trimer, which contact different partners in the signaling unit, and structural differences between the ON and OFF signaling complexes. Our results provide new insights into the signaling mechanism of chemoreceptor arrays and suggest an important functional role for a previously postulated flexible region and glycine hinge in the receptor molecule.

Spatial Restrictions in Chemotaxis Signaling Arrays: A Role for Chemoreceptor Flexible Hinges across Bacterial Diversity

The chemotactic sensory system enables motile bacteria to move toward favorable environments. Throughout bacterial diversity, the chemoreceptors that mediate chemotaxis are clustered into densely packed arrays of signaling complexes. In these arrays, rod-shaped receptors are in close proximity, resulting in limited options for orientations. A recent geometric analysis of these limitations in Escherichia coli, using published dimensions and angles, revealed that in this species, straight chemoreceptors would not fit into the available space, but receptors bent at one or both of the recently-documented flexible hinges would fit, albeit over a narrow window of shallow bend angles. We have now expanded our geometric analysis to consider variations in receptor length, orientation and placement, and thus to species in which those parameters are known to be, or might be, different, as well as to the possibility of dynamic variation in those parameters. The results identified significant limitations on the allowed combinations of chemoreceptor dimensions, orientations and placement. For most combinations, these limitations excluded straight chemoreceptors, but allowed receptors bent at a flexible hinge. Thus, our analysis identifies across bacterial diversity a crucial role for chemoreceptor flexible hinges, in accommodating the limitations of molecular crowding in chemotaxis core signaling complexes and their arrays.

Calcium Ions Modulate Amino Acid Sensing of the Chemoreceptor Mlp24 of Vibrio cholerae

Bacteria sense environmental chemicals using chemosensor proteins, most of which are present in the cytoplasmic membrane. Canonical chemoreceptors bind their specific ligands in their periplasmic domain, and the ligand binding creates a molecular stimulus that is transmitted into the cytoplasm, leading to various cellular responses, such as chemotaxis and specific gene expression. Vibrio cholerae, the causative agent of cholera, contains about 44 putative sensor proteins, which are homologous to methyl-accepting chemotaxis proteins involved in chemotaxis. Two of them, Mlp24 and Mlp37, have been identified as chemoreceptors that mediate chemotactic responses to various amino acids. Although most of the residues of Mlp37 involved in ligand binding are conserved in Mlp24, these chemoreceptors bind the same ligands with different affinities. Moreover, they have distinct cellular roles. Here we determined a series of ligand complex structures of the periplasmic domains of Mlp24 (Mlp24p). The structures revealed that Ca²⁺ binds to the loop that forms the upper wall of the ligand-binding pocket. Ca²⁺ does not bind to the corresponding loop of Mlp37, implying that the structural difference of the loop may cause the ligand affinity difference. Isothermal titration calorimetry (ITC) measurements indicated that Ca²⁺ changes the ligand binding affinity of Mlp24p. Furthermore, Ca²⁺ affected chemotactic behaviors to various amino acids mediated by Mlp24. Thus, Ca²⁺ is suggested to serve as a cosignal for the primary signal mediated by Mlp24p, and V. cholerae fine-tunes its chemotactic behavior depending on the Ca²⁺ concentration by modulating the ligand sensitivity of Mlp24.IMPORTANCE Mlp24 and Mlp37 are homologous chemoreceptors of Vibrio cholerae that bind various amino acids. Although most of the residues involved in ligand interaction are conserved, these chemoreceptors show different affinities for the same ligand and play different cellular roles. A series of ligand complex structures of the periplasmic region of Mlp24 (Mlp24p) and following ITC analysis revealed that Ca²⁺ binds to the loop of Mlp24p and modulates the ligand binding affinity of Mlp24p. Moreover, Ca²⁺ changes the chemotactic behaviors mediated by Mlp24. We propose that Ca²⁺ acts as a cosignal that modulates the affinity of Mlp24 for the primary signal, thereby changing the chemotactic behavior of V. cholerae.

Towards the Response Threshold for p-Hydroxyacetophenone in the Denitrifying Bacterium "Aromatoleum aromaticum" EbN1

The denitrifying betaproteobacterium "Aromatoleum aromaticum" EbN1 regulates the capacity to anaerobically degrade p-ethylphenol (via p-hydroxyacetophenone) with high substrate specificity. This process is mediated by the σ⁵⁴-dependent transcriptional regulator EtpR, which apparently recognizes both aromatic compounds, yielding congruent expression profiles. The responsiveness of this regulatory system was studied with p-hydroxyacetophenone, which is more easily administered to cultures and traced analytically. Cultures of A. aromaticum EbN1 were initially cultivated under nitrate-reducing conditions with a growth-limiting supply of benzoate, upon the complete depletion of which p-hydroxyacetophenone was added at various concentrations (from 500 μM down to 0.1 nM). Depletion profiles of this aromatic substrate and presumptive effector were determined by highly sensitive micro-high-performance liquid chromatography (microHPLC). Irrespective of the added concentration of p-hydroxyacetophenone, depletion commenced after less than 5 min and suggested a response threshold of below 10 nM. This approximation was corroborated by time-resolved transcript profiles (quantitative reverse transcription-PCR) of selected degradation and efflux relevant genes (e.g., pchF, encoding a subunit of predicted p-ethylphenol methylenehydroxylase) and narrowed down to a range of 10 to 1 nM. The most pronounced transcriptional response was observed, as expected, for genes located at the beginning of the two operon-like structures, related to catabolism (i.e., acsA) and potential efflux (i.e., ebA335).IMPORTANCE Aromatic compounds are widespread microbial growth substrates with natural as well as anthropogenic sources, albeit with their in situ concentrations and their bioavailabilities varying over several orders of magnitude. Even though degradation pathways and underlying regulatory systems have long been studied with aerobic and, to a lesser extent, with anaerobic bacteria, comparatively little is known about the effector concentration-dependent responsiveness. A. aromaticum EbN1 is a model organism for the anaerobic degradation of aromatic compounds with the architecture of the catabolic network and its substrate-specific regulation having been intensively studied by means of differential proteogenomics. The present study aims at unraveling the minimal concentration of an aromatic growth substrate (p-hydroxyacetophenone here) required to initiate gene expression for its degradation pathway and to learn in principle about the lower limit of catabolic responsiveness of an anaerobic degradation specialist.

Adenylate cyclases: Receivers, transducers, and generators of signals

Class III adenylate cyclases (ACs) are widespread signaling proteins, which translate diverse intracellular and extracellular stimuli into a uniform intracellular signal. They are typically composed of an N-terminal array of input domains and transducers, followed C-terminally by a catalytic domain, which, as a dimer, generates the second messenger cAMP. The input domains, which receive stimuli, and the transducers, which propagate the signals, are often found in other signaling proteins. The nature of stimuli and the regulatory mechanisms of ACs have been studied experimentally in only a few cases, and even in these, important questions remain open, such as whether eukaryotic ACs regulated by G protein-coupled receptors can also receive stimuli through their own membrane domains. Here we survey the current knowledge on regulation and intramolecular signal propagation in ACs and draw comparisons to other signaling proteins. We highlight the pivotal role of a recently identified cyclase-specific transducer element located N-terminally of many AC catalytic domains, suggesting an intramolecular signaling capacity.

Logistic Regression of Ligands of Chemotaxis Receptors Offers Clues about Their Recognition by Bacteria

Because of relative simplicity of signal transduction pathway, bacterial chemotaxis sensory systems have been expected to be applied to biosensor. Tar and Tsr receptors mediate chemotaxis of Escherichia coli and have been studied extensively as models of chemoreception by bacterial two-transmembrane receptors. Such studies are typically conducted using two canonical ligands: l-aspartate for Tar and l-serine for Tsr. However, Tar and Tsr also recognize various analogs of aspartate and serine; it remains unknown whether the mechanism by which the canonical ligands are recognized is also common to the analogs. Moreover, in terms of engineering, it is important to know a single species of receptor can recognize various ligands to utilize bacterial receptor as the sensor for wide range of substances. To answer these questions, we tried to extract the features that are common to the recognition of the different analogs by constructing classification models based on machine-learning. We computed 20 physicochemical parameters for each of 38 well-known attractants that act as chemoreception ligands, and 15 known non-attractants. The classification models were generated by utilizing one or more of the seven physicochemical properties as descriptors. From the classification models, we identified the most effective physicochemical parameter for classification: the minimum electron potential. This descriptor that occurred repeatedly in classification models with the highest accuracies, This descriptor used alone could accurately classify 42/53 of compounds. Among the 11 misclassified compounds, eight contained two carboxyl groups, which is analogous to the structure of characteristic of aspartate analog. When considered separately, 16 of the 17 aspartate analogs could be classified accurately based on the distance between their two carboxyl groups. As shown in these results, we succeed to predict the ligands for bacterial chemoreceptors using only a few descriptors; single descriptor for single receptor. This result might be due to the relatively simple topology of bacterial two-transmembrane receptors compared to the G-protein-coupled receptors of seven-transmembrane receptors. Moreover, this distance between carboxyl groups correlated with the receptor binding affinity of the aspartate analogs. In view of this correlation, we propose a common mechanism underlying ligand recognition by Tar of compounds with two carboxyl groups.

Molecular Modeling of Chemoreceptor:Ligand Interactions

Docking algorithms have been widely used to elucidate ligand:receptor interactions that are important in biological function. Here, we introduce an in-house developed docking-refinement protocol that combines the following innovative features. (1) The use of multiple short molecular dynamics (MD) docking simulations, with residues within the binding pocket of the receptor unconstrained, so that the binding modes of the ligand in the binding pocket may be exhaustively examined. (2) The initial positioning of the ligand within the binding pocket based on complementary shape, and the use of both harmonic and quartic spherical potentials to constrain the ligand in the binding pocket during multiple short docking simulations. (3) The selection of the most probable binding modes generated by the short docking simulations using interaction energy calculations, as well as the subsequent application of all-atom MD simulations and physical-chemistry based free energy calculations to elucidate the most favorable binding mode of the ligand in complex with the receptor. In this chapter, we provide step-by-step instructions on how to computationally investigate the binding of small-molecule ligands to protein receptors by examining as control and test cases, respectively, the binding of L-serine and R-3,4-dihydroxymandelic acid (R-DHMA) to the Escherichia coli chemoreceptor Tsr. Similar computational strategies can be used for the molecular modeling of a series of ligand:protein receptor interactions.

Mutational Analysis of Binding Protein-Chemoreceptor Interactions

Interactions between ligands and chemoreceptors in Escherichia coli and Salmonella enterica can be studied through genetic manipulation of the actors involved. Sequence analysis and modeling can reveal potential sites of interaction, and these sites can be deleted or mutated and the effects tested through various in vivo chemotaxis assays to ascertain their importance during interaction. Here, the approach for analysis of the interaction between a major E. coli chemoreceptor and its binding protein ligand is described.

Sensory Repertoire of Bacterial Chemoreceptors

Chemoreceptors in bacteria detect a variety of signals and feed this information into chemosensory pathways that represent a major mode of signal transduction. The five chemoreceptors from Escherichia coli have served as traditional models in the study of this protein family. Genome analyses revealed that many bacteria contain much larger numbers of chemoreceptors with broader sensory capabilities. Chemoreceptors differ in topology, sensing mode, cellular location, and, above all, the type of ligand binding domain (LBD). Here, we highlight LBD diversity using well-established and emerging model organisms as well as genomic surveys. Nearly a hundred different types of protein domains that are found in chemoreceptor sequences are known or predicted LBDs, but only a few of them are ubiquitous. LBDs of the same class recognize different ligands, and conversely, the same ligand can be recognized by structurally different LBDs; however, recent studies began to reveal common characteristics in signal-LBD relationships. Although signals can stimulate chemoreceptors in a variety of different ways, diverse LBDs appear to employ a universal transmembrane signaling mechanism. Current and future studies aim to establish relationships between LBD types, the nature of signals that they recognize, and the mechanisms of signal recognition and transduction.

Chemotactic Behaviors of Vibrio cholerae Cells

Vibrio cholerae, the causative agent of cholera, swims in aqueous environments with a single polar flagellum. In a spatial gradient of a chemical, the bacterium can migrate in "favorable" directions, a property that is termed chemotaxis. The chemotaxis of V. cholerae is not only critical for survival in various environments and but also is implicated in pathogenicity. In this chapter, we describe how to characterize the chemotactic behaviors of V. cholerae: these methods include swarm assay, temporal stimulation assay, capillary assay, and receptor methylation assay.

Structural basis for ligand recognition by a Cache chemosensory domain that mediates carboxylate sensing in Pseudomonas syringae

Chemoreceptors enable bacteria to detect chemical signals in the environment and navigate towards niches that are favourable for survival. The sensor domains of chemoreceptors function as the input modules for chemotaxis systems, and provide sensory specificity by binding specific ligands. Cache-like domains are the most common extracellular sensor module in prokaryotes, however only a handful have been functionally or structurally characterised. Here, we have characterised a chemoreceptor Cache-like sensor domain (PscD-SD) from the plant pathogen Pseudomonas syringae pv. actinidiae (Psa). High-throughput fluorescence thermal shift assays, combined with isothermal thermal titration calorimetry, revealed that PscD-SD binds specifically to C₂ (glycolate and acetate) and C₃ (propionate and pyruvate) carboxylates. We solved the structure of PscD-SD in complex with propionate using X-ray crystallography. The structure reveals the key residues that comprise the ligand binding pocket and dictate the specificity of this sensor domain for C₂ and C₃ carboxylates. We also demonstrate that all four carboxylate ligands are chemoattractants for Psa, but only two of these (acetate and pyruvate) are utilisable carbon sources. This result suggests that in addition to guiding the bacteria towards nutrients, another possible role for carboxylate sensing is in locating potential sites of entry into the host plant.

Engineering Hybrid Chemotaxis Receptors in Bacteria

Most bacteria use transmembrane sensors to detect a wide range of environmental stimuli. A large class of such sensors are the chemotaxis receptors used by motile bacteria to follow environmental chemical gradients. In Escherichia coli, chemotaxis receptors are known to mediate highly sensitive responses to ligands, making them potentially useful for biosensory applications. However, with only four ligand-binding chemotaxis receptors, the natural ligand spectrum of E. coli is limited. The design of novel chemoreceptors to extend the sensing capabilities of E. coli is therefore a critical aspect of chemotaxis-based biosensor development. One path for novel sensor design is to harvest the large natural diversity of chemosensory functions found in bacteria by creating hybrids that have the signaling domain from E. coli chemotaxis receptors and sensory domains from other species. In this work, we demonstrate that the E. coli receptor Tar can be successfully combined with most typical sensory domains found in chemotaxis receptors and in evolutionary-related two-component histidine kinases. We show that such functional hybrids can be generated using several different fusion points. Our work further illustrates how hybrid receptors could be used to quantitatively characterize ligand specificity of chemotaxis receptors and histidine kinases using standardized assays in E. coli.

Cache Domains That are Homologous to, but Different from PAS Domains Comprise the Largest Superfamily of Extracellular Sensors in Prokaryotes

Cellular receptors usually contain a designated sensory domain that recognizes the signal. Per/Arnt/Sim (PAS) domains are ubiquitous sensors in thousands of species ranging from bacteria to humans. Although PAS domains were described as intracellular sensors, recent structural studies revealed PAS-like domains in extracytoplasmic regions in several transmembrane receptors. However, these structurally defined extracellular PAS-like domains do not match sequence-derived PAS domain models, and thus their distribution across the genomic landscape remains largely unknown. Here we show that structurally defined extracellular PAS-like domains belong to the Cache superfamily, which is homologous to, but distinct from the PAS superfamily. Our newly built computational models enabled identification of Cache domains in tens of thousands of signal transduction proteins including those from important pathogens and model organisms. Furthermore, we show that Cache domains comprise the dominant mode of extracellular sensing in prokaryotes.

Cell-surface receptors control multiple cellular functions and are attractive targets for drug design. These receptors often have dedicated extracellular domains that bind signaling molecules, such as hormones and nutrients. Computational identification of these ligand-binding domains in genomic sequences is a pre-requisite for their further experimental characterization. Using available three-dimensional structures of several bacterial cell-surface receptors, we built computational models that enabled identification of the Cache domain, as the most common extracellular sensor module in prokaryotes, including many important pathogens. We also demonstrated that the Cache domain is homologous to, but sufficiently different from the most common intracellular sensor module, the PAS domain. These findings provide a unified view on molecular principles of signal recognition by extra- and intracellular receptors.

Identification of a Vibrio cholerae chemoreceptor that senses taurine and amino acids as attractants

Vibrio cholerae, the etiological agent of cholera, was found to be attracted by taurine (2-aminoethanesulfonic acid), a major constituent of human bile. Mlp37, the closest homolog of the previously identified amino acid chemoreceptor Mlp24, was found to mediate taxis to taurine as well as L-serine, L-alanine, L-arginine, and other amino acids. Methylation of Mlp37 was enhanced upon the addition of taurine and amino acids. Isothermal titration calorimetry demonstrated that a purified periplasmic fragment of Mlp37 binds directly to taurine, L-serine, L-alanine and L-arginine. Crystal structures of the periplamic domain of Mlp37 revealed that L-serine and taurine bind to the membrane-distal PAS domain in essentially in the same way. The structural information was supported by characterising the in vivo properties of alanine-substituted mutant forms of Mlp37. The fact that the ligand-binding domain of the L-serine complex had a small opening, which would accommodate a larger R group, accounts for the broad ligand specificity of Mlp37 and allowed us to visualise ligand binding to Mlp37 with fluorescently labelled L-serine. Taken together, we conclude that Mlp37 serves as the major chemoreceptor for taurine and various amino acids.

Chemotactic Signaling by Single-Chain Chemoreceptors

Bacterial chemoreceptors of the methyl-accepting chemotaxis protein (MCP) family operate in commingled clusters that enable cells to detect and track environmental chemical gradients with high sensitivity and precision. MCP homodimers of different detection specificities form mixed trimers of dimers that facilitate inter-receptor communication in core signaling complexes, which in turn assemble into a large signaling network. The two subunits of each homodimeric receptor molecule occupy different locations in the core complexes. One subunit participates in trimer-stabilizing interactions at the trimer axis, the other lies on the periphery of the trimer, where it can interact with two cytoplasmic proteins: CheA, a signaling autokinase, and CheW, which couples CheA activity to receptor control. As a possible tool for independently manipulating receptor subunits in these two structural environments, we constructed and characterized fused genes for the E. coli serine chemoreceptor Tsr that encoded single-chain receptor molecules in which the C-terminus of the first Tsr subunit was covalently connected to the N-terminus of the second with a polypeptide linker. We showed with soft agar assays and with a FRET-based in vivo CheA kinase assay that single-chain Tsr~Tsr molecules could promote serine sensing and chemotaxis responses. The length of the connection between the joined subunits was critical. Linkers nine residues or shorter locked the receptor in a kinase-on state, most likely by distorting the native structure of the receptor HAMP domain. Linkers 22 or more residues in length permitted near-normal Tsr function. Few single-chain molecules were found as monomer-sized proteolytic fragments in cells, indicating that covalently joined receptor subunits were responsible for mediating the signaling responses we observed. However, cysteine-directed crosslinking, spoiling by dominant-negative Tsr subunits, and rearrangement of ligand-binding site lesions revealed subunit swapping interactions that will need to be taken into account in experimental applications of single-chain chemoreceptors.

Single-cell E. coli response to an instantaneously applied chemotactic signal

In response to an attractant or repellant, an Escherichia coli cell controls the rotational direction of its flagellar motor by a chemotaxis system. When an E. coli cell senses an attractant, a reduction in the intracellular concentration of a chemotaxis protein, phosphorylated CheY (CheY-P), induces counterclockwise (CCW) rotation of the flagellar motor, and this cellular response is thought to occur in several hundred milliseconds. Here, to measure the signaling process occurring inside a single E. coli cell, including the recognition of an attractant by a receptor cluster, the inactivation of histidine kinase CheA, and the diffusion of CheY and CheY-P molecules, we applied a serine stimulus by instantaneous photorelease from a caged compound and examined the cellular response at a temporal resolution of several hundred microseconds. We quantified the clockwise (CW) and CCW durations immediately after the photorelease of serine as the response time and the duration of the response, respectively. The results showed that the response time depended on the distance between the receptor and motor, indicating that the decreased CheY-P concentration induced by serine propagates through the cytoplasm from the receptor-kinase cluster toward the motor with a timing that is explained by the diffusion of CheY and CheY-P molecules. The response time included 240 ms for enzymatic reactions in addition to the time required for diffusion of the signaling molecule. The measured response time and duration of the response also revealed that the E. coli cell senses a similar serine concentration regardless of whether the serine concentration is increasing or decreasing. These detailed quantitative findings increase our understanding of the signal transduction process that occurs inside cells during bacterial chemotaxis.

Both piston-like and rotational motions are present in bacterial chemoreceptor signaling

Bacterial chemotaxis signaling is triggered by binding of chemo-effectors to the membrane-bound chemoreceptor dimers. Though much is known about the structure of the chemoreceptors, details of the receptor dynamics and their effects on signaling are still unclear. Here, by using molecular dynamics simulations and principle component analysis, we study the dynamics of the periplasmic domain of aspartate chemoreceptor Tar dimer and its conformational changes when binding to different ligands (attractant, antagonist, and two attractant molecules). We found two dominant components (modes) in the receptor dynamics: a relative rotation of the two Tar monomers and a piston-like up-and-down sliding movement of the α4 helix. These two modes are highly correlated. Binding of one attractant molecule to the Tar dimer induced both significant piston-like downward movements of the α4 helix and strong relative rotations of the two Tar monomers, while binding of an antagonist or the symmetric binding of two attractant molecules to a Tar dimer suppresses both modes. The anti-symmetric effects of the relative rotation mode also explained the negative cooperativity between the two binding pockets. Our results suggest a mechanism of coupled rotation and piston-like motion for bacterial chemoreceptor signaling.

Two-tiered histidine kinase pathway involved in heat shock and salt sensing in the general stress response of Sphingomonas melonis Fr1

The general stress response (GSR) allows bacteria to monitor and defend against a broad set of unrelated, adverse environmental conditions. In Alphaproteobacteria, the key step in GSR activation is phosphorylation of the response regulator PhyR. In Sphingomonas melonis Fr1, seven PhyR-activating kinases (Paks), PakA to PakG, are thought to directly phosphorylate PhyR under different stress conditions, but the nature of the activating signals remains obscure. PakF, a major sensor of NaCl and heat shock, lacks a putative sensor domain but instead harbors a single receiver (REC) domain (PakFREC) N-terminal to its kinase catalytic core. Such kinases are called "hybrid response regulators" (HRRs). How HRRs are able to perceive signals in the absence of a true sensor domain has remained largely unexplored. In the present work, we show that stresses are actually sensed by another kinase, KipF (kinase of PakF), which phosphorylates PakFREC and thereby activates PakF. KipF is a predicted transmembrane kinase, harboring a periplasmic CHASE3 domain flanked by two transmembrane helices in addition to its cytoplasmic kinase catalytic core. We demonstrate that KipF senses different salts through its CHASE3 domain but is not a sensor of general osmotic stress. While salt sensing depends on the CHASE3 domain, heat shock sensing does not, suggesting that these stresses are perceived by different mechanisms. In summary, our results establish a two-tiered histidine kinase pathway involved in activation of the GSR in S. melonis Fr1 and provide the first experimental evidence for the so far uncharacterized CHASE3 domain as a salt sensor.

IMPORTANCE

Hybrid response regulators (HRRs) represent a particular class of histidine kinases harboring an N-terminal receiver (REC) domain instead of a true sensor domain. This suggests that the actual input for HRRs may be phosphorylation of the REC domain. In the present study, we addressed this question by using the HRR PakF. Our results suggest that PakF is activated through phosphorylation of its REC domain and that this is achieved by another kinase, KipF. KipF senses heat shock and salt stress, with the latter requiring the periplasmic CHASE3 domain. This work not only suggests that HRRs work in two-tiered histidine kinase pathways but also provides the first experimental evidence for a role of the so far uncharacterized CHASE3 domain in salt sensing.

Bacterial chemoreceptors and chemoeffectors

Bacteria use chemotaxis signaling pathways to sense environmental changes. Escherichia coli chemotaxis system represents an ideal model that illustrates fundamental principles of biological signaling processes. Chemoreceptors are crucial signaling proteins that mediate taxis toward a wide range of chemoeffectors. Recently, in deep study of the biochemical and structural features of chemoreceptors, the organization of higher-order clusters in native cells, and the signal transduction mechanisms related to the on-off signal output provides us with general insights to understand how chemotaxis performs high sensitivity, precise adaptation, signal amplification, and wide dynamic range. Along with the increasing knowledge, bacterial chemoreceptors can be engineered to sense novel chemoeffectors, which has extensive applications in therapeutics and industry. Here we mainly review recent advances in the E. coli chemotaxis system involving structure and organization of chemoreceptors, discovery, design, and characterization of chemoeffectors, and signal recognition and transduction mechanisms. Possible strategies for changing the specificity of bacterial chemoreceptors to sense novel chemoeffectors are also discussed.

Chemotaxis of Escherichia coli to norepinephrine (NE) requires conversion of NE to 3,4-dihydroxymandelic acid

Norepinephrine (NE), the primary neurotransmitter of the sympathetic nervous system, has been reported to be a chemoattractant for enterohemorrhagic Escherichia coli (EHEC). Here we show that nonpathogenic E. coli K-12 grown in the presence of 2 μM NE is also attracted to NE. Growth with NE induces transcription of genes encoding the tyramine oxidase, TynA, and the aromatic aldehyde dehydrogenase, FeaB, whose respective activities can, in principle, convert NE to 3,4-dihydroxymandelic acid (DHMA). Our results indicate that the apparent attractant response to NE is in fact chemotaxis to DHMA, which was found to be a strong attractant for E. coli. Only strains of E. coli K-12 that produce TynA and FeaB exhibited an attractant response to NE. We demonstrate that DHMA is sensed by the serine chemoreceptor Tsr and that the chemotaxis response requires an intact serine-binding site. The threshold concentration for detection is ≤5 nM DHMA, and the response is inhibited at DHMA concentrations above 50 μM. Cells producing a heterodimeric Tsr receptor containing only one functional serine-binding site still respond like the wild type to low concentrations of DHMA, but their response persists at higher concentrations. We propose that chemotaxis to DHMA generated from NE by bacteria that have already colonized the intestinal epithelium may recruit E. coli and other enteric bacteria that possess a Tsr-like receptor to preferred sites of infection.

Purification, crystallization and preliminary crystallographic analysis of the ligand-binding regions of the PctA and PctB chemoreceptors from Pseudomonas aeruginosa in complex with amino acids

Pseudomonas aeruginosa is an opportunistic pathogen and one of the major model organisms for the study of chemotaxis. The bacterium harbours 26 genes encoding chemoreceptors, most of which have not been annotated with a function. The paralogous chemoreceptors PctA and PctB (Pseudomonas chemotactic transducer A and B) were found to mediate chemotaxis towards L-amino acids. However, the ligand spectrum of the receptors is quite different since the recombinant ligand-binding region (LBR) of PctA binds 17 different L-amino acids whereas that of PctB recognizes only five. To determine the molecular basis underlying this ligand specificity, PctA-LBR and PctB-LBR have been purified and crystals have been produced after pre-incubation with L-Ile and L-Arg, respectively. Initial crystallization conditions have been identified by the counter-diffusion method and X-ray data have been collected at 2.5 Å (PctA-LBR bound to L-Ile) and 3.14 Å (PctB-LBR bound to L-Arg) resolution. Crystals belonged to space groups P2(1)2(1)2(1) and P3(1)2(1), with unit-cell parameters a = 72.2, b = 78.5, c = 116.6 Å and a = b = 111.6, c = 117.4, respectively, for PctA-LBR and PctB-LBR. Molecular-replacement methods will be pursued for structural determination.