Direct sensing and signal transduction during bacterial chemotaxis toward aromatic compounds in Comamonas testosteroni

Micro‐organisms sense and chemotactically respond to aromatic compounds. Although the existence of chemoreceptors that bind to aromatic attractants and subsequently trigger chemotaxis have long been speculated, such a chemoreceptor has not been demonstrated. In this report, we demonstrated that the chemoreceptor MCP2901 from Comamonas testosteroni CNB‐1 binds to aromatic compounds and initiates downstream chemotactic signaling in addition to its ability to trigger chemotaxis via citrate binding. The function of gene MCP2901 was investigated by genetic deletion from CNB‐1 and genetic complementation of the methyl‐accepting chemotaxis protein (MCP)‐null mutant CNB‐1Δ20. Results showed that the expression of MCP2901 in the MCP‐null mutant restored chemotaxis toward nine tested aromatic compounds and nine carboxylic acids. Isothermal titration calorimetry (ITC) analyses demonstrated that the ligand‐binding domain of MCP2901 (MCP2901LBD) bound to citrate, and weakly to gentisate and 4‐hydroxybenzoate. Additionally, ITC assays indicated that MCP2901LBD bound strongly to 2,6‐dihydroxybenzoate and 2‐hydroxybenzoate, which are isomers of gentisate and 4‐hydroxybenzoate respectively that are not metabolized by CNB‐1. Agarose‐in‐plug and capillary assays showed that these two molecules serve as chemoattractants for CNB‐1. Through constructing membrane‐like MCP2901‐inserted Nanodiscs and phosphorelay activity assays, we demonstrated that 2,6‐dihydroxybenzoate and 2‐hydroxybenzoate altered kinase activity of CheA. This is the first evidence of an MCP binding to an aromatic molecule and triggering signal transduction for bacterial chemotaxis.     

Identification of a chemoreceptor that specifically mediates chemotaxis toward metabolizable purine derivatives

Chemotaxis is an essential mechanism that enables bacteria to move toward favorable ecological niches. Escherichia coli, the historical model organism for studying chemotaxis, has five well‐studied chemoreceptors. However, many bacteria with different lifestyle have more chemoreceptors, most of unknown function. Using a high throughput screening approach, we identified a chemoreceptor from Pseudomonas putida KT2440, named McpH, which specifically recognizes purine and its derivatives, adenine, guanine, xanthine, hypoxanthine and uric acid. The latter five compounds form part of the purine degradation pathway, permitting their use as sole nitrogen sources. Isothermal titration calorimetry studies show that these six compounds bind McpH‐Ligand Binding Domain (LBD) with very similar affinity. In contrast, non‐metabolizable purine derivatives (caffeine, theophylline, theobromine), nucleotides, nucleosides or pyrimidines are unable to bind McpH‐LBD. Mutation of mcpH abolished chemotaxis toward the McpH ligands identified – a phenotype that is restored by complementation. This is the first report on bacterial chemotaxis to purine derivatives and McpH the first chemoreceptor described that responds exclusively to intermediates of a catabolic pathway, illustrating a clear link between metabolism and chemotaxis. The evolution of McpH may reflect a saprophytic lifestyle, which would have exposed the studied bacterium to high concentrations of purines produced by nucleic acid degradation.     

Elucidation of the multiple roles of CheD in Bacillus subtilis chemotaxis

Chemotaxis by Bacillus subtilis requires the CheD protein for proper function. In a cheD mutant when McpB was the sole chemoreceptor in B. subtilis, chemotaxis to asparagine was quite good. When McpC was the sole chemoreceptor in a cheD mutant, chemotaxis to proline was very poor. The reason for the difference between the chemoreceptors is because CheD deamidates Q609 in McpC and does not deamidate McpB. When mcpC‐Q609E is expressed as the sole chemoreceptor in a cheD background, chemotaxis is almost fully restored. Concomitantly, in vitro McpC activates the CheA kinase poorly, whereas McpC‐Q609E activates it much more. Moreover, CheD, which activates chemoreceptors, binds better to McpC‐Q609E compared with unmodified McpC. Using hydroxyl radical susceptibility in the presence or absence of CheD, the most likely sites of CheD binding were the modification sites where CheD, CheB and CheR carry out their catalytic activities. Thus, CheD appears to have two separate roles in B. subtilis chemotaxis – to bind to chemoreceptors to activate them as part of the CheC/CheD/CheYp adaptation system and to deamidate selected residues to activate the chemoreceptors and enable them to mediate amino acid chemotaxis.     

Chemoreceptors from the commensal gut Roseburia rectibacter bind to mucin and trigger chemotaxis

Chemotaxis is crucial for bacterial adherence and colonization of the host gastrointestinal tract. Previous studies have demonstrated that chemotaxis affects the virulence of causative pathogens and the infection in the host. However, the chemotactic abilities of non-pathogenic and commensal gut bacteria have rarely been explored. We observed that Roseburia rectibacter NSJ-69 exhibited flagella-dependent motility and chemotaxis to a variety of molecules, including mucin and propionate. A genome-wide analysis revealed that NSJ-69 has 28 putative chemoreceptors, 15 of which have periplasmic ligand-binding domains (LBDs). These LBD-coding genes were chemically synthesized and expressed heterologously in Escherichia coli. Intensive screening of ligands revealed four chemoreceptors bound to mucin and two bound to propionate. When expressed in Comamonas testosteroni or E. coli, these chemoreceptors elicited chemotaxis toward mucin and propionate. Hybrid chemoreceptors were constructed, and results showed that the chemotactic responses to mucin and propionate were dependent on the LBDs of R. rectibacter chemoreceptors. Our study identified and characterized R. rectibacter chemoreceptors. These results will facilitate further investigations on the involvement of microbial chemotaxis in host colonization.     

Differential backbone dynamics of companion helices in the extended helical coiled‐coil domain of a bacterial chemoreceptor

Cytoplasmic domains of transmembrane bacterial chemoreceptors are largely extended four‐helix coiled coils. Previous observations suggested the domain was structurally dynamic. We probed directly backbone dynamics of this domain of the transmembrane chemoreceptor Tar from Escherichia coli using site‐directed spin labeling and electron paramagnetic resonance (EPR) spectroscopy. Spin labels were positioned on solvent‐exposed helical faces because EPR spectra for such positions reflect primarily polypeptide backbone movements. We acquired spectra for spin‐labeled, intact receptor homodimers solubilized in detergent or inserted into native E. coli lipid bilayers in Nanodiscs, characterizing 16 positions distributed throughout the cytoplasmic domain and on both helices of its helical hairpins, one amino terminal to the membrane‐distal tight turn (N‐helix), and the other carboxyl terminal (C‐helix). Detergent solubilization increased backbone dynamics for much of the domain, suggesting that loss of receptor activities upon solubilization reflects wide‐spread destabilization. For receptors in either condition, we observed an unanticipated difference between the N‐ and C‐helices. For bilayer‐inserted receptors, EPR spectra from sites in the membrane‐distal protein‐interaction region and throughout the C‐helix were typical of well‐structured helices. In contrast, for approximately two‐thirds of the N‐helix, from its origin as the AS‐2 helix of the membrane‐proximal HAMP domain to the beginning of the membrane‐distal protein‐interaction region, spectra had a significantly mobile component, estimated by spectral deconvolution to average approximately 15%. Differential helical dynamics suggests a four‐helix bundle organization with a pair of core scaffold helices and two more dynamic partner helices. This newly observed feature of chemoreceptor structure could be involved in receptor function.     

Structural Basis for Polyamine Binding at the dCACHE Domain of the McpU Chemoreceptor from Pseudomonas putida

Many bacteria can move chemotactically to a variety of compounds and the recognition of chemoeffectors by the chemoreceptor ligand binding domain (LBD) defines the specificity of response. Many chemoreceptors were found to recognize different amino and organic acids, but the McpU chemoreceptor from Pseudomonas putida was identified as the first chemoreceptor that bound specifically polyamines. We report here the three-dimensional structure of McpU-LBD in complex with putrescine at a resolution of 2.4 Å, which fitted well a solution structure generated by small-angle X-ray scattering. Putrescine bound to a negatively charged pocket in the membrane distal module of McpU-LBD. Similarities exist in the binding of putrescine to McpU-LBD and taurine to the LBD of the Mlp37 chemoreceptor of Vibrio cholerae. In both structures, the primary amino group of the respective ligand is recognized by hydrogen bonds established by two aspartate and a tyrosine side chain. This feature may be used to predict the ligands of chemoreceptors with unknown function. Analytical ultracentrifugation revealed that McpU-LBD is monomeric in solution and that ligand binding does not alter this oligomeric state. This sensing mode thus differs from that of the well-characterised four-helix bundle domains where ligands bind to two sites at the LBD dimer interface. Although there appear to be different sensing modes, results are discussed in the context of data, indicating that chemoreceptors employ the same mechanism of transmembrane signaling. This work enhances our understanding of CACHE domains, which are the most abundant sensor domains in bacterial chemoreceptors and sensor kinases.     

Correlation between signal input and output in PctA and PctB amino acid chemoreceptor of Pseudomonas aeruginosa

The PctA and PctB chemoreceptors of Pseudomonas aeruginosa mediate chemotaxis toward amino acids. A general feature of signal transduction processes is that a signal input is converted into an output. We have generated chimeras combining the Tar signaling domain with either the PctA or PctB ligand binding domain (LBD). Escherichia coli harboring either PctA‐Tar or PctB‐Tar mediated chemotaxis toward amino acids. The responses of both chimeras were determined using fluorescence resonance energy transfer, and the derived EC₅₀ values are a measure of output. PctA‐Tar and PctB‐Tar responded to 19 and 11 L‐amino acids respectively. The EC₅₀ values of PctA‐Tar responses differed by more than three orders of magnitude, whereas PctB‐Tar responded preferentially to L‐Gln. The comparison of amino acid binding constants and the corresponding EC₅₀ values for both receptors revealed statistically significant correlations between inputs and outputs. PctA and PctB possess a double PDC (PhoQ‐DcuS‐CitA) LBD – a family of binding domain found in various other amino acid chemoreceptors. Similarly, various chemoreceptors share the preferential response to certain amino acids (e.g. L‐Cys, L‐Ser and L‐Thr) that we observed for PctA. Defining the specific inputs and outputs of these chemoreceptors is an important step toward better understanding of their physiological role.     

Diagnostic cross-linking of paired cysteine pairs demonstrates homologous structures for two chemoreceptor domains with low sequence identity

Hundreds of bacterial chemoreceptors from many species have periplasmic, ligand‐recognition domains of approximately the same size, but little or no sequence identity. The only structure determined is for the periplasmic domain of chemoreceptor Tar from Salmonella and Escherichia coli. Do sequence‐divergent but similarly sized chemoreceptor periplasmic domains have related structures? We addressed this issue for the periplasmic domain of chemoreceptor Trg_E from E. coli, which has a low level of sequence similarity to Tar, by combining homology modeling and diagnostic cross‐linking between pairs of introduced cysteines. A homology model of the Trg_E domain was created using the homodimeric, four‐helix bundle structure of the Tar_S domain from Salmonella. In this model, we chose four pairs of positions at which introduced cysteines would be sufficiently close to form disulfides across each of four different helical interfaces. For each pair we chose a second pair, in which one cysteine of the original pair was shifted by one position around the helix and thus would be less favorably placed for disulfide formation. We created genes coding for proteins containing four such pairs of cysteine pairs and investigated disulfide formation in vivo as well as functional consequences of the substitutions and disulfides between neighboring helices. Results of the experimental tests provided strong support for the accuracy of the model, indicating that the Trg_E periplasmic domain is very similar to the Tar_S domain. Diagnostic cross‐linking of paired pairs of introduced cysteines could be applied generally as a stringent test of homology models.     

Bacterial chemoreceptors of different length classes signal independently

Bacterial chemoreceptors sense environmental stimuli and govern cell movement by transmitting the information to the flagellar motors. The highly conserved cytoplasmic domain of chemoreceptors consists in an alpha‐helical hairpin that forms in the homodimer a coiled‐coil four‐helix bundle. Several classes of chemoreceptors that differ in the length of the coiled‐coil structure were characterized. Many bacterial species code for chemoreceptors that belong to different classes, but how these receptors are organized and function in the same cell remains an open question. E. coli cells normally code for single class chemoreceptors that form extended arrays based on trimers of dimers interconnected by the coupling protein CheW and the kinase CheA. This structure promotes effective coupling between the different receptors in the modulation of the kinase activity. In this work, we engineered functional derivatives of the Tsr chemoreceptor of E. coli that mimic receptors whose cytoplasmic domain is longer by two heptads. We found that these long Tsr receptors did not efficiently mix with the native receptors and appeared to function independently. Our results suggest that the assembly of membrane‐bound receptors of different specificities into mixed clusters is dictated by the length‐class to which the receptors belong, ensuring cooperative function only between receptors of the same class.     

A high‐throughput screen for ligand binding reveals the specificities of three amino acid chemoreceptors from Pseudomonas syringae pv. actinidiae

Chemoreceptors play a central role in chemotaxis, allowing bacteria to detect chemical gradients and bias their swimming behavior in order to navigate toward favorable environments. The genome of the kiwifruit pathogen, Pseudomonas syringae pv. actinidiae (Psa) strain NZ‐V13 encodes 43 predicted chemoreceptors, none of which has been characterized. We developed a high‐throughput fluorescence‐based thermal shift assay for identifying the signal molecules that are recognized by a given chemoreceptor ligand binding domain (LBD). Using this assay, we characterized the ligand binding profiles of three Psa homologs of the P. aeruginosa PAO1 amino acid chemoreceptors PctA, PctB and PctC. Each recombinant LBD was screened against 95 potential ligands. The three Psa homologs, named pscA, pscB and pscC (P s a c hemoreceptors A , B and C ) bound 3, 10 and 3 amino acids respectively. In each case, their binding profiles were distinct from their P. aeruginosa PAO1 homologs. Notably, Psa PscA‐LBD only bound the acidic amino acids l ‐aspartate, d ‐aspartate and l ‐glutamate, whereas P. aeruginosa PctA‐LBD binds all of the l ‐proteinogenic amino acids except for l ‐aspartate and l ‐glutamate. A combination of homology modeling, site‐directed mutagenesis and functional screening identified a single amino acid residue in the Psa PscA‐LBD (Ala146) that is critically important for determining its narrow specificity.     

Polar localization of Escherichia coli chemoreceptors requires an intact Tol–Pal complex

Subcellular biomolecular localization is critical for the metabolic and structural properties of the cell. The functional implications of the spatiotemporal distribution of protein complexes during the bacterial cell cycle have long been acknowledged; however, the molecular mechanisms for generating and maintaining their dynamic localization in bacteria are not completely understood. Here we demonstrate that the trans‐envelope Tol–Pal complex, a widely conserved component of the cell envelope of Gram‐negative bacteria, is required to maintain the polar positioning of chemoreceptor clusters in Escherichia coli. Localization of the chemoreceptors was independent of phospholipid composition of the membrane and the curvature of the cell wall. Instead, our data indicate that chemoreceptors interact with components of the Tol–Pal complex and that this interaction is required to polarly localize chemoreceptor clusters. We found that disruption of the Tol–Pal complex perturbs the polar localization of chemoreceptors, alters cell motility, and affects chemotaxis. We propose that the E. coli Tol–Pal complex restricts mobility of the chemoreceptor clusters at the cell poles and may be involved in regulatory mechanisms that co‐ordinate cell division and segregation of the chemosensory machinery.     

Characterization of methyl-accepting chemotaxis proteins (MCPs) for amino acids in plant-growth-promoting rhizobacterium Pseudomonas protegens CHA0 and enhancement of amino acid chemotaxis by MCP genes overexpression

ABSTRACT

Pseudomonas protegens CHA0, known as plant-growth-promoting rhizobacterium, showed positive chemotactic responses toward proteinaceous L-amino acids. Genomic analysis revealed that P. protegens CHA0 possesses four putative chemoreceptors for amino acids (designated CtaA, CtaB, CtaC, and CtaD, respectively). Pseudomonas aeruginosa PCT2, a mutant defective in chemotaxis to amino acids, harboring a plasmid containing each of ctaA, ctaB, ctaC, and ctaD showed chemotactic responses to 20, 4, 4, and 11 types of amino acids, respectively. To enhance chemotaxis toward amino acids, we introduced the plasmids containing ctaA, ctaB, ctaC, or ctaD into P. protegens CHA0. By overexpression of the genes, we succeeded in enhancing chemotaxis toward more than half of the tested ligands. However, unexpectedly, the P. protegens CHA0 transformants showed unchanged or decreased responses to some amino acids when compared to wild-type CHA0. We speculate that alternation of expression of a chemoreceptor may affect the abundance of other chemoreceptors.     

Not too loose, not too tight--just right. Biphasic control of the Tsr HAMP domain

HAMP domains communicate between input and output signalling modules in a wide variety of bacterial sensor proteins. In the Tsr chemoreceptor, they convert a signal initiated by binding of serine to the periplasmic domain of the protein into regulation of receptor control of the CheA kinase, and ultimately of the direction of flagellar rotation. In this issue, Zhou et al. report an extensive mutational analysis of the Tsr HAMP domain that shows that it can assume a number of different signalling states, which presumably correspond to a variety of different conformations. The two conformational extremes of a tightly packed and a loosely packed HAMP four‐helix bundle support only low levels of CheA activity. Thus, Tsr HAMP does not function as a simple on‐off, two‐state device but rather as a dynamic structure with biphasic control. The normal physiological operating range of Tsr is proposed to be at intermediate degrees of packing of the HAMP four‐helix bundle, but HAMP domains in other proteins could occupy different portions of the conformational spectrum.     

Chemotaxis of Escherichia coli to pyrimidines: a new role for the signal transducer tap

Escherichia coli exhibits chemotactic responses to sugars, amino acids, and dipeptides, and the responses are mediated by methyl-accepting chemotaxis proteins (MCPs). Using capillary assays, we demonstrated that Escherichia coli RP437 is attracted to the pyrimidines thymine and uracil and the response was constitutively expressed under all tested growth conditions. All MCP mutants lacking the MCP Tap protein showed no response to pyrimidines, suggesting that Tap, which is known to mediate dipeptide chemotaxis, is required for pyrimidine chemotaxis. In order to confirm the role of Tap in pyrimidine chemotaxis, we constructed chimeric chemoreceptors (Tapsr and Tsrap), in which the periplasmic and cytoplasmic domains of Tap and Tsr were switched. When Tapsr and Tsrap were individually expressed in an E. coli strain lacking all four native MCPs, Tapsr mediated chemotaxis toward pyrimidines and dipeptides, but Tsrap did not complement the chemotaxis defect. The addition of the C-terminal 19 amino acids from Tsr to the C terminus of Tsrap resulted in a functional chemoreceptor that mediated chemotaxis to serine but not pyrimidines or dipeptides. These results indicate that the periplasmic domain of Tap is responsible for detecting pyrimidines and the Tsr signaling domain confers on Tapsr the ability to mediate efficient chemotaxis. A mutant lacking dipeptide binding protein (DBP) was wild type for pyrimidine taxis, indicating that DBP, which is the primary chemoreceptor for dipeptides, is not responsible for detecting pyrimidines. It is not yet known whether Tap detects pyrimidines directly or via an additional chemoreceptor protein.     

Surface sensing and lateral subcellular localization of WspA,the receptor in a chemosensory‐like system leading to c‐di‐GMP production

Pseudomonas aeruginosa responds to growth on agar surfaces to produce cyclic‐di‐GMP, which stimulates biofilm formation. This is mediated by an alternative cellular function chemotaxis‐like system called Wsp. The receptor protein WspA, is bioinformatically indistinguishable from methyl‐accepting chemotaxis proteins. However, unlike standard chemoreceptors, WspA does not form stable clusters at cell poles. Rather, it forms dynamic clusters at both polar and lateral subcellular locations. To begin to study the mechanism of Wsp signal transduction in response to surfaces, we carried out a structure–function study of WspA and found that its C‐terminus is important for its lateral subcellular localization and function. When this region was replaced with that of a chemoreceptor for amino acids, WspA became polarly localized. In addition, introduction of mutations in the C‐terminal region of WspA that rendered this protein able to form more stable receptor–receptor interactions, also resulted in a WspA protein that was less capable of activating signal transduction. Receptor chimeras with a WspA C‐terminus and N‐terminal periplasmic domains from chemoreceptors that sense amino acids or malate responded to surfaces to produce c‐di‐GMP. Thus, the amino acid sequence of the WspA periplasmic region did not need to be conserved for the Wsp system to respond to surfaces.     

Identification and characterization of the chemotactic transducer in Pseudomonas aeruginosa PAO1 for positive chemotaxis to trichloroethylene

Pseudomonas aeruginosa PAO1 is repelled by trichloroethylene (TCE), and the methyl-accepting chemotaxis proteins PctA, PctB, and PctC serve as the major chemoreceptors for negative chemotaxis to TCE. In this study, we found that the pctABC triple mutant of P. aeruginosa PAO1 was attracted by TCE. Chemotaxis assays of a set of mutants containing deletions in 26 potential mcp genes revealed that mcpA (PA0180) is the chemoreceptor for positive chemotaxis to TCE. McpA also detects tetrachloroethylene and dichloroethylene isomers as attractants.     

The receptor-CheW binding interface in bacterial chemotaxis

The basic structural unit of the signaling complex in bacterial chemotaxis consists of the chemotaxis kinase CheA, the coupling protein CheW, and chemoreceptors. These complexes play an important role in regulating the kinase activity of CheA and in turn controlling the rotational bias of the flagellar motor. Although individual three-dimensional structures of CheA, CheW, and chemoreceptors have been determined, the interaction between chemoreceptor and CheW is still unclear. We used nuclear magnetic resonance to characterize the interaction modes of chemoreceptor and CheW from Thermotoga maritima. We find that chemoreceptor binding surface is located near the highly conserved tip region of the N-terminal helix of the receptor, whereas the binding interface of CheW is placed between the β-strand 8 of domain 1 and the β-strands 1 and 3 of domain 2. The receptor-CheW complex shares a similar binding interface to that found in the "trimer-of-dimers" oligomer interface seen in the crystal structure of cytoplasmic domains of chemoreceptors from Escherichia coli. Based on the association constants inferred from fast exchange chemical shifts associated with receptor-CheW titrations, we estimate that CheW binds about four times tighter to its first binding site of the receptor dimer than to its second binding site. This apparent anticooperativity in binding may reflect the close proximity of the two CheW binding surfaces near the receptor tip or further, complicating the events at this highly conserved region of the receptor. This work describes the first direct observation of the interaction between chemoreceptor and CheW.     

Two CheW coupling proteins are essential in a chemosensory pathway of Borrelia burgdorferi

In the model organism Escherichia coli, the coupling protein CheW, which bridges the chemoreceptors and histidine kinase CheA, is essential for chemotaxis. Unlike the situation in E. coli, Borrelia burgdorferi, the causative agent of Lyme disease, has three cheW homologues (cheW₁, cheW₂ and cheW₃). Here, a comprehensive approach is utilized to investigate the roles of the three cheWs in chemotaxis of B. burgdorferi. First, genetic studies indicated that both the cheW₁ and cheW₃ genes are essential for chemotaxis, as the mutants had altered swimming behaviours and were non‐chemotactic. Second, immunofluorescence and cryo‐electron tomography studies suggested that both CheW₁ and CheW₃ are involved in the assembly of chemoreceptor arrays at the cell poles. In contrast to cheW₁ and cheW₃, cheW₂ is dispensable for chemotaxis and assembly of the chemoreceptor arrays. Finally, immunoprecipitation studies demonstrated that the three CheWs interact with different CheAs: CheW₁ and CheW₃ interact with CheA₂ whereas CheW₂ binds to CheA₁. Collectively, our results indicate that CheW₁ and CheW₃ are incorporated into one chemosensory pathway that is essential for B. burgdorferi chemotaxis. Although many bacteria have more than one homologue of CheW, to our knowledge, this report provides the first experimental evidence that two CheW proteins coexist in one chemosensory pathway and that both are essential for chemotaxis.     

Identification of CtpL as a Chromosomally Encoded Chemoreceptor for 4-Chloroaniline and Catechol in Pseudomonas aeruginosa PAO1

Bacterial chemotaxis influences the ability of bacteria to survive and thrive in most environments, including polluted ones. Despite numerous reports of the phenotypic characterization of chemotactic bacteria, only a few molecular details of chemoreceptors for aromatic pollutants have been described. In this study, the molecular basis of chemotaxis toward an environmentally toxic chlorinated aromatic pollutant, 4-chloroaniline (4CA), was evaluated. Among the three Pseudomonas spp. tested, Pseudomonas aeruginosa PAO1 exhibited positive chemotaxis both to the nonmetabolizable 4CA, where 4-chloroacetanilide was formed as a dead-end transformation product, and to the metabolizable catechol. Molecular analysis of all 26 mutants with a disrupted methyl-accepting chemotaxis gene revealed that CtpL, a chromosomally encoded chemoreceptor, was responsible for the positive chemotactic response toward 4CA. Since CtpL has previously been described to be a major chemoreceptor for inorganic phosphate at low concentrations in PAO1, this report describes a fortuitous ability of CtpL to function toward aromatic pollutants. In addition, its regulation not only was dependent on the presence of the chemoattractant inducer but also was regulated by conditions of phosphate starvation. These results expand the range of known chemotactic transducers and their function in the environmental bacterium PAO1.