Genetic analysis of the catalytic domain of the chemotaxis-associated histidine kinase CheA

Campylobacter jejuni BumSR directs a response to butyrate via sensor phosphatase activity to impact transcription and colonization

Campylobacter jejuni monitors intestinal metabolites produced by the host and microbiota to initiate intestinal colonization of avian and animal hosts for commensalism and infection of humans for diarrheal disease. We previously discovered that C. jejuni has the capacity to spatially discern different intestinal regions by sensing lactate and the short-chain fatty acids acetate and butyrate and then alter transcription of colonization factors appropriately for in vivo growth. In this study, we identified the C. jejuni butyrate-modulated regulon and discovered that the BumSR two-component signal transduction system (TCS) directs a response to butyrate by identifying mutants in a genetic screen defective for butyrate-modulated transcription. The BumSR TCS, which is important for infection of humans and optimal colonization of avian hosts, senses butyrate likely by indirect means to alter transcription of genes encoding important colonization determinants. Unlike many canonical TCSs, the predicted cytoplasmic sensor kinase BumS lacked in vitro autokinase activity, which would normally lead to phosphorylation of the cognate BumR response regulator. Instead, BumS has likely evolved mutations to naturally function as a phosphatase whose activity is influenced by exogenous butyrate to control the level of endogenous phosphorylation of BumR and its ability to alter transcription of target genes. To our knowledge, the BumSR TCS is the only bacterial signal transduction system identified so far that mediates responses to the microbiota-generated intestinal metabolite butyrate, an important factor for host intestinal health and homeostasis. Our findings suggest that butyrate sensing by this system is vital for C. jejuni colonization of multiple hosts.

Conformational Transitions that Enable Histidine Kinase Autophosphorylation and Receptor Array Integration

During bacterial chemotaxis, transmembrane chemoreceptor arrays regulate autophosphorylation of the dimeric histidine kinase CheA. The five domains of CheA (P1-P5) each play a specific role in coupling receptor stimulation to CheA activity. Biochemical and X-ray scattering studies of thermostable CheA from Thermotoga maritima determine that the His-containing substrate domain (P1) is sequestered by interactions that depend upon P1 of the adjacent subunit. Non-hydrolyzable ATP analogs (but not ATP or ADP) release P1 from the protein core (domains P3P4P5) and increase its mobility. Detachment of both P1 domains or removal of one within a dimer increases net autophosphorylation substantially at physiological temperature (55°C). However, nearly all activity is lost without the dimerization domain (P3). The linker length between P1 and P3 dictates intersubunit (trans) versus intrasubunit (cis) autophosphorylation, with the trans reaction requiring a minimum length of 47 residues. A new crystal structure of the most active dimerization-plus-kinase unit (P3P4) reveals trans directing interactions between the tether connecting P3 to P2-P1 and the adjacent ATP-binding (P4) domain. The orientation of P4 relative to P3 in the P3P4 structure supports a planar CheA conformation that is required by membrane array models, and it suggests that the ATP lid of CheA may be poised to interact with receptors and coupling proteins. Collectively, these data suggest that the P1 domains are restrained in the off-state as a result of cross-subunit interactions. Perturbations at the nucleotide-binding pocket increase P1 mobility and access of the substrate His to P4-bound ATP.

Crystallization and preliminary X-ray crystallographic analysis of Escherichia coli CheA P3 dimerization domain

The chemotaxis histidine kinase CheA assembles into a dimer in which the P3 dimerization domain forms a four-helix bundle by the parallel association of two α-helical hairpins from each subunit. Ligand occupancy of the chemoreceptor regulates signal transduction by controlling the autophosphorylation activity of CheA. Autophosphorylation of CheA occurs in trans, i.e. one subunit phosphorylates the other. The P3 domain of CheA from Escherichia coli has been overexpressed in E. coli and crystallized at 298 K using PEG as a precipitant. X-ray diffraction data to 2.80 Å resolution have been collected at 100 K using synchrotron radiation. The crystal belonged to space group P1, with unit-cell parameters a = 59.271, b = 67.674, c = 82.815 Å, α = 77.568, β = 86.073, γ = 64.436°. The asymmetric unit may contain up to ten dimeric units of P3 four-helix bundles.

FimL regulates cAMP synthesis in Pseudomonas aeruginosa

Pseudomonas aeruginosa, a ubiquitous bacteria found in diverse ecological niches, is an important cause of acute infections in immunocompromised individuals and chronic infections in patients with Cystic Fibrosis. One signaling molecule required for the coordinate regulation of virulence factors associated with acute infections is 3′, 5′-cyclic adenosine monophosphate, (cAMP), which binds to and activates a catabolite repressor homolog, Vfr. Vfr controls the transcription of many virulence factors, including those associated with Type IV pili (TFP), the Type III secretion system (T3SS), the Type II secretion system, flagellar-mediated motility, and quorum sensing systems. We previously identified FimL, a protein with histidine phosphotransfer-like domains, as a regulator of Vfr-dependent processes, including TFP-dependent motility and T3SS function. In this study, we carried out genetic and physiologic studies to further define the mechanism of action of FimL. Through a genetic screen designed to identify suppressors of FimL, we found a putative cAMP-specific phosphodiesterase (CpdA), suggesting that FimL regulates cAMP levels. Inactivation of CpdA increases cAMP levels and restores TFP-dependent motility and T3SS function to fimL mutants, consistent with in vivo phosphodiesterase activity. By constructing combinations of double and triple mutants in the two adenylate cyclase genes (cyaA and cyaB), fimL, and cpdA, we show that ΔfimL mutants resemble ΔcyaB mutants in TM defects, decreased T3SS transcription, and decreased cAMP levels. Similar to some of the virulence factors that they regulate, we demonstrate that CyaB and FimL are polarly localized. These results reveal new complexities in the regulation of diverse virulence pathways associated with acute P. aeruginosa infections.

Genetic analysis of the regulation of type IV pilus function by the Chp chemosensory system of Pseudomonas aeruginosa

The virulence of the opportunistic pathogen Pseudomonas aeruginosa involves the coordinate expression of many virulence factors, including type IV pili, which are required for colonization of host tissues and for twitching motility. Type IV pilus function is controlled in part by the Chp chemosensory system, which includes a histidine kinase, ChpA, and two CheY-like response regulators, PilG and PilH. How the Chp components interface with the type IV pilus motor proteins PilB, PilT, and PilU is unknown. We present genetic evidence confirming the role of ChpA, PilG, and PilB in the regulation of pilus extension and the role of PilH and PilT in regulating pilus retraction. Using informative double and triple mutants, we show that (i) ChpA, PilG, and PilB function upstream of PilH, PilT, and PilU; (ii) that PilH enhances PilT function; and (iii) that PilT and PilB retain some activity in the absence of signaling input from components of the Chp system. By site-directed mutagenesis, we demonstrate that the histidine kinase domain of ChpA and the phosphoacceptor sites of both PilG and PilH are required for type IV pilus function, suggesting that they form a phosphorelay system important in the regulation of pilus extension and retraction. Finally, we present evidence suggesting that pilA transcription is regulated by intracellular PilA levels. We show that PilA is a negative regulator of pilA transcription in P. aeruginosa and that the Chp system functionally regulates pilA transcription by controlling PilA import and export.

Differential affinity and catalytic activity of CheZ in E. coli chemotaxis

Push–pull networks, in which two antagonistic enzymes control the activity of a messenger protein, are ubiquitous in signal transduction pathways. A classical example is the chemotaxis system of the bacterium Escherichia coli, in which the kinase CheA and the phosphatase CheZ regulate the phosphorylation level of the messenger protein CheY. Recent experiments suggest that both the kinase and the phosphatase are localized at the receptor cluster, and Vaknin and Berg recently demonstrated that the spatial distribution of the phosphatase can markedly affect the dose–response curves. We argue, using mathematical modeling, that the canonical model of the chemotaxis network cannot explain the experimental observations of Vaknin and Berg. We present a new model, in which a small fraction of the phosphatase is localized at the receptor cluster, while the remainder freely diffuses in the cytoplasm; moreover, the phosphatase at the cluster has a higher binding affinity for the messenger protein and a higher catalytic activity than the phosphatase in the cytoplasm. This model is consistent with a large body of experimental data and can explain many of the experimental observations of Vaknin and Berg. More generally, the combination of differential affinity and catalytic activity provides a generic mechanism for amplifying signals that could be exploited in other two-component signaling systems. If this model is correct, then a number of recent modeling studies, which aim to explain the chemotactic gain in terms of the activity of the receptor cluster, should be reconsidered.

In both prokaryotes and eukaryotes, extra- and intracellular signals are often processed by biochemical networks in which two enzymes together control the activity of a messenger protein via opposite modification reactions. A well-known example is the chemotaxis network of Escherichia coli that controls the swimming behavior of the bacterium in response to chemical stimuli. Recent experiments suggest that the two counteracting enzymes in this network are colocalized at the receptor cluster, while experiments by Vaknin and Berg indicate that the spatial distribution of the enzymes by itself can markedly affect the response of the network. We argue using mathematical modeling that the most widely used model of the chemotaxis network is inconsistent with these experimental observations. We then present an alternative model in which part of one enzyme is colocalized with the other enzyme at the receptor cluster, while the remainder freely diffuses in the cytoplasm; moreover, the fraction at the cluster both binds more strongly to the messenger protein and modifies it faster. This model is consistent with a large number of experimental observations and provides a generic mechanism for amplifying signals.

Changing cellular location of CheZ predicted by molecular simulations

In the chemotaxis pathway of the bacterium Escherichia coli, signals are carried from a cluster of receptors to the flagellar motors by the diffusion of the protein CheY-phosphate (CheYp) through the cytoplasm. A second protein, CheZ, which promotes dephosphorylation of CheYp, partially colocalizes with receptors in the plasma membrane. CheZ is normally dimeric in solution but has been suggested to associate into highly active oligomers in the presence of CheYp. A model is presented here and supported by Brownian dynamics simulations, which accounts for these and other experimental data: A minority component of the receptor cluster (dimers of CheA_short) nucleates CheZ oligomerization and CheZ molecules move from the cytoplasm to a bound state at the receptor cluster depending on the current level of cellular stimulation. The corresponding simulations suggest that dynamic CheZ localization will sharpen cellular responses to chemoeffectors, increase the range of detectable ligand concentrations, and make adaptation more precise and robust. The localization and activation of CheZ constitute a negative feedback loop that provides a second tier of adaptation to the system. Subtle adjustments of this kind are likely to be found in many other signaling pathways.

In order to function effectively, a living cell must not only synthesize the correct molecules but also put them in the correct place. Understanding how this positioning occurs, and what its consequences are, is a matter of great interest and concern to contemporary biologists. The author here proposes a novel mechanism that will enhance the ability of a bacterial cell to perform chemotaxis—the ability to swim toward sources of food or away from noxious substances. In this hypothesis, a key protein in the chemotaxis pathway moves dynamically between the membrane and the cytoplasm depending on the presence of attractants or repellents. This idea is explored and tested by means of detailed molecular simulations in which all of the relevant molecules are shown in their correct location in the cell. The simulations show that the proposed shift in location of the key molecule will improve the speed, range, and robustness of the cell's response. It seems likely that similar movements of proteins will occur in many other signaling pathways.

Analysis of ATP binding to CheA containing tryptophan substitutions near the active site

Signal transduction in the chemotaxis system of Escherichia coli involves an autophosphorylating protein histidine kinase, CheA. At the active site of CheA, phenylalanine residues 455 and 459 occupy positions near the ATP-binding pocket, immediately adjacent to one of the hinge regions of a loop that undergoes an ATP-induced conformational change ("lid closure") that has been characterized previously in X-ray crystal structures [Bilwes et al. (2001) Nat. Struct. Biol. 8, 353-360]. We generated versions of CheA carrying F455W and F459W replacements and investigated whether the fluorescence properties of the introduced tryptophan side chains were affected by nucleotide binding in a manner that would provide a signal for investigating the dynamics of active site events, such as ATP binding and lid closure. Our results indicate that CheA(F455W) is useful in this regard, but CheA(F459W) is not. CheA(F455W) retained full catalytic activity and exhibited easily monitored fluorescence changes upon binding nucleotide: we observed a 25-30% decrease in CheA(F455W) fluorescence emission intensity at 330 nm upon binding ATP in the absence of Mg(2+); in the presence of Mg(2+), the emission spectrum of the CheA(F455W):ATP complex was red-shifted by 5 nm and exhibited an increased intensity (approximately 20% higher at 345 nm relative to that of uncomplexed CheA(F455W)). Different fluorescence changes were observed when two ATP analogues (ADPNP and ADPCP) were used instead of ATP and when Mn(2+) or Ca(2+) was used in place of Mg(2+). We exploited the fluorescence changes induced by Mg(2+)-ATP to explore the kinetics and mechanism of nucleotide binding by CheA(F455W). In the absence of Mg(2+), binding appears to involve a simple one-step equilibrium (k(assn) = 0.7 microM(-1) s(-1) and k(dissn) = 270 s(-1) at 4 degrees C). In the presence of Mg(2+), the binding mechanism involves at least two steps: (i) rapid, relatively weak binding followed by (ii) a rapid, reversible step (k(forward) = 300 s(-1) and k(reverse) =15 s(-1) at 4 degrees C) that enhanced the overall affinity of the complex and generated an increase in W455 fluorescence. This second step could reflect a conformational change at the CheA active site, such as lid closure. These results provide the first insight into the dynamics of nucleotide binding and substrate-induced conformational changes at the active site of a protein histidine kinase.

Signal transduction in bacterial chemotaxis

Motile bacteria respond to environmental cues to move to more favorable locations. The components of the chemotaxis signal transduction systems that mediate these responses are highly conserved among prokaryotes including both eubacterial and archael species. The best-studied system is that found in Escherichia coli. Attractant and repellant chemicals are sensed through their interactions with transmembrane chemoreceptor proteins that are localized in multimeric assemblies at one or both cell poles together with a histidine protein kinase, CheA, an SH3-like adaptor protein, CheW, and a phosphoprotein phosphatase, CheZ. These multimeric protein assemblies act to control the level of phosphorylation of a response regulator, CheY, which dictates flagellar motion. Bacterial chemotaxis is one of the most-understood signal transduction systems, and many biochemical and structural details of this system have been elucidated. This is an exciting field of study because the depth of knowledge now allows the detailed molecular mechanisms of transmembrane signaling and signal processing to be investigated.

Subunit exchange by CheA histidine kinases from the mesophile Escherichia coli and the thermophile Thermotoga maritima

Dimerization of the chemotaxis histidine kinase CheA is required for intersubunit autophosphorylation [Swanson, R. V., Bourret, R. B., and Simon, M. I. (1993) Mol. Microbiol. 8, 435-441]. Here we show that CheA dimers exchange subunits by the rate-limiting dissociation of a central four-helix bundle association domain (P3), despite the high stability of P3 versus unfolding. P3 alone determines the stability and exchange properties of the CheA dimer. For CheA proteins from the mesophile Escherichia coli and the thermophile Thermotoga maritima, subunit dissociation activates at temperatures where the respective organisms live (37 and 80 degrees C). Under destabilizing conditions, P3 dimer dissociation is cooperative with unfolding. Chemical denaturation is reversible for both EP3 and TP3. Aggregation accompanies thermal unfolding for both proteins under most conditions, but thermal unfolding is reversible and two-state for EP3 at low protein concentrations. Residue differences within interhelical loops may account for the contrasted thermodynamic properties of structurally similar EP3 and TP3 (41% sequence identity). Under stabilizing conditions, greater correlation between activation energy for dimer dissociation and P3 stability suggests more unfolding in the dissociation of EP3 than TP3. Furthermore, destabilization of extended conformations by glycerol slows relative dissociation rates more for EP3 than for TP3. Nevertheless, at physiological temperatures, neither protein likely unfolds completely during subunit exchange. EP3 and TP3 will not exchange subunits with each other. The receptor coupling protein CheW reduces the subunit dissociation rate of the T. maritima CheA dimer by interacting with the regulatory domain P5.

The role of the G2 box, a conserved motif in the histidine kinase superfamily, in modulating the function of EnvZ

Histidine kinase EnvZ, a transmembrane osmotic sensor for Escherichia coli, is a bifunctional enzyme having OmpR (its cognate response regulator) kinase and phosphorylated OmpR (OmpR-P) phosphatase activities. Its cytoplasmic domain consists of domain A responsible for dimerization of EnvZ, histidine phosphotransfer and phosphatase activities, and domain B responsible for ATP binding. Here, we have constructed a number of substitution mutations at the G2 box, one of the conserved motifs in domain B, and demonstrated that they influence the phosphatase activity of EnvZ over a wide range. The effects of ADP, a cofactor for the phosphatase activity, were found to be substantially different depending upon the mutations. The effects of these mutations were also examined in vivo using a chimeric Tar-EnvZ construct (Taz1-1), and the results agreed with the in vitro data for the phosphatase and kinase activities for all mutations. Using Taz1-1 carrying the T402A mutation, three independent intragenic suppressor mutations (T235M, S269L and E276K) were isolated, and all were found in domain A. Together, the present results demonstrate for the first time that domain A and domain B are functionally co-ordinated and topologically arranged in a specific manner. The G2 box may modulate the interaction between these two domains in response to extracellular osmolarity.

Active site mutations in CheA, the signal-transducing protein kinase of the chemotaxis system in Escherichia coli

We investigated the functional roles of putative active site residues in Escherichia coli CheA by generating nine site-directed mutants, purifying the mutant proteins, and quantifying the effects of those mutations on autokinase activity and binding affinity for ATP. We designed these mutations to alter key positions in sequence motifs conserved in the protein histidine kinase family, including the N box (H376 and N380), the G1 box (D420 and G422), the F box (F455 and F459), the G2 box (G470, G472, and G474), and the "GT block" (T499), a motif identified by comparison of CheA to members of the GHL family of ATPases. Four of the mutant CheA proteins exhibited no detectable autokinase activity (Kin(-)). Of these, three (N380D, D420N, and G422A) exhibited moderate decreases in their affinities for ATP in the presence or absence of Mg(2+). The other Kin(-) mutant (G470A/G472A/G474A) exhibited wild-type affinity for ATP in the absence of Mg(2+), but reduced affinity (relative to that of wild-type CheA) in the presence of Mg(2+). The other five mutants (Kin(+)) autophosphorylated at rates slower than that exhibited by wild-type CheA. Of these, three mutants (H376Q, D420E, and F455Y/F459Y) exhibited severely reduced k(cat) values, but preserved K(M)(ATP) and K(d)(ATP) values close to those of wild-type CheA. Two mutants (T499S and T499A) exhibited only small effects on k(cat) and K(M)(ATP). Overall, these results suggest that conserved residues in the N box, G1 box, G2 box, and F box contribute to the ATP binding site and autokinase active site in CheA, while the GT block makes little, if any, contribution. We discuss the effects of specific mutations in relation to the three-dimensional structure of CheA and to binding interactions that contribute to the stability of the complex between CheA and Mg(2+)-bound ATP in both the ground state and the transition state for the CheA autophosphorylation reaction.

Selection for in vivo regulators of bacterial virulence

We devised a noninvasive genetic selection strategy to identify positive regulators of bacterial virulence genes during actual infection of an intact animal host. This strategy combines random mutagenesis with a switch-like reporter of transcription that confers antibiotic resistance in the off state and sensitivity in the on state. Application of this technology to the human intestinal pathogen Vibrio cholerae identified several regulators of cholera toxin and a central virulence gene regulator that are operative during infection. These regulators function in chemotaxis, signaling pathways, transport across the cell envelope, biosynthesis, and adherence. We show that phenotypes that appear genetically independent in cell culture become interrelated in the host milieu.

Selection for in vivo regulators of bacterial virulence

We devised a noninvasive genetic selection strategy to identify positive regulators of bacterial virulence genes during actual infection of an intact animal host. This strategy combines random mutagenesis with a switch-like reporter of transcription that confers antibiotic resistance in the off state and sensitivity in the on state. Application of this technology to the human intestinal pathogen Vibrio cholerae identified several regulators of cholera toxin and a central virulence gene regulator that are operative during infection. These regulators function in chemotaxis, signaling pathways, transport across the cell envelope, biosynthesis, and adherence. We show that phenotypes that appear genetically independent in cell culture become interrelated in the host milieu.

Polar clustering of the chemoreceptor complex in Escherichia coli occurs in the absence of complete CheA function

Bacterial chemotaxis requires a phosphorelay system initiated by the interaction of a ligand with its chemoreceptor and culminating in a change in the directional bias of flagellar rotation. Chemoreceptor-CheA-CheW ternary complexes mediate transduction of the chemotactic signal. In vivo, these complexes cluster predominantly in large groups at the cell poles. The function of chemoreceptor clustering is currently unknown. To gain insight into the relationship between signaling and chemoreceptor clustering, we examined these properties in several Escherichia coli mutant strains that produce CheA variants altered in their ability to mediate chemotaxis, autophosphorylate, or bind ATP. We show here that polar clustering of chemoreceptor complexes does not require functional CheA protein, although maximal clustering occurred only in chemotactically competent cells. Surprisingly, in cells containing a minimum of 13 gold particles at the cell pole, a significant level of clustering was observed in the absence of CheA, demonstrating that CheA is not absolutely essential for chemoreceptor clustering. Nonchemotactic cells expressing only CheA(S), a C-terminal CheA deletion, or CheA bearing a mutation in the ATP-binding site mediated slightly less than maximal chemoreceptor clustering. Cells expressing only full-length CheA (CheA(L)) from either a chromosomal or a plasmid-encoded allele displayed a methyl-accepting chemotaxis protein localization pattern indistinguishable from that of strains carrying both CheA(L) and CheA(S), demonstrating that CheA(L) alone can mediate polar clustering.

TNP-ATP and TNP-ADP as probes of the nucleotide binding site of CheA, the histidine protein kinase in the chemotaxis signal transduction pathway of Escherichia coli

The interaction of CheA with ATP has important consequences in the chemotaxis signal transduction pathway of Escherichia coli. This interaction results in autophosphorylation of CheA, a histidine protein kinase. Autophosphorylation of CheA sets in motion a chain of biochemical events that enables the chemotaxis receptor proteins to communicate with the flagellar motors. As a result of this communication, CheA allows the receptors to control the cell swimming pattern in response to gradients of attractant and repellent chemicals. To probe CheA interactions with ATP, we investigated the interaction of CheA with the fluorescent nucleotide analogues TNP-ATP [2'(3')-O-(2,4,6-trinitrophenyl)adenosine 5'-triphosphate] and TNP-ADP. Spectroscopic studies indicated that CheA bound TNP-ATP and TNP-ADP with high affinity (micromolar Kd values) and caused a marked enhancement of the fluorescence of the TNP moiety of these modified nucleotides. Analysis of titration experiments indicated a binding stoichiometry of two molecules of TNP-ATP (TNP-ADP) per CheA dimer and suggested that the two binding sites on the CheA dimer operate independently. Binding of TNP-ATP to CheA was inhibited by ATP, and analysis of this inhibition indicated that the CheA dimer binds 2 molecules of ATP. Competition experiments also indicated that CheA binds TNP-ATP considerably more tightly than it binds unmodified ATP. Binding of TNP-ADP to CheA was inhibited by ADP in a similar manner. TNP-ATP was not a substrate for CheA and served as a potent inhibitor of CheA autophosphorylation (Ki < 1 microM). The glycine-rich regions (G1 and G2) of CheA and other histidine protein kinases have been presumed to play important roles in ATP binding and/or catalysis of CheA autophosphorylation, although few experimental tests of these functional assignments have been made. Here, we demonstrate that a CheA mutant protein with Gly-->Ala substitutions in G1 and G2 has a markedly reduced affinity for ATP and ADP, as measured by Hummel-Dreyer chromatography. This mutant protein also bound TNP-ATP and TNP-ADP very poorly and had no detectable autokinase activity. Surprisingly, a distinct single-site substitution in G2 (Gly470-->Lys) had no observable effect on the affinity of CheA for ATP and ADP, despite the fact that it rendered CheA completely inactive as an autokinase. This mutant protein also bound TNP-ATP and TNP-ADP with affinities and stoichiometries that were indistinguishable from those observed with wild-type CheA. These results provide some preliminary insight into the possible functional roles of G1 and G2, and they suggest that TNP-nucleotides are useful tools for exploring the effects of additional mutations on the active site of CheA.

Signal transduction in bacteria: molecular mechanisms of stimulus-response coupling

In bacteria, adaptive responses to changing environmental conditions are mediated by signal transduction systems that involve modular protein domains. Despite great diversity in the integration of domains into different systems, studies of individual components have revealed molecular strategies that are widely applicable. Studies of receptors have advanced our understanding of how information is transmitted across membranes, the determination of three-dimensional structures of domains of histidine protein kinase domains and response regulator proteins has begun to reveal the molecular basis of signaling via two-component phosphoryltransfer pathways, and the description of 'eukaryotic-like' protein domains involved in bacterial signaling has emphasized the universality of intracellular signaling mechanisms.