Binding of the chemotaxis response regulator CheY to the isolated, intact switch complex of the bacterial flagellar motor: lack of cooperativity

Purification, crystallization and crystallographic analysis of the PorX response regulator associated with the type IX secretion system

Pathogenic bacteria utilize specialized macromolecular secretion systems to transport virulence factors across membrane(s) and manipulate their infected host. To date, 11 secretion systems have been identified, including the type IX secretion system (T9SS) associated with human, avian and farmed-fish diseases. As a bacterial secretion system, the T9SS also facilitates gliding motility and the degradation of different macromolecules by the secretion of metabolic enzymes in nonpathogenic bacteria. PorX is a highly conserved protein that regulates the transcription of essential T9SS components and additionally mediates the function of T9SS via direct interaction with PorL, the rotary motor protein of the T9SS. PorX is also a member of a two-component system regulatory cascade, where it serves as the response regulator that relays a signal transduced from a conserved sensor histidine kinase, PorY, to a designated sigma factor. Here, the recombinant expression and purification of PorX homologous proteins from the pathogenic bacterium Porphyromonas gingivalis and the nonpathogenic bacterium Flavobacterium johnsoniae are reported. A bioinformatical characterization of the different domains comprising the PorX protein is also provided, and the crystallization and X-ray analysis of PorX from F. johnsoniae are reported.

The switching mechanism of the bacterial rotary motor combines tight regulation with inherent flexibility

Regulatory switches are wide spread in many biological systems. Uniquely among them, the switch of the bacterial flagellar motor is not an on/off switch but rather controls the motor’s direction of rotation in response to binding of the signaling protein CheY. Despite its extensive study, the molecular mechanism underlying this switch has remained largely unclear. Here, we resolved the functions of each of the three CheY‐binding sites at the switch in E. coli, as well as their different dependencies on phosphorylation and acetylation of CheY. Based on this, we propose that CheY motor switching activity is potentiated upon binding to the first site. Binding of potentiated CheY to the second site produces unstable switching and at the same time enables CheY binding to the third site, an event that stabilizes the switched state. Thereby, this mechanism exemplifies a unique combination of tight motor regulation with inherent switching flexibility.

Allosteric Priming of E. coli CheY by the Flagellar Motor Protein FliM

Phosphorylation of Escherichia coli CheY protein transduces chemoreceptor stimulation to a highly cooperative flagellar motor response. CheY binds to the N-terminal peptide of the FliM motor protein (FliM_N). Constitutively active D13K-Y106W CheY has been an important tool for motor physiology. The crystal structures of CheY and CheY ⋅ FliM_N with and without D13K-Y106W have shown FliM_N-bound CheY contains features of both active and inactive states. We used molecular dynamics (MD) simulations to characterize the CheY conformational landscape accessed by FliM_N and D13K-Y106W. Mutual information measures identified the central features of the long-range CheY allosteric network between D57 phosphorylation site and the FliM_N interface, namely the closure of the α4-β4 hinge and inward rotation of Y- or W106 with W58. We used hydroxy-radical foot printing with mass spectroscopy (XFMS) to track the solvent accessibility of these and other side chains. The solution XFMS oxidation rate correlated with the solvent-accessible area of the crystal structures. The protection of allosteric relay side chains reported by XFMS confirmed the intermediate conformation of the native CheY ⋅ FliM_N complex, the inactive state of free D13K-Y106W CheY, and the MD-based network architecture. We extended the MD analysis to determine temporal coupling and energetics during activation. Coupled aromatic residue rotation was a graded rather than a binary switch, with Y- or W106 side-chain burial correlated with increased FliM_N affinity. Activation entrained CheY fold stabilization to FliM_N affinity. The CheY network could be partitioned into four dynamically coordinated sectors. Residue substitutions mapped to sectors around D57 or the FliM_N interface according to phenotype. FliM_N increased sector size and interactions. These sectors fused between the substituted K13-W106 residues to organize a tightly packed core and novel surfaces that may bind additional sites to explain the cooperative motor response. The community maps provide a more complete description of CheY priming than proposed thus far.

The Architectural Dynamics of the Bacterial Flagellar Motor Switch

The rotary bacterial flagellar motor is remarkable in biochemistry for its highly synchronized operation and amplification during switching of rotation sense. The motor is part of the flagellar basal body, a complex multi-protein assembly. Sensory and energy transduction depends on a core of six proteins that are adapted in different species to adjust torque and produce diverse switches. Motor response to chemotactic and environmental stimuli is driven by interactions of the core with small signal proteins. The initial protein interactions are propagated across a multi-subunit cytoplasmic ring to switch torque. Torque reversal triggers structural transitions in the flagellar filament to change motile behavior. Subtle variations in the core components invert or block switch operation. The mechanics of the flagellar switch have been studied with multiple approaches, from protein dynamics to single molecule and cell biophysics. The architecture, driven by recent advances in electron cryo-microscopy, is available for several species. Computational methods have correlated structure with genetic and biochemical databases. The design principles underlying the basis of switch ultra-sensitivity and its dependence on motor torque remain elusive, but tantalizing clues have emerged. This review aims to consolidate recent knowledge into a unified platform that can inspire new research strategies.

Mechanisms and Dynamics of the Bacterial Flagellar Motor

Many bacteria are able to actively propel themselves through their complex environment, in search of resources and suitable niches. The source of this propulsion is the Bacterial Flagellar Motor (BFM), a molecular complex embedded in the bacterial membrane which rotates a flagellum. In this chapter we review the known physical mechanisms at work in the motor. The BFM shows a highly dynamic behavior in its power output, its structure, and in the stoichiometry of its components. Changes in speed, rotation direction, constituent protein conformations, and the number of constituent subunits are dynamically controlled in accordance to external chemical and mechanical cues. The mechano-sensitivity of the motor is likely related to the surface-sensing ability of bacteria, relevant in the initial stage of biofilm formation.

Assembly, Functions and Evolution of Archaella, Flagella and Cilia

Cells from all three domains of life on Earth utilize motile macromolecular devices that protrude from the cell surface to generate forces that allow them to swim through fluid media. Research carried out on archaea during the past decade or so has led to the recognition that, despite their common function, the motility devices of the three domains display fundamental differences in their properties and ancestry, reflecting a striking example of convergent evolution. Thus, the flagella of bacteria and the archaella of archaea employ rotary filaments that assemble from distinct subunits that do not share a common ancestor and generate torque using energy derived from distinct fuel sources, namely chemiosmotic ion gradients and FlaI motor-catalyzed ATP hydrolysis, respectively. The cilia of eukaryotes, however, assemble via kinesin-2-driven intraflagellar transport and utilize microtubules and ATP-hydrolyzing dynein motors to beat in a variety of waveforms via a sliding filament mechanism. Here, with reference to current structural and mechanistic information about these organelles, we briefly compare the evolutionary origins, assembly and tactic motility of archaella, flagella and cilia.

A Mechanism of Modulating the Direction of Flagellar Rotation in Bacteria by Fumarate and Fumarate Reductase

Fumarate, an electron acceptor in anaerobic respiration of Escherichia coli, has an additional function of assisting the flagellar motor to shift from counterclockwise to clockwise rotation, with a consequent modulation of the bacterial swimming behavior. Fumarate transmits its effect to the motor via the fumarate reductase complex (FrdABCD), shown to bind to FliG-one of the motor's switch proteins. How binding of the FrdABCD respiratory enzyme to FliG enhances clockwise rotation and how fumarate is involved in this activity have remained puzzling. Here we show that the FrdA subunit in the presence of fumarate is sufficient for binding to FliG and for clockwise enhancement. We further demonstrate by in vitro binding assays and super-resolution microscopy in vivo that the mechanism by which fumarate-occupied FrdA enhances clockwise rotation involves its preferential binding to the clockwise state of FliG (FliG_cw). Continuum electrostatics combined with docking analysis and conformational sampling endorsed the experimental conclusions and suggested that the FrdA-FliG_cw interaction is driven by the positive electrostatic potential generated by FrdA and the negatively charged areas of FliG. They further demonstrated that fumarate changes FrdA's conformation to one that can bind to FliG_cw. These findings also show that the reason for the failure of the succinate dehydrogenase flavoprotein SdhA (an almost-identical analog of FrdA shown to bind to FliG equally well) to enhance clockwise rotation is that it has no binding preference for FliG_cw. We suggest that this mechanism is physiologically important as it can modulate the magnitude of ΔG⁰ between the clockwise and counterclockwise states of the motor to tune the motor to the growth conditions of the bacteria.

Flagella-Driven Motility of Bacteria

The bacterial flagellum is a helical filamentous organelle responsible for motility. In bacterial species possessing flagella at the cell exterior, the long helical flagellar filament acts as a molecular screw to generate thrust. Meanwhile, the flagella of spirochetes reside within the periplasmic space and not only act as a cytoskeleton to determine the helicity of the cell body, but also rotate or undulate the helical cell body for propulsion. Despite structural diversity of the flagella among bacterial species, flagellated bacteria share a common rotary nanomachine, namely the flagellar motor, which is located at the base of the filament. The flagellar motor is composed of a rotor ring complex and multiple transmembrane stator units and converts the ion flux through an ion channel of each stator unit into the mechanical work required for motor rotation. Intracellular chemotactic signaling pathways regulate the direction of flagella-driven motility in response to changes in the environments, allowing bacteria to migrate towards more desirable environments for their survival. Recent experimental and theoretical studies have been deepening our understanding of the molecular mechanisms of the flagellar motor. In this review article, we describe the current understanding of the structure and dynamics of the bacterial flagellum.

One Basic Blueprint, Many Different Motors

The cytoplasmic C ring of the bacterial flagellum is known as the switch complex. It binds the response regulator phospho-CheY to control the direction of flagellar rotation. The C ring of enteric bacteria is well characterized. However, no Gram-positive switch complex had been modeled. Ward et al. (E. Ward, E. A. Kim, J. Panushka, T. Botelho, et al., J Bacteriol 201:e00626-18, 2019, https://doi.org/10.1128/JB.00626-18) propose a structure for the Bacillus subtilis switch complex based on extensive biochemical studies. The work demonstrates that a similar architecture can accommodate different proteins and a reversed signaling logic.

Organization of the Flagellar Switch Complex of Bacillus subtilis

While the protein complex responsible for controlling the direction (clockwise [CW] or counterclockwise [CCW]) of flagellar rotation has been fairly well studied in Escherichia coli and Salmonella, less is known about the switch complex in Bacillus subtilis or other Gram-positive species. Two component proteins (FliG and FliM) are shared between E. coli and B. subtilis, but in place of the protein FliN found in E. coli, the B. subtilis complex contains the larger protein FliY. Notably, in B. subtilis the signaling protein CheY-phosphate induces a switch from CW to CCW rotation, opposite to its action in E. coli Here, we have examined the architecture and function of the switch complex in B. subtilis using targeted cross-linking, bacterial two-hybrid protein interaction experiments, and characterization of mutant phenotypes. In major respects, the B. subtilis switch complex appears to be organized similarly to that in E. coli The complex is organized around a ring built from the large middle domain of FliM; this ring supports an array of FliG subunits organized in a similar way to that of E. coli, with the FliG C-terminal domain functioning in the generation of torque via conserved charged residues. Key differences from E. coli involve the middle domain of FliY, which forms an additional, more outboard array, and the C-terminal domains of FliM and FliY, which are organized into both FliY homodimers and FliM heterodimers. Together, the results suggest that the CW and CCW conformational states are similar in the Gram-negative and Gram-positive switches but that CheY-phosphate drives oppositely directed movements in the two cases.IMPORTANCE Flagellar motility plays key roles in the survival of many bacteria and in the harmful action of many pathogens. Bacterial flagella rotate; the direction of flagellar rotation is controlled by a multisubunit protein complex termed the switch complex. This complex has been extensively studied in Gram-negative model species, but little is known about the complex in Bacillus subtilis or other Gram-positive species. Notably, the switch complex in Gram-positive species responds to its effector CheY-phosphate (CheY-P) by switching to CCW rotation, whereas in E. coli or Salmonella CheY-P acts in the opposite way, promoting CW rotation. In the work here, the architecture of the B. subtilis switch complex has been probed using cross-linking, protein interaction measurements, and mutational approaches. The results cast light on the organization of the complex and provide a framework for understanding the mechanism of flagellar direction control in B. subtilis and other Gram-positive species.

[Research progress on the type Ⅸ secretion system of Porphyromonas gingivalis]

In recent years, the study found that Porphyromonas gingivalis type Ⅸ secretion system (T9SS) is a novel protein secretion system, also known as Por secretion system (PorSS). Unlike the eight protein secretion systems found in the past, the system is a polyprotein complex found only in Bacteroides. The secreted proteins have both N- and C-terminus, where the former includes Sec-dependent type Ⅰ signals peptide, and the latter contains conserved domains (C-terminal conserved domain, CTD). Porphyromonas gingivalis T9SS includes proteins such as intima, outer membrane, cytoplasm, and cell cycle, including at least 34 proteins containing CTD. Porphyromonas gingivalis T9SS is involved in regulating associated virulence factors including gingivin, fimbriae, lipopolysaccharide, HBP35, CPG70 protein and peptidyl-arginine deiminase. These CTD-containing virulence proteins are localized by T9SS and then released to the extracellular domain, thereby destroying periodontal tissue. Therefore, this review summarizes the research progress on the T9SS of Porphyromonas gingivalis.

Involvement of Two-Component Signaling on Bacterial Motility and Biofilm Development

Two-component signaling is a specialized mechanism that bacteria use to respond to changes in their environment. Nonpathogenic strains of Escherichia coli K-12 harbor 30 histidine kinases and 32 response regulators, which form a network of regulation that integrates many other global regulators that do not follow the two-component signaling mechanism, as well as signals from central metabolism. The output of this network is a multitude of phenotypic changes in response to changes in the environment. Among these phenotypic changes, many two-component systems control motility and/or the formation of biofilm, sessile communities of bacteria that form on surfaces. Motility is the first reversible attachment phase of biofilm development, followed by a so-called swim or stick switch toward surface organelles that aid in the subsequent phases. In the mature biofilm, motility heterogeneity is generated by a combination of evolutionary and gene regulatory events.

The Type IX Secretion System (T9SS): Highlights and Recent Insights into Its Structure and Function

Protein secretion systems are vital for prokaryotic life, as they enable bacteria to acquire nutrients, communicate with other species, defend against biological and chemical agents, and facilitate disease through the delivery of virulence factors. In this review, we will focus on the recently discovered type IX secretion system (T9SS), a complex translocon found only in some species of the Bacteroidetes phylum. T9SS plays two roles, depending on the lifestyle of the bacteria. It provides either a means of movement (called gliding motility) for peace-loving environmental bacteria or a weapon for pathogens. The best-studied members of these two groups are Flavobacterium johnsoniae, a commensal microorganism often found in water and soil, and Porphyromonas gingivalis, a human oral pathogen that is a major causative agent of periodontitis. In P. gingivalis and some other periodontopathogens, T9SS translocates proteins, especially virulence factors, across the outer membrane (OM). Proteins destined for secretion bear a conserved C-terminal domain (CTD) that directs the cargo to the OM translocon. At least 18 proteins are involved in this still enigmatic process, with some engaged in the post-translational modification of T9SS cargo proteins. Upon translocation across the OM, the CTD is removed by a protease with sortase-like activity and an anionic LPS is attached to the newly formed C-terminus. As a result, a cargo protein could be secreted into the extracellular milieu or covalently attached to the bacterial surface. T9SS is regulated by a two-component system; however, the precise environmental signal that triggers it has not been identified. Exploring unknown systems contributing to bacterial virulence is exciting, as it may eventually lead to new therapeutic strategies. During the past decade, the major components of T9SS were identified, as well as hints suggesting the possible mechanism of action. In addition, the list of characterized cargo proteins is constantly growing. The actual structure of the translocon, situated in the OM of bacteria, remains the least explored area; however, new technical approaches and increasing scientific attention have resulted in a growing body of data. Therefore, we present a compact up-to-date review of this topic.

CheY acetylation is required for ordinary adaptation time in Escherichia coli chemotaxis

Recent studies demonstrated the dependence of speed adaptation in Escherichia coli on acetylation of the chemotaxis signaling molecule CheY. Here, we examined whether CheY acetylation is involved in chemotactic adaptation. A mutant lacking the acetylating enzyme acetyl-CoA synthetase (Acs) requires more time to adapt to attractant stimulation, and vice versa to repellent stimulation. This effect is avoided by conditions that favor production of acetyl-CoA, thus enabling Acs-independent CheY autoacetylation, or reversed by expressing Acs from a plasmid. These findings suggest that CheY should be acetylated for ordinary adaptation time, and that the function of this acetylation in adaptation is to enable the motor to shift its rotation to clockwise. We further identify the enzyme phosphotransacetylase as a third deacetylase of CheY in E. coli.

The Type IX Secretion System (T9SS): Highlights and Recent Insights into Its Structure and Function

Protein secretion systems are vital for prokaryotic life, as they enable bacteria to acquire nutrients, communicate with other species, defend against biological and chemical agents, and facilitate disease through the delivery of virulence factors. In this review, we will focus on the recently discovered type IX secretion system (T9SS), a complex translocon found only in some species of the Bacteroidetes phylum. T9SS plays two roles, depending on the lifestyle of the bacteria. It provides either a means of movement (called gliding motility) for peace-loving environmental bacteria or a weapon for pathogens. The best-studied members of these two groups are Flavobacterium johnsoniae, a commensal microorganism often found in water and soil, and Porphyromonas gingivalis, a human oral pathogen that is a major causative agent of periodontitis. In P. gingivalis and some other periodontopathogens, T9SS translocates proteins, especially virulence factors, across the outer membrane (OM). Proteins destined for secretion bear a conserved C-terminal domain (CTD) that directs the cargo to the OM translocon. At least 18 proteins are involved in this still enigmatic process, with some engaged in the post-translational modification of T9SS cargo proteins. Upon translocation across the OM, the CTD is removed by a protease with sortase-like activity and an anionic LPS is attached to the newly formed C-terminus. As a result, a cargo protein could be secreted into the extracellular milieu or covalently attached to the bacterial surface. T9SS is regulated by a two-component system; however, the precise environmental signal that triggers it has not been identified. Exploring unknown systems contributing to bacterial virulence is exciting, as it may eventually lead to new therapeutic strategies. During the past decade, the major components of T9SS were identified, as well as hints suggesting the possible mechanism of action. In addition, the list of characterized cargo proteins is constantly growing. The actual structure of the translocon, situated in the OM of bacteria, remains the least explored area; however, new technical approaches and increasing scientific attention have resulted in a growing body of data. Therefore, we present a compact up-to-date review of this topic.

The PorX Response Regulator of the Porphyromonas gingivalis PorXY Two-Component System Does Not Directly Regulate the Type IX Secretion Genes but Binds the PorL Subunit

The Type IX secretion system (T9SS) is a versatile multi-protein complex restricted to bacteria of the Bacteriodetes phylum and responsible for the secretion or cell surface exposition of diverse proteins that participate to S-layer formation, gliding motility or pathogenesis. The T9SS is poorly characterized but a number of proteins involved in the assembly of the secretion apparatus in the oral pathogen Porphyromonas gingivalis have been identified based on genome substractive analyses. Among these proteins, PorY, and PorX encode typical two-component system (TCS) sensor and CheY-like response regulator respectively. Although the porX and porY genes do not localize at the same genetic locus, it has been proposed that PorXY form a bona fide TCS. Deletion of porX in P. gingivalis causes a slight decrease of the expression of a number of other T9SS genes, including sov, porT, porP, porK, porL, porM, porN, and porY. Here, we show that PorX and the soluble cytoplasmic domain of PorY interact. Using electrophoretic mobility shift, DNA-protein co-purification and heterologous host expression assays, we demonstrate that PorX does not bind T9SS gene promoters and does not directly regulate expression of the T9SS genes. Finally, we show that PorX interacts with the cytoplasmic domain of PorL, a component of the T9SS membrane core complex and propose that the CheY-like PorX protein might be involved in the dynamics of the T9SS.

The Gearbox of the Bacterial Flagellar Motor Switch

Switching of flagellar motor rotation sense dictates bacterial chemotaxis. Multi-subunit FliM-FliG rotor rings couple signal protein binding in FliM with reversal of a distant FliG C-terminal (FliG_C) helix involved in stator contacts. Subunit dynamics were examined in conformer ensembles generated by molecular simulations from the X-ray structures. Principal component analysis extracted collective motions. Interfacial loop immobilization by complex formation coupled elastic fluctuations of the FliM middle (FliM_M) and FliG middle (FliG_M) domains. Coevolved mutations captured interfacial dynamics as well as contacts. FliG_M rotation was amplified via two central hinges to the FliG_C helix. Intrinsic flexibility, reported by the FliG_MC ensembles, reconciled conformers with opposite FliG_C helix orientations. FliG domain stacking deformed the inter-domain linker and reduced flexibility; but conformational changes were not triggered by engineered linker deletions that cause a rotation-locked phenotype. These facts suggest that binary rotation states arise from conformational selection by stacking interactions.

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Switch complex exploits differential subunit stiffness for mechanical amplification

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Distinct rotor protein X-ray structures generate overlapping conformer ensembles

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Stacking constraints on a flexible helix linker could select diverse rotation states

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Non-contact elastic couplings at the subunit interface in the complex have coevolved

Fundamental constraints on the abundances of chemotaxis proteins

Flagellated bacteria, such as Escherichia coli, perform directed motion in gradients of concentration of attractants and repellents in a process called chemotaxis. The E. coli chemotaxis signaling pathway is a model for signal transduction, but it has unique features. We demonstrate that the need for fast signaling necessitates high abundances of the proteins involved in this pathway. We show that further constraints on the abundances of chemotaxis proteins arise from the requirements of self-assembly both of flagellar motors and of chemoreceptor arrays. All these constraints are specific to chemotaxis, and published data confirm that chemotaxis proteins tend to be more highly expressed than their homologs in other pathways. Employing a chemotaxis pathway model, we show that the gain of the pathway at the level of the response regulator CheY increases with overall chemotaxis protein abundances. This may explain why, at least in one E. coli strain, the abundance of all chemotaxis proteins is higher in media with lower nutrient content. We also demonstrate that the E. coli chemotaxis pathway is particularly robust to abundance variations of the motor protein FliM.

Flagellar Motor Switching in Caulobacter Crescentus Obeys First Passage Time Statistics

A Caulobacter crescentus swarmer cell is propelled by a helical flagellum, which is rotated by a motor at its base. The motor alternates between rotating in clockwise and counterclockwise directions and spends variable intervals of time in each state. We measure the distributions of these intervals for cells either free swimming or tethered to a glass slide. A peak time of around one second is observed in the distributions for both motor directions with counterclockwise intervals more sharply peaked and clockwise intervals displaying a larger tail at long times. We show that distributions of rotation intervals fit first passage time statistics for a biased random walker and the dynamic binding of CheY-P to FliM motor subunits accounts for this behavior. Our results also suggest that the presence of multiple CheY proteins in C. crescentus may be responsible for differences between its switching behavior and that of the extensively studied E. coli.

Assembly states of FliM and FliG within the flagellar switch complex

At the base of the bacterial flagella, a cytoplasmic rotor (the C-ring) generates torque and reverses rotation sense in response to stimuli. The bulk of the C-ring forms from many copies of the proteins FliG, FliM, and FliN, which together constitute the switch complex. To help resolve outstanding issues regarding C-ring architecture, we have investigated interactions between FliM and FliG from Thermotoga maritima with X-ray crystallography and pulsed dipolar ESR spectroscopy (PDS). A new crystal structure of an 11-unit FliG:FliM complex produces a large arc with a curvature consistent with the dimensions of the C-ring. Previously determined structures along with this new structure provided a basis to test switch complex assembly models. PDS combined with mutational studies and targeted cross-linking reveal that FliM and FliG interact through their middle domains to form both parallel and antiparallel arrangements in solution. Residue substitutions at predicted interfaces disrupt higher-order complexes that are primarily mediated by contacts between the C-terminal domain of FliG and the middle domain of a neighboring FliG molecule. Spin separations among multi-labeled components fit a self-consistent model that agree well with electron microscopy images of the C-ring. An activated form of the response regulator CheY destabilizes the parallel arrangement of FliM molecules to perturb FliG alignment in a process that may reflect the onset of rotation switching. These data suggest a model of C-ring assembly in which intermolecular contacts among FliG domains provide a template for FliM assembly and cooperative transitions.

Acetylation reduces the ability of CheY to undergo autophosphorylation

CheY, the response regulator of the chemotaxis system in Escherichia coli, can be regulated by two covalent modifications - phosphorylation and acetylation. Both covalent modifications are involved in chemotaxis, but the mechanism and role of the acetylation are still obscure. While acetylation was shown to repress the binding of CheY to its target proteins, the effect of acetylation on the ability of CheY to undergo autophosphorylate with AcP is not fully investigated. To obtain more information on the function of this acetylation, we successfully expressed and purified CheY protein with a 6 × His-tag on the C-terminus. Subsequently, acetylated CheY (AcCheY) was obtained with AcCoA as the acetyl donor, and the acetylation level of AcCheY was confirmed by Western blotting and then mass spectrometry. Using tryptophan fluorescence intensity measurements as a monitor of phosphorylation, we showed that acetylation reduces the ability of CheY to undergo autophosphorylation.

Second-chance signal transduction explains cooperative flagellar switching

The reversal of flagellar motion (switching) results from the interaction between a switch complex of the flagellar rotor and a torque-generating stationary unit, or stator (motor unit). To explain the steeply cooperative ligand-induced switching, present models propose allosteric interactions between subunits of the rotor, but do not address the possibility of a reaction that stimulates a bidirectional motor unit to reverse direction of torque. During flagellar motion, the binding of a ligand-bound switch complex at the dwell site could excite a motor unit. The probability that another switch complex of the rotor, moving according to steady-state rotation, will reach the same dwell site before that motor unit returns to ground state will be determined by the independent decay rate of the excited-state motor unit. Here, we derive an analytical expression for the energy coupling between a switch complex and a motor unit of the stator complex of a flagellum, and demonstrate that this model accounts for the cooperative switching response without the need for allosteric interactions. The analytical result can be reproduced by simulation when (1) the motion of the rotor delivers a subsequent ligand-bound switch to the excited motor unit, thereby providing the excited motor unit with a second chance to remain excited, and (2) the outputs from multiple independent motor units are constrained to a single all-or-none event. In this proposed model, a motor unit and switch complex represent the components of a mathematically defined signal transduction mechanism in which energy coupling is driven by steady-state and is regulated by stochastic ligand binding. Mathematical derivation of the model shows the analytical function to be a general form of the Hill equation (Hill AV (1910) The possible effects of the aggregation of the molecules of haemoglobin on its dissociation curves. J Physiol 40: iv-vii).

Conformational spread in the flagellar motor switch: a model study

The reliable response to weak biological signals requires that they be amplified with fidelity. In E. coli, the flagellar motors that control swimming can switch direction in response to very small changes in the concentration of the signaling protein CheY-P, but how this works is not well understood. A recently proposed allosteric model based on cooperative conformational spread in a ring of identical protomers seems promising as it is able to qualitatively reproduce switching, locked state behavior and Hill coefficient values measured for the rotary motor. In this paper we undertook a comprehensive simulation study to analyze the behavior of this model in detail and made predictions on three experimentally observable quantities: switch time distribution, locked state interval distribution, Hill coefficient of the switch response. We parameterized the model using experimental measurements, finding excellent agreement with published data on motor behavior. Analysis of the simulated switching dynamics revealed a mechanism for chemotactic ultrasensitivity, in which cooperativity is indispensable for realizing both coherent switching and effective amplification. These results showed how cells can combine elements of analog and digital control to produce switches that are simultaneously sensitive and reliable.

A molecular mechanism of direction switching in the flagellar motor of Escherichia coli

The direction of flagellar rotation is regulated by a rotor-mounted protein assembly, termed the "switch complex," formed from multiple copies of the proteins FliG, FliM, and FliN. The structures of major parts of these proteins are known, and the overall organization of proteins in the complex has been elucidated previously using a combination of protein-binding, mutational, and cross-linking approaches. In Escherichia coli, the switch from counterclockwise to clockwise rotation is triggered by the signaling protein phospho-CheY, which binds to the lower part of the switch complex and induces small movements of FliM and FliN subunits relative to each other. Direction switching also must produce movements in the upper part of the complex, particularly in the C-terminal domain of FliG (FliG(C)), which interacts with the stator to generate the torque for flagellar rotation. In the present study, protein movements in the middle and upper parts of the switch complex have been probed by means of targeted cross-linking and mutational analysis. Switching induces a tilting movement of the FliM domains that form the middle part of the switch and a consequent rotation of the affixed FliG(C) domains that reorients the stator interaction sites by about 90°. In a recently proposed hypothesis for the motor mechanism, such a reorientation of FliG(C) would reverse the direction of motor rotation.

Identification of an anchor residue for CheA-CheY interactions in the chemotaxis system of Escherichia coli

Transfer of a phosphoryl group from autophosphorylated CheA (P-CheA) to CheY is an important step in the bacterial chemotaxis signal transduction pathway. This reaction involves CheY (i) binding to the P2 domain of P-CheA and then (ii) acquiring the phosphoryl group from the P1 domain. Crystal structures indicated numerous side chain interactions at the CheY-P2 binding interface. To investigate the individual contributions of the P2 side chains involved in these contacts, we analyzed the effects of eight alanine substitution mutations on CheA-CheY binding interactions. An F214A substitution in P2 caused ∼1,000-fold reduction in CheA-CheY binding affinity, while Ala substitutions at other P2 positions had small effects (E171A, E178A, and I216A) or no detectable effects (H181A, D202A, D207A, and C213A) on binding affinity. These results are discussed in relation to previous in silico predictions of hot-spot and anchor positions at the CheA-CheY interface. We also investigated the consequences of these mutations for chemotaxis signal transduction in living cells. CheA(F214A) was defective in mediating localization of CheY-YFP to the large clusters of signaling proteins that form at the poles of Escherichia coli cells, while the other CheA variants did not differ from wild-type (wt) CheA (CheA(wt)) in this regard. In our set of mutants, only CheA(F214A) exhibited a markedly diminished ability to support chemotaxis in motility agar assays. Surprisingly, however, in FRET assays that monitored receptor-regulated production of phospho-CheY, CheA(F214A) (and each of the other Ala substitution mutants) performed just as well as CheA(wt). Overall, our findings indicate that F214 serves as an anchor residue at the CheA-CheY interface and makes an important contribution to the binding energy in vitro and in vivo; however, loss of this contribution does not have a large negative effect on the overall ability of the signaling pathway to modulate P-CheY levels in response to chemoattractants.

Allosteric conformational spread: exact results using a simple transfer matrix method

A transfer matrix method is described for the conformational spread (CS) model of allosteric cooperativity within a one-dimensional arrangement of four-state binding sites. Each such binding site can realize one of two possible conformational states. Each of these states can either bind ligand or not bind ligand. Thus, analytical expressions that are exact within the context of the CS model are derived for the grand partition function, for the mean fraction of binding sites occupied by ligand versus ligand concentration, and for the mean fraction of binding sites in a given allosteric state versus ligand concentration. The utility of our analytical results is demonstrated by least-mean-square fitting of prior experimental results obtained on the bacterial flagellar motor for the fraction of FliM/FliG/FliN complexes with CheY-P bound [V. Sourjik and H. C. Berg, Proc. Natl. Acad. Sci. U.S.A. 99, 12669 (2002)] and for the cw bias [P. Cluzel, Science 287, 1652 (2000)], which plausibly may be identified as the fraction of protomers realizing state 2. Finally, the relationships between our analytical results and the classical Monod-Wyman-Changeaux, Koshland-Nemethy-Filmer, and McGhee-Von Hippel treatments of allosteric cooperativity are elucidated, as is the connection to an earlier approximate analytical treatment of the CS model.

Developing stochastic models for spatial inference: bacterial chemotaxis

BACKGROUND

Biological systems are inherently inhomogeneous and spatial effects play a significant role in processes such as pattern formation. At the cellular level proteins are often localised either through static attachment or via a dynamic equilibrium. As well as spatial heterogeneity many cellular processes exhibit stochastic fluctuations and so to make inferences about the location of molecules there is a need for spatial stochastic models. A test case for spatial models has been bacterial chemotaxis which has been studied extensively as a model of signal transduction.

RESULTS

By creating specific models of a cellular system that incorporate the spatial distributions of molecules we have shown how the fit between simulated and experimental data can be used to make inferences about localisation, in the case of bacterial chemotaxis. This method allows the robust comparison of different spatial models through alternative model parameterisations.

CONCLUSIONS

By using detailed statistical analysis we can reliably infer the parameters for the spatial models, and also to evaluate alternative models. The statistical methods employed in this case are particularly powerful as they reduce the need for a large number of simulation replicates. The technique is also particularly useful when only limited molecular level data is available or where molecular data is not quantitative.

A post-translational, c-di-GMP-dependent mechanism regulating flagellar motility

Elevated levels of the second messenger cyclic dimeric GMP, c-di-GMP, promote transition of bacteria from single motile cells to surface-attached multicellular communities. Here we describe a post-translational mechanism by which c-di-GMP initiates this transition in enteric bacteria. High levels of c-di-GMP induce the counterclockwise bias in Escherichia coli flagellar rotation, which results in smooth swimming. Based on co-immunoprecipitation, two-hybrid and mutational analyses, the E. coli c-di-GMP receptor YcgR binds to the FliG subunit of the flagellum switch complex, and the YcgR-FliG interaction is strengthened by c-di-GMP. The central fragment of FliG binds to YcgR as well as to FliM, suggesting that YcgR-c-di-GMP biases flagellum rotation by altering FliG-FliM interactions. The c-di-GMP-induced smooth swimming promotes trapping of motile bacteria in semi-solid media and attachment of liquid-grown bacteria to solid surfaces, whereas c-di-GMP-dependent mechanisms not involving YcgR further facilitate surface attachment. The YcgR-FliG interaction is conserved in the enteric bacteria, and the N-terminal YcgR/PilZN domain of YcgR is required for this interaction. YcgR joins a growing list of proteins that regulate motility via the FliG subunit of the flagellum switch complex, which suggests that FliG is a common regulatory entryway that operates in parallel with the chemotaxis that utilizes the FliM-entryway.

Acetylation represses the binding of CheY to its target proteins

The ability of CheY, the response regulator of bacterial chemotaxis, to generate clockwise rotation is regulated by two covalent modifications - phosphorylation and acetylation. While the function and signal propagation of the former are widely understood, the mechanism and role of the latter are still obscure. To obtain information on the function of this acetylation, we non-enzymatically acetylated CheY to a level similar to that found in vivo, and examined its binding to its kinase CheA, its phosphatase CheZ and the switch protein FliM - its target at the flagellar switch complex. Acetylation repressed the binding to all three proteins. These results suggest that both phosphorylation and acetylation determine CheY's ability to bind to its target proteins, thus providing two levels of regulation, fast and slow respectively. The fast level is modulated by environmental signals (e.g. chemotactic and thermotactic stimuli). The slow one is regulated by the metabolic state of the cell and it determines, at each metabolic state, the fraction of CheY molecules that can participate in signalling.

Conformational spread as a mechanism for cooperativity in the bacterial flagellar switch

The bacterial flagellar switch that controls the direction of flagellar rotation during chemotaxis has a highly cooperative response. This has previously been understood in terms of the classic two-state, concerted model of allosteric regulation. Here, we used high-resolution optical microscopy to observe switching of single motors and uncover the stochastic multistate nature of the switch. Our observations are in detailed quantitative agreement with a recent general model of allosteric cooperativity that exhibits conformational spread--the stochastic growth and shrinkage of domains of adjacent subunits sharing a particular conformational state. We expect that conformational spread will be important in explaining cooperativity in other large signaling complexes.

A molecular mechanism of bacterial flagellar motor switching

The high-resolution structures of nearly all the proteins that comprise the bacterial flagellar motor switch complex have been solved; yet a clear picture of the switching mechanism has not emerged. Here, we used NMR to characterize the interaction modes and solution properties of a number of these proteins, including several soluble fragments of the flagellar motor proteins FliM and FliG, and the response-regulator CheY. We find that activated CheY, the switch signal, binds to a previously unidentified region of FliM, adjacent to the FliM-FliM interface. We also find that activated CheY and FliG bind with mutual exclusivity to this site on FliM, because their respective binding surfaces partially overlap. These data support a model of CheY-driven motor switching wherein the binding of activated CheY to FliM displaces the carboxy-terminal domain of FliG (FliGC) from FliM, modulating the FliGC-MotA interaction, and causing the motor to switch rotational sense as required for chemotaxis.

The nonequilibrium mechanism for ultrasensitivity in a biological switch: sensing by Maxwell's demons

The Escherichia coli flagellar motor senses the intracellular concentration of the response regulator CheY-P and responds by varying the bias between its counterclockwise (CCW) and clockwise (CW) rotational states. The response is ultrasensitive with a large Hill coefficient (approximately 10). Recently, the detailed distribution functions of the CW and the CCW dwell times have been measured for different CW biases. Based on a general result on the properties of the dwell-time statistics for all equilibrium models, we show that the observed dwell-time statistics imply that the flagellar motor switch operates out of equilibrium, with energy dissipation. We propose a dissipative allosteric model that generates dwell-time statistics consistent with the experimental results. Our model reveals a general nonequilibrium mechanism for ultrasensitivity wherein the switch operates with a small energy expenditure to create high sensitivity. In contrast to the conventional equilibrium models, this mechanism does not require one to assume that CheY-P binds to the CCW and CW states with different affinities. The estimated energy consumption by the flagellar motor switch suggests that the transmembrane proton motive force, which drives the motor's rotation, may also power its switching. The existence of net transitional fluxes between microscopic states of the switch is predicted, measurement of these fluxes can test the nonequilibrium model directly. Both the results on the general properties of the dwell-time statistics and the mechanism for ultrasensitivity should be useful for understanding a diverse class of physical and biological systems.

The bacterial flagellar switch complex is getting more complex

The mechanism of function of the bacterial flagellar switch, which determines the direction of flagellar rotation and is essential for chemotaxis, has remained an enigma for many years. Here we show that the switch complex associates with the membrane-bound respiratory protein fumarate reductase (FRD). We provide evidence that FRD binds to preparations of isolated switch complexes, forms a 1:1 complex with the switch protein FliG, and that this interaction is required for both flagellar assembly and switching the direction of flagellar rotation. We further show that fumarate, known to be a clockwise/switch factor, affects the direction of flagellar rotation through FRD. These results not only uncover a new component important for switching and flagellar assembly, but they also reveal that FRD, an enzyme known to be primarily expressed and functional under anaerobic conditions in Escherichia coli, nonetheless, has important, unexpected functions under aerobic conditions.

Flagellar motility in bacteria structure and function of flagellar motor

Bacterial flagella are filamentous organelles that drive cell locomotion. They thrust cells in liquids (swimming) or on surfaces (swarming) so that cells can move toward favorable environments. At the base of each flagellum, a reversible rotary motor, which is powered by the proton- or the sodium-motive force, is embedded in the cell envelope. The motor consists of two parts: the rotating part, or rotor, that is connected to the hook and the filament, and the nonrotating part, or stator, that conducts coupling ion and is responsible for energy conversion. Intensive genetic and biochemical studies of the flagellum have been conducted in Salmonella typhimurium and Escherichia coli, and more than 50 gene products are known to be involved in flagellar assembly and function. The energy-coupling mechanism, however, is still not known. In this chapter, we survey our current knowledge of the flagellar system, based mostly on studies from Salmonella, E. coli, and marine species Vibrio alginolyticus, supplemented with distinct aspects of other bacterial species revealed by recent studies.

complex interplay between type 1 fimbrial expression and flagellum-mediated motility of uropathogenic Escherichia coli

Type 1 fimbriae and flagella have been previously shown to contribute to the virulence of uropathogenic Escherichia coli (UPEC) within the urinary tract. In this study, the relationship between motility and type 1 fimbrial expression was tested for UPEC strain CFT073 by examining the phenotypic effect of fimbrial expression on motility and the effect that induction of motility has on type 1 fimbrial expression. While constitutive expression of type 1 fimbriae resulted in a significant decrease in motility and flagellin expression (P < 0.0001), a loss of type 1 fimbrial expression did not result in increased motility. Additionally, hypermotility and flagellar gene over- and underexpression were not observed to affect the expression of type 1 fimbriae. Hence, it appeared that the relationship between type 1 fimbrial expression and motility is unidirectional, where the overexpression of type 1 fimbriae dramatically affects motility and flagellum expression but not vice versa. Moreover, the constitutive expression of type 1 fimbriae in UPEC cystitis isolate F11 and the laboratory strain E. coli K-12 MG1655 also resulted in decreased motility, suggesting that this phenomenon is not specific to CFT073 or UPEC in general. Lastly, by analyzing the repression of motility caused by constitutive type 1 fimbrial expression, it was concluded that the synthesis and presence of type 1 fimbriae at the bacterial surface is only partially responsible for the repression of motility, as evidenced by the partial restoration of motility in the CFT073 fim L-ON DeltafimAICDFGH mutant. Altogether, these data provide further insight into the complex interplay between type 1 fimbrial expression and flagellum-mediated motility.

Characterization of Escherichia coli MG1655 grown in a low-shear modeled microgravity environment

Background

Extra-cellular shear force is an important environmental parameter that is significant both medically and in the space environment. Escherichia coli cells grown in a low-shear modeled microgravity (LSMMG) environment produced in a high aspect rotating vessel (HARV) were subjected to transcriptional and physiological analysis.

Results

Aerobic LSMMG cultures were grown in rich (LB) and minimal (MOPS + glucose) medium with a normal gravity vector HARV control. Reproducible changes in transcription were seen, but no specific LSMMG responsive genes were identified. Instead, absence of shear and a randomized gravity vector appears to cause local extra-cellular environmental changes, which elicit reproducible cellular responses. In minimal media, the majority of the significantly up- or down-regulated genes of known function were associated with the cell envelope. In rich medium, most LSMMG down-regulated genes were involved in translation. No observable changes in post-culture stress responses and antibiotic sensitivity were seen in cells immediately after exposure to LSMMG. Comparison with earlier studies of Salmonella enterica serovar Typhimurium conducted under similar growth conditions, revealed essentially no similarity in the genes that were significantly up- or down-regulated.

Conclusion

Comparison of these results to previous studies suggests that different organisms may dramatically differ in their responses to medically significant low-shear and space environments. Depending on their specific response, some organisms, such as Salmonella, may become preadapted in a manner that predisposes them to increased virulence.

Structure of FliM provides insight into assembly of the switch complex in the bacterial flagella motor

Bacteria switch the direction their flagella rotate to control movement. FliM, along with FliN and FliG, compose a complex in the motor that, upon binding phosphorylated CheY, reverses the sense of flagellar rotation. The 2.0-A resolution structure of the FliM middle domain (FliM(M)) from Thermotoga maritima reveals a pseudo-2-fold symmetric topology similar to the CheY phosphatases CheC and CheX. A variable structural element, which, in CheC, mediates binding to CheD (alpha2') and, in CheX, mediates dimerization (beta'(x)), has a truncated structure unique to FliM (alpha2'). An exposed helix of FliM(M) (alpha1) does not contain the catalytic residues of CheC and CheX but does include positions conserved in FliM sequences. Cross-linking experiments with site-directed cysteine mutants show that FliM self-associates through residues on alpha1 and alpha2'. CheY activated by BeF(3)(-) binds to FliM with approximately 40-fold higher affinity than CheY (K(d) = 0.04 microM vs. 2 microM). Mapping residue conservation, suppressor mutation sites, binding data, and deletion analysis onto the FliM(M) surface defines regions important for contacts with the stator-interacting protein FliG and for either counterclockwise or clockwise rotation. Association of 33-35 FliM subunits would generate a 44- to 45-nm-diameter disk, consistent with the known dimensions of the C-ring. The localization of counterclockwise- and clockwise-biasing mutations to distinct surfaces suggests that the binding of phosphorylated CheY cooperatively realigns FliM around the ring.

Microbial cell individuality and the underlying sources of heterogeneity

Single cells in genetically homogeneous microbial cultures exhibit marked phenotypic individuality, a biological phenomenon that is considered to bolster the fitness of populations. Major phenotypes that are characterized by heterogeneity span the breadth of microbiology, in fields ranging from pathogenicity to ecology. The cell cycle, cell ageing and epigenetic regulation are proven drivers of heterogeneity in several of the best-known phenotypic examples. However, the full contribution of factors such as stochastic gene expression is yet to be realized.

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.

The fast tumble signal in bacterial chemotaxis

We have analyzed repellent signal processing in Escherichia coli by flash photorelease of leucine from photolabile precursors. We found that 1). response amplitudes of free-swimming cell populations increased with leucine jump concentration, with an apparent Hill coefficient of 1.3 and a half-maximal dose of 14.4 microM; 2). at a 0-0.5 mM leucine concentration jump sufficient to obtain a saturation motile response, the swimming cell response time of approximately 0.05 s was several-fold more rapid than the motor response time of 0.39 +/- 0.18 s measured by following the rotation of cells tethered by a single flagellum to quartz coverslips; and 3). the motor response time of individual cells was correlated with rotation bias but not cell size. These results provide information on amplification, rate-limiting step, and flagellar bundle mechanics during repellent signal processing. The difference between the half-maximal dose for the excitation response and the corresponding value reported for adaptation provides an estimate of the increase in the rate of formation of CheYP, the phosphorylated form of the signal protein CheY. The estimated increase gives a lower limit receptor kinase coupling ratio of 6.0. The magnitude and form of the motor response time distribution argue for it being determined by the poststimulus switching probability rather than CheYP turnover, diffusion, or binding. The temporal difference between the tethered and swimming cell response times to repellents can be quantitatively accounted for and suggests that one flagellum is sufficient to cause a measurable change of direction in which a bacterium swims.

Simulated diffusion of phosphorylated CheY through the cytoplasm of Escherichia coli

We describe the use of a computational model to study the effects of cellular architecture and macromolecular crowding on signal transduction in Escherichia coli chemotaxis. A newly developed program, Smoldyn, allows the movement and interaction of a large number of individual molecules in a structured environment to be simulated (S. S. Andrews and D. Bray, Phys. Biol., in press). With Smoldyn, we constructed a three-dimensional model of an E. coli cell and examined the diffusion of CheYp from the cluster of receptors to the flagellar motors under control conditions and in response to attractant and repellent stimuli. Our simulations agree well with experimental observations of cell swimming responses and are consistent with the diffusive behavior expected in wild-type and mutant cells. The high resolution available to us in the new program allows us to calculate the loci of individual CheYp molecules in a cell and the distribution of their lifetimes under different cellular conditions. We find that the time delay between stimulus and response differs for flagellar motors located at different positions in the cell. We explore different possible locations for the phosphatase CheZ and show conditions under which a gradient of CheYp exists in the cell. The introduction of inert blocks into the cytoplasm, representing impenetrable structures such as the nucleoid and large protein complexes, produces a fall in the apparent diffusion coefficient of CheYp and enhances the differences between motors. These and other results are left as predictions for future experiments.

Making sense of it all: bacterial chemotaxis

Bacteria must be able to respond to a changing environment, and one way to respond is to move. The transduction of sensory signals alters the concentration of small phosphorylated response regulators that bind to the rotary flagellar motor and cause switching. This simple pathway has provided a paradigm for sensory systems in general. However, the increasing number of sequenced bacterial genomes shows that although the central sensory mechanism seems to be common to all bacteria, there is added complexity in a wide range of species.

Co-regulation of Acetylation and Phosphorylation of CheY, A Response Regulator in Chemotaxis of Escherichia coli

CheY, a response regulator of the chemotaxis system in Escherichia coli, can be activated by either phosphorylation or acetylation to generate clockwise rotation of the flagellar motor. Both covalent modifications are involved in chemotaxis, but the function of the latter remains obscure. To understand why two different modifications apparently activate the same function of CheY, we studied the effect that each modification exerts on the other. The phosphodonors of CheY, the histidine kinase CheA and acetyl phosphate, each strongly inhibited both the autoacetylation of the acetylating enzyme, acetyl-CoA synthetase (Acs), and the acetylation of CheY. CheZ, the enzyme that enhances CheY dephosphorylation, had the opposite effect and enhanced Acs autoacetylation and CheY acetylation. These effects of the phosphodonors and CheZ were not caused by their respective activities. Rather, they were caused by their interactions with Acs and, possibly, with CheY. In addition, the presence of Acs elevated the phosphorylation levels of both CheA and CheY, and acetate repressed this stimulation. These observations suggest that CheY phosphorylation and acetylation are linked and co-regulated. We propose that the physiological role of these mutual effects is at two levels: linking chemotaxis to the metabolic state of the cell, and serving as a tuning mechanism that compensates for cell-to-cell variations in the concentrations of CheA and CheZ.

Diversity in chemotaxis mechanisms among the bacteria and archaea

The study of chemotaxis describes the cellular processes that control the movement of organisms toward favorable environments. In bacteria and archaea, motility is controlled by a two-component system involving a histidine kinase that senses the environment and a response regulator, a very common type of signal transduction in prokaryotes. Most insights into the processes involved have come from studies of Escherichia coli over the last three decades. However, in the last 10 years, with the sequencing of many prokaryotic genomes, it has become clear that E. coli represents a streamlined example of bacterial chemotaxis. While general features of excitation remain conserved among bacteria and archaea, specific features, such as adaptational processes and hydrolysis of the intracellular signal CheY-P, are quite diverse. The Bacillus subtilis chemotaxis system is considerably more complex and appears to be similar to the one that existed when the bacteria and archaea separated during evolution, so that understanding this mechanism should provide insight into the variety of mechanisms used today by the broad sweep of chemotactic bacteria and archaea. However, processes even beyond those used in E. coli and B. subtilis have been discovered in other organisms. This review emphasizes those used by B. subtilis and these other organisms but also gives an account of the mechanism in E. coli.