An extreme clockwise switch bias mutation in fliG of Salmonella typhimurium and its suppression by slow-motile mutations in motA and motB

Structural basis of the bacterial flagellar motor rotational switching

The bacterial flagellar motor is a huge bidirectional rotary nanomachine that drives rotation of the flagellum for bacterial motility. The cytoplasmic C ring of the flagellar motor functions as the switch complex for the rotational direction switching from counterclockwise to clockwise. However, the structural basis of the rotational switching and how the C ring is assembled have long remained elusive. Here, we present two high-resolution cryo-electron microscopy structures of the C ring-containing flagellar basal body-hook complex from Salmonella Typhimurium, which are in the default counterclockwise state and in a constitutively active CheY mutant-induced clockwise state, respectively. In both complexes, the C ring consists of four subrings, but is in two different conformations. The CheY proteins are bound into an open groove between two adjacent protomers on the surface of the middle subring of the C ring and interact with the FliG and FliM subunits. The binding of the CheY protein induces a significant upward shift of the C ring towards the MS ring and inward movements of its protomers towards the motor center, which eventually remodels the structures of the FliG subunits and reverses the orientations and surface electrostatic potential of the αtorque helices to trigger the counterclockwise-to-clockwise rotational switching. The conformational changes of the FliG subunits reveal that the stator units on the motor require a relocation process in the inner membrane during the rotational switching. This study provides unprecedented molecular insights into the rotational switching mechanism and a detailed overall structural view of the bacterial flagellar motors.

CryoEM structures reveal how the bacterial flagellum rotates and switches direction

Bacterial chemotaxis requires bidirectional flagellar rotation at different rates. Rotation is driven by a flagellar motor, which is a supercomplex containing multiple rings. Architectural uncertainty regarding the cytoplasmic C-ring, or 'switch', limits our understanding of how the motor transmits torque and direction to the flagellar rod. Here we report cryogenic electron microscopy structures for Salmonella enterica serovar typhimurium inner membrane MS-ring and C-ring in a counterclockwise pose (4.0 Å) and isolated C-ring in a clockwise pose alone (4.6 Å) and bound to a regulator (5.9 Å). Conformational differences between rotational poses include a 180° shift in FliF/FliG domains that rotates the outward-facing MotA/B binding site to inward facing. The regulator has specificity for the clockwise pose by bridging elements unique to this conformation. We used these structures to propose how the switch reverses rotation and transmits torque to the flagellum, which advances the understanding of bacterial chemotaxis and bidirectional motor rotation.

Structure, Assembly, and Function of Flagella Responsible for Bacterial Locomotion

Many motile bacteria use flagella for locomotion under a variety of environmental conditions. Because bacterial flagella are under the control of sensory signal transduction pathways, each cell is able to autonomously control its flagellum-driven locomotion and move to an environment favorable for survival. The flagellum of Salmonella enterica serovar Typhimurium is a supramolecular assembly consisting of at least three distinct functional parts: a basal body that acts as a bidirectional rotary motor together with multiple force generators, each of which serves as a transmembrane proton channel to couple the proton flow through the channel with torque generation; a filament that functions as a helical propeller that produces propulsion; and a hook that works as a universal joint that transmits the torque produced by the rotary motor to the helical propeller. At the base of the flagellum is a type III secretion system that transports flagellar structural subunits from the cytoplasm to the distal end of the growing flagellar structure, where assembly takes place. In recent years, high-resolution cryo-electron microscopy (cryoEM) image analysis has revealed the overall structure of the flagellum, and this structural information has made it possible to discuss flagellar assembly and function at the atomic level. In this article, we describe what is known about the structure, assembly, and function of Salmonella flagella.

Flagellar motor remodeling during swarming requires FliL

FliL is an essential component of the flagellar machinery in some bacteria, but a conditional one in others. The conditional role is for optimal swarming in some bacteria. During swarming, physical forces associated with movement on a surface are expected to exert a higher load on the flagellum, requiring more motor torque to move. FliL was reported to enhance motor output in several bacteria and observed to assemble as a ring around ion-conducting stators that power the motor. In this study we identify a common new function for FliL in diverse bacteria-Escherichia coli, Bacillus subtilis, and Proteus mirabilis. During swarming, all these bacteria show increased cell speed and a skewed motor bias that suppresses cell tumbling. We demonstrate that these altered motor parameters, or "motor remodeling," require FliL. Both swarming and motor remodeling can be restored in an E. coli fliL mutant by complementation with fliL genes from P. mirabilis and B. subtilis, showing conservation of a swarming-associated FliL function across phyla. In addition, we demonstrate that the strong interaction we reported earlier between FliL and the flagellar MS-ring protein FliF is confined to the RBM-3 domain of FliF that links the periplasmic rod to the cytoplasmic C-ring. This interaction may explain several phenotypes associated with the absence of FliL.

Flagellar motor remodeling during swarming requires FliL

FliL is an essential component of the flagellar machinery in some bacteria, but a conditional one in others. The conditional role is for optimal swarming in some bacteria. During swarming, physical forces associated with movement on a surface are expected to exert a higher load on the flagellum, requiring more motor torque to move. Bacterial physiology and morphology are also altered during swarming to cope with the challenges of surface navigation. FliL was reported to enhance motor output in several bacteria and observed to assemble as a ring around ion-conducting stators that power the motor. In this study we identify a common new function for FliL in diverse bacteria - Escherichia coli, Bacillus subtilis and Proteus mirabilis . During swarming, all these bacteria show increased cell speed and a skewed motor bias that suppresses cell tumbling. We demonstrate that these altered motor parameters, or 'motor remodeling', require FliL. Both swarming and motor remodeling can be restored in an E. coli fliL mutant by complementation with fliL genes from P. mirabilis and B. subtilis , showing conservation of swarming-associated FliL function across phyla. In addition, we demonstrate that the strong interaction we reported earlier between FliL and the flagellar MS-ring protein FliF is confined to the RBM-3 domain of FliF that links the periplasmic rod to the cytoplasmic C-ring. This interaction may explain several phenotypes associated with the absence of FliL.

Dynamic proton-dependent motors power type IX secretion and gliding motility in Flavobacterium

Motile bacteria usually rely on external apparatus like flagella for swimming or pili for twitching. By contrast, gliding bacteria do not rely on obvious surface appendages to move on solid surfaces. Flavobacterium johnsoniae and other bacteria in the Bacteroidetes phylum use adhesins whose movement on the cell surface supports motility. In F. johnsoniae, secretion and helicoidal motion of the main adhesin SprB are intimately linked and depend on the type IX secretion system (T9SS). Both processes necessitate the proton motive force (PMF), which is thought to fuel a molecular motor that comprises the GldL and GldM cytoplasmic membrane proteins. Here, we show that F. johnsoniae gliding motility is powered by the pH gradient component of the PMF. We further delineate the interaction network between the GldLM transmembrane helices (TMHs) and show that conserved glutamate residues in GldL TMH2 are essential for gliding motility, although having distinct roles in SprB secretion and motion. We then demonstrate that the PMF and GldL trigger conformational changes in the GldM periplasmic domain. We finally show that multiple GldLM complexes are distributed in the membrane, suggesting that a network of motors may be present to move SprB along a helical path on the cell surface. Altogether, our results provide evidence that GldL and GldM assemble dynamic membrane channels that use the proton gradient to power both T9SS-dependent secretion of SprB and its motion at the cell surface.

Bridging the N-terminal and middle domains in FliG of the flagellar rotor

Flagella are necessary for bacterial movement and contribute to various aspects of virulence. They are complex cylindrical structures built of multiple molecular rings with self-assembly properties. The flagellar rotor is composed of the MS-ring and the C-ring. The FliG protein of the C-ring is central to flagellar assembly and function due to its roles in linking the C-ring with the MS-ring and in torque transmission from stator to rotor. No high-resolution structure of an assembled C-ring has been resolved to date, and the conformation adopted by FliG within the ring is unclear due to variations in available crystallographic data. Here, we use molecular dynamics (MD) simulations to study the conformation and dynamics of FliG in different states of assembly, including both in physiologically relevant and crystallographic lattice environments. We conclude that the linker between the FliG N-terminal and middle domain likely adopts an extended helical conformation in vivo, in contrast with the contracted conformation observed in some previous X-ray studies. We further support our findings with integrative model building of full-length FliG and a FliG ring model that is compatible with cryo-electron tomography (cryo-ET) and electron microscopy (EM) densities of the C-ring. Collectively, our study contributes to a better mechanistic understanding of the flagellar rotor assembly and its function.

Mutations in the stator protein PomA affect switching of rotational direction in bacterial flagellar motor

The flagellar motor rotates bi-directionally in counter-clockwise (CCW) and clockwise (CW) directions. The motor consists of a stator and a rotor. Recent structural studies have revealed that the stator is composed of a pentameric ring of A subunits and a dimer axis of B subunits. Highly conserved charged and neighboring residues of the A subunit interacts with the rotor, generating torque through a gear-like mechanism. The rotational direction is controlled by chemotaxis signaling transmitted to the rotor, with less evidence for the stator being involved. In this study, we report novel mutations that affect the switching of the rotational direction at the putative interaction site of the stator to generate rotational force. Our results highlight an aspect of flagellar motor function that appropriate switching of the interaction states between the stator and rotor is critical for controlling the rotational direction.

Native flagellar MS ring is formed by 34 subunits with 23-fold and 11-fold subsymmetries

The bacterial flagellar MS ring is a transmembrane complex acting as the core of the flagellar motor and template for flagellar assembly. The C ring attached to the MS ring is involved in torque generation and rotation switch, and a large symmetry mismatch between these two rings has been a long puzzle, especially with respect to their role in motor function. Here, using cryoEM structural analysis of the flagellar basal body and the MS ring formed by full-length FliF from Salmonella enterica, we show that the native MS ring is formed by 34 FliF subunits with no symmetry variation. Symmetry analysis of the C ring shows a variation with a peak at 34-fold, suggesting flexibility in C ring assembly. Finally, our data also indicate that FliF subunits assume two different conformations, contributing differentially to the inner and middle parts of the M ring and thus resulting in 23- and 11-fold subsymmetries in the inner and middle M ring, respectively. The internal core of the M ring, formed by 23 subunits, forms a hole of the right size to accommodate the protein export gate.

The bacterial flagellar MS ring is a core transmembrane complex within the flagellar basal body. Here, cryoEM analysis suggests that the MS ring is formed by 34 full-length FliF subunits, with 23- and 11-fold subsymmetries in the inner and middle M ring, respectively.

Molecular structure of the intact bacterial flagellar basal body

The bacterial flagellum is a macromolecular protein complex that enables motility in many species. Bacterial flagella self-assemble a strong, multicomponent drive shaft that couples rotation in the inner membrane to the micrometre-long flagellar filament that powers bacterial swimming in viscous fluids1-3. Here, we present structures of the intact Salmonella flagellar basal body4, encompassing the inner membrane rotor, drive shaft and outer-membrane bushing, solved using cryo-electron microscopy to resolutions of 2.2-3.7 Å. The structures reveal molecular details of how 173 protein molecules of 13 different types assemble into a complex spanning two membranes and a cell wall. The helical drive shaft at one end is intricately interwoven with the rotor component with both the export gate complex and the proximal rod forming interactions with the MS-ring. At the other end, the drive shaft distal rod passes through the LP-ring bushing complex, which functions as a molecular bearing anchored in the outer membrane through interactions with the lipopolysaccharide. The in situ structure of a protein complex capping the drive shaft provides molecular insights into the assembly process of this molecular machine.

GFP Fusion to the N-Terminus of MotB Affects the Proton Channel Activity of the Bacterial Flagellar Motor in Salmonella

The bacterial flagellar motor converts the energy of proton flow through the MotA/MotB complex into mechanical works required for motor rotation. The rotational force is generated by electrostatic interactions between the stator protein MotA and the rotor protein FliG. The Arg-90 and Glu-98 from MotA interact with Asp-289 and Arg-281 of FliG, respectively. An increase in the expression level of the wild-type MotA/MotB complex inhibits motility of the gfp-motBfliG(R281V) mutant but not the fliG(R281V) mutant, suggesting that the MotA/GFP-MotB complex cannot work together with wild-type MotA/MotB in the presence of the fliG(R281V) mutation. However, it remains unknown why. Here, we investigated the effect of the GFP fusion to MotB at its N-terminus on the MotA/MotB function. Over-expression of wild-type MotA/MotB significantly reduced the growth rate of the gfp-motBfliG(R281V) mutant. The over-expression of the MotA/GFP-MotB complex caused an excessive proton leakage through its proton channel, thereby inhibiting cell growth. These results suggest that the GFP tag on the MotB N-terminus affects well-regulated proton translocation through the MotA/MotB proton channel. Therefore, we propose that the N-terminal cytoplasmic tail of MotB couples the gating of the proton channel with the MotA-FliG interaction responsible for torque generation.

Biphasic chemotaxis of Escherichia coli to the microbiota metabolite indole

Bacterial chemotaxis to prominent microbiota metabolites such as indole is important in the formation of microbial communities in the gastrointestinal (GI) tract. However, the basis of chemotaxis to indole is poorly understood. Here, we exposed Escherichia coli to a range of indole concentrations and measured the dynamic responses of individual flagellar motors to determine the chemotaxis response. Below 1 mM indole, a repellent-only response was observed. At 1 mM indole and higher, a time-dependent inversion from a repellent to an attractant response was observed. The repellent and attractant responses were mediated by the Tsr and Tar chemoreceptors, respectively. Also, the flagellar motor itself mediated a repellent response independent of the receptors. Chemotaxis assays revealed that receptor-mediated adaptation to indole caused a bipartite response-wild-type cells were attracted to regions of high indole concentration if they had previously adapted to indole but were otherwise repelled. We propose that indole spatially segregates cells based on their state of adaptation to repel invaders while recruiting beneficial resident bacteria to growing microbial communities within the GI tract.

Effect of PlzD, a YcgR homologue of c-di-GMP-binding protein, on polar flagellar motility in Vibrio alginolyticus

YcgR, a cyclic diguanylate (c-di-GMP)-binding protein expressed in Escherichia coli, brakes flagellar rotation by binding to the motor in a c-di-GMP dependent manner and has been implicated in triggering biofilm formation. Vibrio alginolyticus has a single polar flagellum and encodes YcgR homologue, PlzD. When PlzD or PlzD-GFP was highly over-produced in nutrient-poor condition, the polar flagellar motility of V. alginolyticus was reduced. This inhibitory effect is c-di-GMP independent as mutants substituting putative c-di-GMP-binding residues retain the effect. Moderate over-expression of PlzD-GFP allowed its localization at the flagellated cell pole. Truncation of the N-terminal 12 or 35 residues of PlzD abolished the inhibitory effect and polar localization, and no inhibitory effect was observed by deleting plzD or expressing an endogenous level of PlzD-GFP. Subcellular fractionation showed that PlzD, but not its N-terminally truncated variants, was precipitated when over-produced. Moreover, immunoblotting and N-terminal sequencing revealed that endogenous PlzD is synthesized from Met33. These results suggest that an N-terminal extension allows PlzD to localize at the cell pole but causes aggregation and leads to inhibition of motility. In V. alginolyticus, PlzD has a potential property to associate with the polar flagellar motor but this interaction is too weak to inhibit rotation.

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.

Directional Switching Mechanism of the Bacterial Flagellar Motor

Bacteria sense temporal changes in extracellular stimuli via sensory signal transducers and move by rotating flagella towards into a favorable environment for their survival. Each flagellum is a supramolecular motility machine consisting of a bi-directional rotary motor, a universal joint and a helical propeller. The signal transducers transmit environmental signals to the flagellar motor through a cytoplasmic chemotactic signaling pathway. The flagellar motor is composed of a rotor and multiple stator units, each of which acts as a transmembrane proton channel to conduct protons and exert force on the rotor. FliG, FliM and FliN form the C ring on the cytoplasmic face of the basal body MS ring made of the transmembrane protein FliF and act as the rotor. The C ring also serves as a switching device that enables the motor to spin in both counterclockwise (CCW) and clockwise (CW) directions. The phosphorylated form of the chemotactic signaling protein CheY binds to FliM and FliN to induce conformational changes of the C ring responsible for switching the direction of flagellar motor rotation from CCW to CW. In this mini-review, we will describe current understanding of the switching mechanism of the bacterial flagellar motor.

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.

Switching and Torque Generation in Swarming E. coli

Escherichia coli swarm on semi-solid surfaces with the aid of flagella. It has been hypothesized that swarmer cells overcome the increased viscous drag near surfaces by developing higher flagellar thrust and by promoting surface wetness with the aid of a flagellar switch. The switch enables reversals between clockwise (CW) and counterclockwise (CCW) directions of rotation of the flagellar motor. Here, we measured the behavior of flagellar motors in swarmer cells. Results indicated that although the torque was similar to that in planktonic cells, the tendency to rotate CCW was higher in swarmer cells. This suggested that swarmers likely have a smaller pool of phosphorylated CheY. Results further indicated that the upregulation of the flagellin gene was not critical for flagellar thrust or swarming. Consistent with earlier reports, moisture added to the swarm surface restored swarming in a CCW-only mutant, but not in a FliG mutant that rotated motors CW-only (FliG^CW). Fluorescence assays revealed that FliG^CW cells grown on agar surfaces carried fewer flagella than planktonic FliG^CW cells. The surface-dependent reduction in flagella correlated with a reduction in the number of putative flagellar preassemblies. These results hint toward a possibility that the conformational dynamics of switch proteins play a role in the proper assembly of flagellar complexes and flagellar export, thereby aiding bacterial swarming.

New crystal forms of the integral membrane Escherichia coli quinol:fumarate reductase suggest that ligands control domain movement

Quinol:fumarate reductase (QFR) is an integral membrane protein and a member of the respiratory Complex II superfamily. Although the structure of Escherichia coli QFR was first reported almost twenty years ago, many open questions of catalysis remain. Here we report two new crystal forms of QFR, one grown from the lipidic cubic phase and one grown from dodecyl maltoside micelles. QFR crystals grown from the lipid cubic phase processed as P1, merged to 7.5 Å resolution, and exhibited crystal packing similar to previous crystal forms. Crystals grown from dodecyl maltoside micelles processed as P2₁, merged to 3.35 Å resolution, and displayed a unique crystal packing. This latter crystal form provides the first view of the E. coli QFR active site without a dicarboxylate ligand. Instead, an unidentified anion binds at a shifted position. In one of the molecules in the asymmetric unit, this is accompanied by rotation of the capping domain of the catalytic subunit. In the other molecule, this is associated with loss of interpretable electron density for this same capping domain. Analysis of the structure suggests that the ligand adjusts the position of the capping domain.

Effect of a clockwise-locked deletion in FliG on the FliG ring structure of the bacterial flagellar motor

FliG is a rotor protein of the bacterial flagellar motor. FliG consists of FliG_N , FliG_M and FliG_C domains. Intermolecular FliG_M -FliG_C interactions promote FliG ring formation on the cytoplasmic face of the MS ring. A conformational change in Helix_MC connecting FliG_M and FliG_C is responsible for the switching between the counterclockwise (CCW) and clockwise (CW) rotational states of the FliG ring. However, it remains unknown how it occurs. Here, we carried out in vivo disulfide cross-linking experiments to see the effect of a CW-locked deletion (∆PAA) in FliG on the FliG ring structure in Salmonella enterica. Higher-order oligomers were observed in the membrane fraction of the fliG(∆PAA + G166C/G194C) strain upon oxidation with iodine in a way similar to FliG(G166C/G194C), indicating that the PAA deletion does not inhibit domain-swap polymerization of FliG. FliG(∆PAA + E174C) formed a cross-linked homodimer whereas FliG(E174C) did not, indicating that Glu174 in Helix_MC of one FliG protomer is located much closer to that of its neighboring subunit in the CW motor than in the CCW motor. We will discuss possible helical rearrangements of Helix_MC that induce a structural remodeling of the FliG ring upon flagellar motor switching.

How Biophysics May Help Us Understand the Flagellar Motor of Bacteria Which Cause Infections

Motor proteins are molecules which convert chemical energy to mechanical work and are responsible for motility across all levels: for transport within a cell, for the motion of an individual cell in its surroundings, and for movement in multicellular aggregates, such as muscles. The bacterial flagellar motor is one of the canonical examples of a molecular complex made from several motor proteins, which self-assembles on demand and provides the locomotive force for bacteria. This locomotion provides a key aspect of bacteria's prevalence. Here, we outline the biophysics behind the assembly, the energetics, the switching and the rotation of this remarkable nanoscale electric motor that is Nature's first wheel.

Genetic analysis of revertants isolated from the rod-fragile fliF mutant of Salmonella

FliF is the protein comprising the MS-ring of the bacterial flagellar basal body, which is the base for the assembly of flagellar axial structures. From a fliF mutant that easily releases the rod-hook-filament in viscous environments, more than 400 revertants that recovered their swarming ability in viscous conditions, were isolated. The second-site mutations were determined for approximately 70% of them. There were three regions where the mutations were localized: two in Region I, 112 in Region II, and 71 in Region III including the true reversion. In Region I, second-site mutations were found in FlgC and FlgF of the proximal rod, suggesting that they affect the interaction between the MS-ring and the rod. In Region II, there were 69 and 42 mutations in MotA and MotB, respectively, suggesting that the second-site mutations in MotA and MotB may decrease the rotational speed of the flagellar motor to reduce the probability of releasing the rod under this condition. One exception is a mutation in FlhC that caused a down regulation of the flagellar proteins production but it may directly affect transcription or translation of motA and motB. In Region III, there were 44, 24, and 3 mutations in FliG, FliM, and FliF, respectively. There were no second-site mutations identified in FliN although it is involved in torque generation as a component of the C-ring. Many of the mutations were involved in the motor rotation, and it is suggested that such reduced speeds result in stabilizing the filament attachment to the motor.

Switching of bacterial flagellar motors [corrected] triggered by mutant FliG

Binding of the chemotaxis response regulator CheY-P promotes switching between rotational states in flagellar motors of the bacterium Escherichia coli. Here, we induced switching in the absence of CheY-P by introducing copies of a mutant FliG locked in the clockwise (CW) conformation (FliG(CW)). The composition of the mixed FliG ring was estimated via fluorescence imaging, and the probability of CW rotation (CWbias) was determined from the rotation of tethered cells. The results were interpreted in the framework of a 1D Ising model. The data could be fit by assuming that mutant subunits are more stable in the CW conformation than in the counterclockwise conformation. We found that CWbias varies depending on the spatial arrangement of the assembled subunits in the FliG ring. This offers a possible explanation for a previous observation of hysteresis in the switch function in analogous mixed FliM motors-in motors containing identical fractions of mutant FliM(CW) in otherwise wild-type motors, the CWbias differed depending on whether mutant subunits were expressed in strains with native motors or native subunits were expressed in strains with mutant motors.

Effect of FliG three amino acids deletion in Vibrio polar-flagellar rotation and formation

Most of bacteria can swim by rotating flagella bidirectionally. The C ring, located at the bottom of the flagellum and in the cytoplasmic space, consists of FliG, FliM and FliN, and has an important function in flagellar protein secretion, torque generation and rotational switch of the motor. FliG is the most important part of the C ring that interacts directly with a stator subunit. Here, we introduced a three-amino acids in-frame deletion mutation (ΔPSA) into FliG from Vibrio alginolyticus, whose corresponding mutation in Salmonella confers a switch-locked phenotype, and examined its phenotype. We found that this FliG mutant could not produce flagellar filaments in a fliG null strain but the FliG(ΔPSA) protein could localize at the cell pole as does the wild-type protein. Unexpectedly, when this mutant was expressed in a wild-type strain, cells formed flagella efficiently but the motor could not rotate. We propose that this different phenotype in Vibrio and Salmonella might be due to distinct interactions between FliG mutant and FliM in the C ring between the bacterial species.

Conformational change in the periplamic region of the flagellar stator coupled with the assembly around the rotor

The torque of the bacterial flagellum is generated by the rotor-stator interaction coupled with the ion flow through the channel in the stator. Anchoring the stator unit to the peptidoglycan layer with proper orientation around the rotor is believed to be essential for smooth rotation of the flagellar motor. The stator unit of the sodium-driven flagellar motor of Vibrio is composed of PomA and PomB, and is thought to be fixed to the peptidoglycan layer and the T-ring by the C-terminal periplasmic region of PomB. Here, we report the crystal structure of a C-terminal fragment of PomB (PomBC) at 2.0-Å resolution, and the structure suggests a conformational change in the N-terminal region of PomBC for anchoring the stator. On the basis of the structure, we designed double-Cys replaced mutants of PomB for in vivo disulfide cross-linking experiments and examined their motility. The motility can be controlled reproducibly by reducing reagent. The results of these experiments suggest that the N-terminal disordered region (121-153) and following the N-terminal two-thirds of α1(154-164) in PomBC changes its conformation to form a functional stator around the rotor. The cross-linking did not affect the localization of the stator nor the ion conductivity, suggesting that the conformational change occurs in the final step of the stator assembly around the rotor.

Mechanisms and regulation of surface interactions and biofilm formation in Agrobacterium

For many pathogenic bacteria surface attachment is a required first step during host interactions. Attachment can proceed to invasion of host tissue or cells or to establishment of a multicellular bacterial community known as a biofilm. The transition from a unicellular, often motile, state to a sessile, multicellular, biofilm-associated state is one of the most important developmental decisions for bacteria. Agrobacterium tumefaciens genetically transforms plant cells by transfer and integration of a segment of plasmid-encoded transferred DNA (T-DNA) into the host genome, and has also been a valuable tool for plant geneticists. A. tumefaciens attaches to and forms a complex biofilm on a variety of biotic and abiotic substrates in vitro. Although rarely studied in situ, it is hypothesized that the biofilm state plays an important functional role in the ecology of this organism. Surface attachment, motility, and cell division are coordinated through a complex regulatory network that imparts an unexpected asymmetry to the A. tumefaciens life cycle. In this review, we describe the mechanisms by which A. tumefaciens associates with surfaces, and regulation of this process. We focus on the transition between flagellar-based motility and surface attachment, and on the composition, production, and secretion of multiple extracellular components that contribute to the biofilm matrix. Biofilm formation by A. tumefaciens is linked with virulence both mechanistically and through shared regulatory molecules. We detail our current understanding of these and other regulatory schemes, as well as the internal and external (environmental) cues mediating development of the biofilm state, including the second messenger cyclic-di-GMP, nutrient levels, and the role of the plant host in influencing attachment and biofilm formation. A. tumefaciens is an important model system contributing to our understanding of developmental transitions, bacterial cell biology, and biofilm formation.

Structure and function of the bi-directional bacterial flagellar motor

The bacterial flagellum is a locomotive organelle that propels the bacterial cell body in liquid environments. The flagellum is a supramolecular complex composed of about 30 different proteins and consists of at least three parts: a rotary motor, a universal joint, and a helical filament. The flagellar motor of Escherichia coli and Salmonella enterica is powered by an inward-directed electrochemical potential difference of protons across the cytoplasmic membrane. The flagellar motor consists of a rotor made of FliF, FliG, FliM and FliN and a dozen stators consisting of MotA and MotB. FliG, FliM and FliN also act as a molecular switch, enabling the motor to spin in both counterclockwise and clockwise directions. Each stator is anchored to the peptidoglycan layer through the C-terminal periplasmic domain of MotB and acts as a proton channel to couple the proton flow through the channel with torque generation. Highly conserved charged residues at the rotor–stator interface are required not only for torque generation but also for stator assembly around the rotor. In this review, we will summarize our current understanding of the structure and function of the proton-driven bacterial flagellar motor.

ATP synthase: the right size base model for nanomotors in nanomedicine

Nanomedicine results from nanotechnology where molecular scale minute precise nanomotors can be used to treat disease conditions. Many such biological nanomotors are found and operate in living systems which could be used for therapeutic purposes. The question is how to build nanomachines that are compatible with living systems and can safely operate inside the body? Here we propose that it is of paramount importance to have a workable base model for the development of nanomotors in nanomedicine usage. The base model must placate not only the basic requirements of size, number, and speed but also must have the provisions of molecular modulations. Universal occurrence and catalytic site molecular modulation capabilities are of vital importance for being a perfect base model. In this review we will provide a detailed discussion on ATP synthase as one of the most suitable base models in the development of nanomotors. We will also describe how the capabilities of molecular modulation can improve catalytic and motor function of the enzyme to generate a catalytically improved and controllable ATP synthase which in turn will help in building a superior nanomotor. For comparison, several other biological nanomotors will be described as well as their applications for nanotechnology.

Identification and characterization of a novel flagellum-dependent Salmonella-infecting bacteriophage, iEPS5

A novel flagellatropic phage of Salmonella enterica serovar Typhimurium, called iEPS5, was isolated and characterized. iEPS5 has an icosahedral head and a long noncontractile tail with a tail fiber. Genome sequencing revealed a double-stranded DNA of 59,254 bp having 73 open reading frames (ORFs). To identify the receptor for iEPS5, Tn5 transposon insertion mutants of S. Typhimurium SL1344 that were resistant to the phage were isolated. All of the phage-resistant mutants were found to have mutations in genes involved in flagellar formation, suggesting that the flagellum is the adsorption target of this phage. Analysis of phage infection using the ΔmotA mutant, which is flagellated but nonmotile, demonstrated the requirement of flagellar rotation for iEPS5 infection. Further analysis of phage infection using the ΔcheY mutant revealed that iEPS5 could infect host bacteria only when the flagellum is rotating counterclockwise (CCW). These results suggested that the CCW-rotating flagellar filament is essential for phage adsorption and required for successful infection by iEPS5. In contrast to the well-studied flagellatropic phage Chi, iEPS5 cannot infect the ΔfliK mutant that makes a polyhook without a flagellar filament, suggesting that these two flagellatropic phages utilize different infection mechanisms. Here, we present evidence that iEPS5 injects its DNA into the flagellar filament for infection by assessing DNA transfer from SYBR gold-labeled iEPS5 to the host bacteria.

High hydrostatic pressure induces counterclockwise to clockwise reversals of the Escherichia coli flagellar motor

The bacterial flagellar motor is a reversible rotary machine that rotates a left-handed helical filament, allowing bacteria to swim toward a more favorable environment. The direction of rotation reverses from counterclockwise (CCW) to clockwise (CW), and vice versa, in response to input from the chemotaxis signaling circuit. CW rotation is normally caused by binding of the phosphorylated response regulator CheY (CheY-P), and strains lacking CheY are typically locked in CCW rotation. The detailed mechanism of switching remains unresolved because it is technically difficult to regulate the level of CheY-P within the concentration range that produces flagellar reversals. Here, we demonstrate that high hydrostatic pressure can induce CW rotation even in the absence of CheY-P. The rotation of single flagellar motors in Escherichia coli cells with the cheY gene deleted was monitored at various pressures and temperatures. Application of >120 MPa pressure induced a reversal from CCW to CW at 20°C, although at that temperature, no motor rotated CW at ambient pressure (0.1 MPa). At lower temperatures, pressure-induced changes in direction were observed at pressures of <120 MPa. CW rotation increased with pressure in a sigmoidal fashion, as it does in response to increasing concentrations of CheY-P. Application of pressure generally promotes the formation of clusters of ordered water molecules on the surfaces of proteins. It is possible that hydration of the switch complex at high pressure induces structural changes similar to those caused by the binding of CheY-P.

Mechanism for adaptive remodeling of the bacterial flagellar switch

The bacterial flagellar motor has been shown in previous work to adapt to changes in the steady-state concentration of the chemotaxis signaling molecule, CheY-P, by changing the FliM content. We show here that the number of FliM molecules in the motor and the fraction of FliM molecules that exchange depend on the direction of flagellar rotation, not on CheY-P binding per se. Our results are consistent with a model in which the structural differences associated with the direction of rotation modulate the strength of FliM binding. When the motor spins counterclockwise, FliM binding strengthens, the fraction of FliM molecules that exchanges decreases, and the ring content increases. The larger number of CheY-P binding sites enhances the motor's sensitivity, i.e., the motor adapts. An interesting unresolved question is how additional copies of FliM might be accommodated.

The hydrophobic core of FliG domain II is the stabilizer in the Salmonella flagellar motor

The flagellar protein FliG is the major component of the flagellar torque generator, and consists of two separate domains, I and II. Domain I is essential for flagellar assembly, while domain II in the C-terminal region is not essential for flagellar assembly but is dedicated to torque generation. Previously, we found that some fliG mutants were temperature-hypersensitive (hyper-TS) and identified three residues (F236V, D244Y and K273E) on domain II responsible for the temperature-sensitive (TS) phenotype. In this study, we substituted the three residues with all 20 amino acids (X) and analysed the behaviour of the variants at various temperatures. Each group of F236X, D244X and K273X variants gave rise to several hyper-TS mutants. In F236X, only substitution with F and W gave rise to wild-type, while other hydrophobic residues resulted in hyper-TS mutants and hydrophilic residues resulted in non-motile variants. The atomic arrangement around the F236 residue indicated that F236 together with neighbouring residues forms a hydrophobic core in the centre of domain II, which is well conserved among many species. These data suggest that the hydrophobic core may play an essential role in stabilizing the whole structure of domain II, so that changes of physiological conditions in the microenvironment of domain II do not perturb torque generation.

Energy complexes are apparently associated with the switch-motor complex of bacterial flagella

Recently, the switch-motor complex of bacterial flagella was found to be associated with a number of non-flagellar proteins, which, in spite of not being known as belonging to the chemotaxis system, affect the function of the flagella. The observation that one of these proteins, fumarate reductase, is essentially involved in electron transport under anaerobic conditions raised the question of whether other energy-linked enzymes are associated with the switch-motor complex as well. Here, we identified two additional such enzymes in Escherichia coli. Employing fluorescence resonance energy transfer in vivo and pull-down assays invitro, we provided evidence for the interaction of F(0)F(1) ATP synthase via its β subunit with the flagellar switch protein FliG and for the interaction of NADH-ubiquinone oxidoreductase with FliG, FliM, and possibly FliN. Furthermore, we measured higher rates of ATP synthesis, ATP hydrolysis, and electron transport from NADH to oxygen in membrane areas adjacent to the flagellar motor than in other membrane areas. All these observations suggest the association of energy complexes with the flagellar switch-motor complex. Finding that deletion of the β subunit in vivo affected the direction of flagellar rotation and switching frequency further implied that the interaction of F(0)F(1) ATP synthase with FliG is important for the function of the switch of bacterial flagella.

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.

Characterization of the periplasmic region of PomB, a Na+-driven flagellar stator protein in Vibrio alginolyticus

The stator proteins PomA and PomB form a complex that couples Na(+) influx to torque generation in the polar flagellar motor of Vibrio alginolyticus. This stator complex is anchored to an appropriate place around the rotor through a putative peptidoglycan-binding (PGB) domain in the periplasmic region of PomB (PomB(C)). To investigate the function of PomB(C), a series of N-terminally-truncated and in-frame mutants with deletions between the transmembrane (TM) segment and the PGB domain of PomB was constructed. A PomB(C) fragment consisting of residues 135 to 315 (PomB(C₅) formed a stable homodimer and significantly inhibited the motility of wild-type cells when overexpressed in the periplasm. A fragment with an in-frame deletion (PomB(ΔL)) of up to 80 residues retained function, and its overexpression with PomA impaired cell growth. This inhibitory effect was suppressed by a mutation at the functionally critical Asp (D24N) in the TM segment of PomB, suggesting that a high level of Na(+) influx through the mutant stator causes the growth impairment. The overproduction of functional PomA/PomB(ΔL) stators also reduced the motile fractions of the cells. That effect could be slightly relieved by a mutation (L168P) in the putative N-terminal α-helix that connects to the PGB domain without affecting the growth inhibition, suggesting that a conformational change of the region including the PGB domain affects stator assembly. Our results reveal common features of the periplasmic region of PomB/MotB and demonstrate that a flexible linker that contains a "plug" segment is important for the control of Na(+) influx through the stator complex as well as for stator assembly.

Structural insight into the rotational switching mechanism of the bacterial flagellar motor

Structural analysis of a clockwise-biased rotation mutant of the bacterial flagellar rotor protein FliG provides a new model for the arrangement of FliG subunits in the motor, and novel insights into rotation switching.

The bacterial flagellar motor can rotate either clockwise (CW) or counterclockwise (CCW). Three flagellar proteins, FliG, FliM, and FliN, are required for rapid switching between the CW and CCW directions. Switching is achieved by a conformational change in FliG induced by the binding of a chemotaxis signaling protein, phospho-CheY, to FliM and FliN. FliG consists of three domains, FliG_N, FliG_M, and FliG_C, and forms a ring on the cytoplasmic face of the MS ring of the flagellar basal body. Crystal structures have been reported for the FliG_MC domains of Thermotoga maritima, which consist of the FliG_M and FliG_C domains and a helix E that connects these two domains, and full-length FliG of Aquifex aeolicus. However, the basis for the switching mechanism is based only on previously obtained genetic data and is hence rather indirect. We characterized a CW-biased mutant (fliG(ΔPAA)) of Salmonella enterica by direct observation of rotation of a single motor at high temporal and spatial resolution. We also determined the crystal structure of the FliG_MC domains of an equivalent deletion mutant variant of T. maritima (fliG(ΔPEV)). The FliG(ΔPAA) motor produced torque at wild-type levels under a wide range of external load conditions. The wild-type motors rotated exclusively in the CCW direction under our experimental conditions, whereas the mutant motors rotated only in the CW direction. This result suggests that wild-type FliG is more stable in the CCW state than in the CW state, whereas FliG(ΔPAA) is more stable in the CW state than in the CCW state. The structure of the TM-FliG_MC(ΔPEV) revealed that extremely CW-biased rotation was caused by a conformational change in helix E. Although the arrangement of FliG_C relative to FliG_M in a single molecule was different among the three crystals, a conserved FliG_M-FliG_C unit was observed in all three of them. We suggest that the conserved FliG_M-FliG_C unit is the basic functional element in the rotor ring and that the PAA deletion induces a conformational change in a hinge-loop between FliG_M and helix E to achieve the CW state of the FliG ring. We also propose a novel model for the arrangement of FliG subunits within the motor. The model is in agreement with the previous mutational and cross-linking experiments and explains the cooperative switching mechanism of the flagellar motor.

The bacterial flagellum is a rotating organelle that governs cell motility. At the base of each flagellum is a motor powered by the electrochemical potential difference of specific ions across the cytoplasmic membrane. In response to environmental stimuli, rotation of the motor switches between counterclockwise and clockwise, with a corresponding effect on the swimming direction of the cell. Switching is triggered by the binding of the signaling protein phospho-CheY to FliM and FliN, and achieved by conformational changes in the rotor protein FliG. The actual switching mechanism, however, remains unclear. In this study, we characterized a fliG mutant of Salmonella that shows an extreme clockwise-biased rotation, and determined the structure of a fragment of FliG (FliG_MC) of the equivalent mutant variant of Thermotoga maritima. FliG_MC is composed of two domains and covers the regions essential for torque generation and FliM binding. We showed that the mutant structure has a conformational change in the helix connecting the two domains, leading to a domain orientation distinct from that of the wild-type FliG. On the basis of this structure, we propose a new model for the arrangement of FliG subunits in the rotor that is consistent with the previous mutational studies and explains how cooperative switching occurs in the motor.

The flagellar protein FliL is essential for swimming in Rhodobacter sphaeroides

In this work we characterize the function of the flagellar protein FliL in Rhodobacter sphaeroides. Our results show that FliL is essential for motility in this bacterium and that in its absence flagellar rotation is highly impaired. A green fluorescent protein (GFP)-FliL fusion forms polar and lateral fluorescent foci that show different spatial dynamics. The presence of these foci is dependent on the expression of the flagellar genes controlled by the master regulator FleQ, suggesting that additional components of the flagellar regulon are required for the proper localization of GFP-FliL. Eight independent pseudorevertants were isolated from the fliL mutant strain. In each of these strains a single nucleotide change in motB was identified. The eight mutations affected only three residues located on the periplasmic side of MotB. Swimming of the suppressor mutants was not affected by the presence of the wild-type fliL allele. Pulldown and yeast two-hybrid assays showed that that the periplasmic domain of FliL is able to interact with itself but not with the periplasmic domain of MotB. From these results we propose that FliL could participate in the coupling of MotB with the flagellar rotor in an indirect fashion.

Structure of the torque ring of the flagellar motor and the molecular basis for rotational switching

The flagellar motor drives the rotation of flagellar filaments at hundreds of revolutions per second, efficiently propelling bacteria through viscous media. The motor uses the potential energy from an electrochemical gradient of cations across the cytoplasmic membrane to generate torque. A rapid switch from anticlockwise to clockwise rotation determines whether a bacterium runs smoothly forward or tumbles to change its trajectory. A protein called FliG forms a ring in the rotor of the flagellar motor that is involved in the generation of torque through an interaction with the cation-channel-forming stator subunit MotA. FliG has been suggested to adopt distinct conformations that induce switching but these structural changes and the molecular mechanism of switching are unknown. Here we report the molecular structure of the full-length FliG protein, identify conformational changes that are involved in rotational switching and uncover the structural basis for the formation of the FliG torque ring. This allows us to propose a model of the complete ring and switching mechanism in which conformational changes in FliG reverse the electrostatic charges involved in torque generation.

Role of a conserved prolyl residue (Pro173) of MotA in the mechanochemical reaction cycle of the proton-driven flagellar motor of Salmonella

The MotA/B complex acts as the stator of the proton-driven bacterial flagellar motor. Proton translocation through the stator complex is efficiently coupled with torque generation by the stator-rotor interactions. In Salmonella enterica serovar Typhimurium, the highly conserved Pro173 residue of MotA is close to the absolutely conserved Asp33 residue of MotB, which is believed to be a proton-binding site. Pro173 is postulated to be involved in coupling proton influx to torque generation. However, it remains unknown what critical function Pro173 carries out. Here, we characterize the motility and the torque-speed relation of the flagellar motor of the slow motile motA(P173A) mutant of Salmonella. Stall torque produced by the mutant motor was at the wild-type level, indicating that neither the number of stators in the motor nor the rotor-stator interaction is affected by the P173A substitution. In agreement with this, the motA(P173A) allele exerted a strong dominant-negative effect on wild-type motility. In contrast, high-speed rotation at low load was significantly impaired by the mutation, suggesting that the maximum rate of torque generation cycle is severely limited. Simulation of the torque-speed curve by a simple kinetic model indicated that the mutation reduces the rate of conformational changes of the MotA/B complex that switches the exposure of Asp33 to the outside and the inside of the cell, thereby slowing down the mechanochemical reaction cycle. Based on these results, we propose that Pro173 plays an important role in facilitating the conformational dynamics of the stator complex for rapid proton translocation and torque generation cycle.

Stator assembly and activation mechanism of the flagellar motor by the periplasmic region of MotB

Torque generation in the Salmonella flagellar motor is coupled to translocation of H(+) ions through the proton-conducting channel of the Mot protein stator complex. The Mot complex is believed to be anchored to the peptidoglycan (PG) layer by the putative peptidoglycan-binding (PGB) domain of MotB. Proton translocation is activated only when the stator is installed into the motor. We report the crystal structure of a C-terminal periplasmic fragment of MotB (MotB(C)) that contains the PGB domain and includes the entire periplasmic region essential for motility. Structural and functional analyses indicate that the PGB domains must dimerize in order to form the proton-conducting channel. Drastic conformational changes in the N-terminal portion of MotB(C) are required both for PG binding and the proton channel activation.

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.

Effect of intracellular pH on the torque-speed relationship of bacterial proton-driven flagellar motor

Bacterial flagella responsible for motility are driven by rotary motors powered by the electrochemical potential difference of specific ions across the cytoplasmic membrane. The stator of proton-driven flagellar motor converts proton influx into mechanical work. However, the energy conversion mechanism remains unclear. Here, we show that the motor is sensitive to intracellular proton concentration for high-speed rotation at low load, which was considerably impaired by lowering intracellular pH, while zero-speed torque was not affected. The change in extracellular pH did not show any effect. These results suggest that a high intracellular proton concentration decreases the rate of proton translocation and therefore that of the mechanochemical reaction cycle of the motor but not the actual torque generation step within the cycle by the stator-rotor interactions.

Suppressor analysis of the MotB(D33E) mutation to probe bacterial flagellar motor dynamics coupled with proton translocation

MotA and MotB form the stator of the proton-driven bacterial flagellar motor, which conducts protons and couples proton flow with motor rotation. Asp-33 of Salmonella enterica serovar Typhimurium MotB, which is a putative proton-binding site, is critical for torque generation. However, the mechanism of energy coupling remains unknown. Here, we carried out genetic and motility analysis of a slowly motile motB(D33E) mutant and its pseudorevertants. We first confirmed that the poor motility of the motB(D33E) mutant is due to neither protein instability, mislocalization, nor impaired interaction with MotA. We isolated 17 pseudorevertants and identified the suppressor mutations in the transmembrane helices TM2 and TM3 of MotA and in TM and the periplasmic domain of MotB. The stall torque produced by the motB(D33E) mutant motor was about half of the wild-type level, while those for the pseudorevertants were recovered nearly to the wild-type levels. However, the high-speed rotations of the motors under low-load conditions were still significantly impaired, suggesting that the rate of proton translocation is still severely limited at high speed. These results suggest that the second-site mutations recover a torque generation step involving stator-rotor interactions coupled with protonation/deprotonation of Glu-33 but not maximum proton conductivity.