Rotary Nanomotors in the Rear View Mirror.

Rotation is part of our everyday lives. For most of human history, rotation was considered a uniquely human invention, something beyond the anatomical capabilities of organisms. In 1973, Howard Berg made the audacious proposal that the common gut bacterium Escherichia coli swims by rotating helical flagellar filaments. In 1987, Paul Boyer suggested that the FoF1 ATP synthase of E. coli is also a rotary device. Now we know that rotating nanomachines evolved independently at least three times. They power a wide variety of cellular processes. Here, the study of flagellar rotation in E. coli is briefly summarized. In 2020, the Cryo-EM structure of the MotAB stator element of the bacterial flagellum was described. The structure strongly suggests that the MotAB stator rotates to drive flagellar rotation. Similar motors are coupled to other diverse processes. The following articles in this issue review the current knowledge and speculation about rotating biological nanomachines.

Howard Berg (1934-2021).

David Blair and Michael Manson commemorate the late Howard Berg, who studied, among other things, the biophysics of bacterial motion.

Mechanosensitive recruitment of stator units promotes binding of the response regulator CheY-P to the flagellar motor.

Reversible switching of the bacterial flagellar motor between clockwise (CW) and counterclockwise (CCW) rotation is necessary for chemotaxis, which enables cells to swim towards favorable chemical habitats. Increase in the viscous resistance to the rotation of the motor (mechanical load) inhibits switching. However, cells must maintain homeostasis in switching to navigate within environments of different viscosities. The mechanism by which the cell maintains optimal chemotactic function under varying loads is not understood. Here, we show that the flagellar motor allosterically controls the binding affinity of the chemotaxis response regulator, CheY-P, to the flagellar switch complex by modulating the mechanical forces acting on the rotor. Mechanosensitive CheY-P binding compensates for the load-induced loss of switching by precisely adapting the switch response to a mechanical stimulus. The interplay between mechanical forces and CheY-P binding tunes the chemotactic function to match the load. This adaptive response of the chemotaxis output to mechanical stimuli resembles the proprioceptive feedback in the neuromuscular systems of insects and vertebrates.

Howard Berg's Random Walk through Biology.

This minireview presents the career of biophysicist Howard Berg from his first interest in bacterial chemotaxis and motility through the present. After a summary of some of his early work, a series of reminiscences of students, postdocs, colleagues, and family members is presented. In sum, these recollections capture the effect that Howard's scientific life has had on the field of bacterial chemotaxis and motility and on the careers and lives of those who have interacted with him.

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.

pH Sensing in Bacillus subtilis: a New Path to a Common Goal.

pH is one of the most fundamental properties of the environments in which microorganisms live. It is, therefore, not surprising that bacteria have evolved mechanisms to sense and respond to pH. One aspect of this response for motile bacteria is to migrate to areas of optimal pH. The paper by P. Tohidifar, M. J. Plutz, G. W. Ordal, and C. V. Rao (J Bacteriol 202:e00491-19, 2020, https://doi.org/10.1128/JB.00491-19) describes how Bacillus subtilis uses bidirectional chemotaxis mediated by four closely related dCACHE_1 chemoreceptors to migrate to regions of neutral pH.

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

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

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.

The Diversity of Bacterial Chemosensing.

Bacteria have a continuous and urgent need to inform themselves about the chemistry of their surroundings. They must rapidly adjust their patterns of gene expression, their metabolic and transport functions, and their behavior to cope with every challenge and opportunity with which they are presented. This volume collates the most recent methods developed to monitor and manipulate the processes by which bacteria sense and respond to their chemical environment.

Transmembrane Signal Transduction in Bacterial Chemosensing.

Like all living organisms, bacteria must communicate with the world around them. As they typically live as single cells, the communication with their environment must occur at the cell membrane, both in moving molecules in and out and in transmitting information about their surroundings to response elements within the cell. This volume is devoted primarily to methods used to study either the behavior of bacteria in response to their environment or methods used to study events that involve signaling pathways that are initiated by events at the cell membrane. The topics are arranged according to the scale of the events described: (1) Methods for studying bacterial chemotaxis at the population and whole-cell levels; (2) In vivo analysis of receptor function; (3) Cryo-EM methods for studying chemoreceptor structure; (4) Monitoring the intracellular movement of chemosensory proteins; (5) High-throughput methods for screening novel chemoeffectors; (6) Creating chemical tools for studying chemosensory signal transduction; (7) Computerized analysis of chemotaxis. Every effort has been made to get the most experienced and proficient practitioners of each of the methods described, and the editor is indebted to all who agreed to participate.

Conversion of Norepinephrine to 3,4-Dihdroxymandelic Acid in Escherichia coli Requires the QseBC Quorum-Sensing System and the FeaR Transcription Factor.

The detection of norepinephrine (NE) as a chemoattractant by Escherichia coli strain K-12 requires the combined action of the TynA monoamine oxidase and the FeaB aromatic aldehyde dehydrogenase. The role of these enzymes is to convert NE into 3,4-dihydroxymandelic acid (DHMA), which is a potent chemoattractant sensed by the Tsr chemoreceptor. These two enzymes must be induced by prior exposure to NE, and cells that are exposed to NE for the first time initially show minimal chemotaxis toward it. The induction of TynA and FeaB requires the QseC quorum-sensing histidine kinase, and the signaling cascade requires new protein synthesis. Here, we demonstrate that the cognate response regulator for QseC, the transcription factor QseB, is also required for induction. The related quorum-sensing kinase QseE appears not to be part of the signaling pathway, but its cognate response regulator, QseF, which is also a substrate for phosphotransfer from QseC, plays a nonessential role. The promoter of the feaR gene, which encodes a transcription factor that has been shown to be essential for the expression of tynA and feaB, has two predicted QseB-binding sites. One of these sites appears to be in an appropriate position to stimulate transcription from the P1 promoter of the feaR gene. This study unites two well-known pathways: one for expression of genes regulated by catecholamines (QseBC) and one for expression of genes required for metabolism of aromatic amines (FeaR, TynA, and FeaB). This cross talk allows E. coli to convert the host-derived and chemotactically inert NE into the potent bacterial chemoattractant DHMA.IMPORTANCE The chemotaxis of E. coli K-12 to norepinephrine (NE) requires the conversion of NE to 3,4-dihydroxymandleic acid (DHMA), and DHMA is both an attractant and inducer of virulence gene expression for a pathogenic enterohemorrhagic E. coli (EHEC) strain. The induction of virulence by DHMA and NE requires QseC. The results described here show that the cognate response regulator for QseC, QseB, is also required for conversion of NE into DHMA. Production of DHMA requires induction of a pathway involved in the metabolism of aromatic amines. Thus, the QseBC sensory system provides a direct link between virulence and chemotaxis, suggesting that chemotaxis to host signaling molecules may require that those molecules are first metabolized by bacterial enzymes to generate the actual chemoattractant.

Chemotaxis to self-generated AI-2 promotes biofilm formation in Escherichia coli.

Responses to the interspecies quorum-sensing signal autoinducer-2 (AI-2) regulate the patterns of gene expression that promote biofilm development. Escherichia coli also senses AI-2 as a chemoattractant, a response that requires the periplasmic AI-2-binding protein LsrB and the chemoreceptor Tsr. Here, we confirm, as previously observed, that under static conditions highly motile E. coli cells self-aggregate and form surface-adherent structures more readily than cells lacking LsrB and Tsr, or than ΔluxS cells unable to produce AI-2. This difference is observed both at 37 and 30 °C. Cells deleted for the genes encoding the lsrACDBFG operon repressor (ΔlsrR), or the AI-2 kinase (ΔlsrK), or an AI-2 uptake channel protein (ΔlsrC), or an AI-2 metabolism enzyme (ΔlsrG) are also defective in biofilm formation. The Δtsr and ΔlsrB cells are totally defective in AI-2 chemotaxis, whereas the other mutants show normal or near-normal chemotaxis to external gradients of AI-2. These data demonstrate that chemotaxis to external AI-2 is necessary but not sufficient to induce the full range of density-dependent behaviours that are required for optimal biofilm formation. We also demonstrate that, compared to other binding-protein-dependent chemotaxis systems in E. coli, low levels (on the order of ~250 molecules of periplasmic LsrB per wild-type cell and as low as ~50 molecules per cell in some mutants) are adequate for a strong chemotaxis response to external gradients of AI-2.

The Norepinephrine Metabolite 3,4-Dihydroxymandelic Acid Is Produced by the Commensal Microbiota and Promotes Chemotaxis and Virulence Gene Expression in Enterohemorrhagic Escherichia coli.

Enterohemorrhagic Escherichia coli (EHEC) is a commonly occurring foodborne pathogen responsible for numerous multistate outbreaks in the United States. It is known to infect the host gastrointestinal tract, specifically, in locations associated with lymphoid tissue. These niches serve as sources of enteric neurotransmitters, such as epinephrine and norepinephrine, that are known to increase virulence in several pathogens, including enterohemorrhagic E. coli The mechanisms that allow pathogens to target these niches are poorly understood. We previously reported that 3,4-dihydroxymandelic acid (DHMA), a metabolite of norepinephrine produced by E. coli, is a chemoattractant for the nonpathogenic E. coli RP437 strain. Here we report that DHMA is also a chemoattractant for EHEC. In addition, DHMA induces the expression of EHEC virulence genes and increases attachment to intestinal epithelial cells in vitro in a QseC-dependent manner. We also show that DHMA is present in murine gut fecal contents and that its production requires the presence of the commensal microbiota. On the basis of its ability to both attract and induce virulence gene expression in EHEC, we propose that DHMA acts as a molecular beacon to target pathogens to their preferred sites of infection in vivo.

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

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

Cryoelectron tomography reveals the sequential assembly of bacterial flagella in Borrelia burgdorferi.

Periplasmic flagella are essential for the distinctive morphology, motility, and infectious life cycle of the Lyme disease spirochete Borrelia burgdorferi. In this study, we genetically trapped intermediates in flagellar assembly and determined the 3D structures of the intermediates to 4-nm resolution by cryoelectron tomography. We provide structural evidence that secretion of rod substrates triggers remodeling of the central channel in the flagellar secretion apparatus from a closed to an open conformation. This open channel then serves as both a gateway and a template for flagellar rod assembly. The individual proteins assemble sequentially to form a modular rod. The hook cap initiates hook assembly on completion of the rod, and the filament cap facilitates filament assembly after formation of the mature hook. Cryoelectron tomography and mutational analysis thus combine synergistically to provide a unique structural blueprint of the assembly process of this intricate molecular machine in intact cells.

Residues at the cytoplasmic end of transmembrane helix 2 determine the signal output of the TarEc chemoreceptor.

Baseline signal output and communication between the periplasmic and cytoplasmic domains of the Escherichia coli aspartate chemoreceptor Tar(Ec) are both strongly influenced by residues at the C-terminus of transmembrane helix 2 (TM2). In particular, the cytoplasmic aromatic anchor, composed of residues Trp-209 and Tyr-210 in wild-type Tar(Ec), is important for determining the CheA kinase-stimulating activity of the receptor and its ability to respond to chemoeffector-induced stimuli. Here, we have studied the effect on Tar(Ec) function of the six-residue sequence at positions 207-212. Moving various combinations of aromatic residues among these positions generates substantial changes in receptor activity. Trp has the largest effect on function, both in maintaining normal activity and in altering activity when it is moved. Tyr has a weaker effect, and Phe has the weakest; however, all three aromatic residues can alter signal output when they are placed in novel positions. We also find that Gly-211 plays an important role in receptor function, perhaps because of the flexibility it introduces into the TM2-HAMP domain connector. The conservation of this Gly residue in the high-abundance chemoreceptors of E. coli and Salmonella enterica suggests that it may be important for the nuanced, bidirectional transmembrane signaling that occurs in these proteins.

Molecular architecture of chemoreceptor arrays revealed by cryoelectron tomography of Escherichia coli minicells.

The chemoreceptors of Escherichia coli localize to the cell poles and form a highly ordered array in concert with the CheA kinase and the CheW coupling factor. However, a high-resolution structure of the array has been lacking, and the molecular basis of array assembly has thus remained elusive. Here, we use cryoelectron tomography of flagellated E. coli minicells to derive a 3D map of the intact array. Docking of high-resolution structures into the 3D map provides a model of the core signaling complex, in which a CheA/CheW dimer bridges two adjacent receptor trimers via multiple hydrophobic interactions. A further, hitherto unknown, hydrophobic interaction between CheW and the homologous P5 domain of CheA in an adjacent core complex connects the complexes into an extended array. This architecture provides a structural basis for array formation and could explain the high sensitivity and cooperativity of chemotaxis signaling in E. coli.

The residue composition of the aromatic anchor of the second transmembrane helix determines the signaling properties of the aspartate/maltose chemoreceptor Tar of Escherichia coli.

Repositioning of the tandem aromatic residues (Trp-209 and Tyr-210) at the cytoplasmic end of the second transmembrane helix (TM2) modulates the signal output of the aspartate/maltose chemoreceptor of Escherichia coli (Tar(Ec)). Here, we directly assessed the effect of the residue composition of the aromatic anchor by studying the function of a library of Tar(Ec) variants that possess all possible combinations of Ala, Phe, Tyr, and Trp at positions 209 and 210. We identified three important properties of the aromatic anchor. First, a Trp residue at position 209 was required to maintain clockwise (CW) signal output in the absence of adaptive methylation, but adaptive methylation restored the ability of all of the mutant receptors to generate CW rotation. Second, when the aromatic anchor was replaced with tandem Ala residues, signaling was less compromised than when an Ala residue occupied position 209 and an aromatic residue occupied position 210. Finally, when Trp was present at position 209, the identity of the residue at position 210 had little effect on baseline signal output or aspartate chemotaxis, although maltose taxis was significantly affected by some substitutions at position 210. All of the mutant receptors we constructed supported some level of aspartate and maltose taxis in semisolid agar swim plates, but those without Trp at position 209 were overmethylated in their baseline signaling state. These results show the importance of the cytoplasmic aromatic anchor of TM2 in maintaining the baseline Tar(Ec) signal output and responsiveness to attractant signaling.