A novel type bacterial flagellar motor that can use divalent cations as a coupling ion

Ion-Powered Rotary Motors: Where Did They Come from and Where They Are Going?

Molecular motors are found in many living organisms. One such molecular machine, the ion-powered rotary motor (IRM), requires the movement of ions across a membrane against a concentration gradient to drive rotational movement. The bacterial flagellar motor (BFM) is an example of an IRM which relies on ion movement through the stator proteins to generate the rotation of the flagella. There are many ions which can be used by the BFM stators to power motility and different ions can be used by a single bacterium expressing multiple stator variants. The use of ancestral sequence reconstruction (ASR) and functional analysis of reconstructed stators shows promise for understanding how these proteins evolved and when the divergence in ion use may have occurred. In this review, we discuss extant BFM stators and the ions that power them as well as recent examples of the use of ASR to study ion-channel selectivity and how this might be applied to further study of the BFM stator complex.

Physiological importance and role of Mg2+ in improving bacterial resistance to cesium

Cesium (Cs) is an alkali metal with radioactive isotopes such as ¹³⁷Cs and ¹³⁴Cs. ¹³⁷Cs, a product of uranium fission, has garnered attention as a radioactive contaminant. Radioactive contamination remediation using microorganisms has been the focus of numerous studies. We investigated the mechanism underlying Cs⁺ resistance in Microbacterium sp. TS-1 and other representative microorganisms, including Bacillus subtilis. The addition of Mg²⁺ effectively improved the Cs⁺ resistance of these microorganisms. When exposed to high concentrations of Cs⁺, the ribosomes of Cs⁺-sensitive mutants of TS-1 collapsed. Growth inhibition of B. subtilis in a high-concentration Cs⁺ environment was because of a drastic decrease in the intracellular potassium ion concentration and not the destabilization of the ribosomal complex. This is the first study demonstrating that the toxic effect of Cs⁺ on bacterial cells differs based on the presence of a Cs⁺ efflux mechanism. These results will aid in utilizing high-concentration Cs⁺-resistant microorganisms for radioactive contamination remediation in the future.

A magnesium transporter is involved in the cesium ion resistance of the high-concentration cesium ion-resistant bacterium Microbacterium sp. TS-1

Cesium ion (Cs⁺) resistance has been reported in bacteria but is poorly understood as reports on Cs⁺-resistant bacteria have been limited. We previously reported a novel Cs⁺/H⁺ antiporter CshA implicated in Cs⁺-resistance in Microbacterium sp. TS-1. The present study used the same screening method to isolate novel Cs⁺-sensitive mutants and their revertants from TS-1. A comparative mutation site analysis using whole-genome sequencing revealed that MTS1_03028 encodes the Mg²⁺ transporter MgtE and is a candidate Cs⁺ resistance-related gene. We performed a bioinformatic analysis of MTS1_03028 and complementation experiments on Cs⁺ resistance in the TS-1 MTS1_03028 mutants Mut5 and Mut7 as well as Escherichia coli expressing MTS1_03028 in the presence of Mg²⁺. We established the role of MgtE in Cs⁺ resistance through a functional analysis of TS-1. Enhancing Mg²⁺ transport by expression of MTS_03028 conferred increased Cs⁺ resistance. When this strain was exposed to Cs⁺ concentrations exceeding 200 mM, CshA consistently lowered the intracellular Cs⁺ concentration. To our knowledge, the present study is the first to clarify the mechanism of Cs⁺ resistance in certain bacteria. The study findings offer important insights into the mechanism of bacterial resistance to excess Cs⁺ in the environment, suggesting the potential for bioremediation in high Cs-contaminated areas.

Bacterial Proprioception: Can a Bacterium Sense Its Movement?

The evolution of the bacterial flagellum gave rise to motility and repurposing of a signaling network, now termed the chemotaxis network, enabled biasing of cell movements. This made it possible for the bacterium to seek out favorable chemical environments. To enable chemotaxis, the chemotaxis network sensitively detects extracellular chemical stimuli and appropriately modulates flagellar functions. Additionally, the flagellar motor itself is capable of detecting mechanical stimuli and adapts its structure and function in response, likely triggering a transition from planktonic to surface-associated lifestyles. Recent work has shown a link between the flagellar motor's response to mechanical stimuli and the chemotactic output. Here, we elaborate on this link and discuss how it likely helps the cell sense and adapt to changes in its swimming speeds in different environments. We discuss the mechanism whereby the motor precisely tunes its chemotaxis output under different mechanical loads, analogous to proprioception in higher order organisms. We speculate on the roles bacterial proprioception might play in a variety of phenomena including the transition to surface-associated lifestyles such as swarming and biofilms.

Novel Cesium Resistance Mechanism of Alkaliphilic Bacterium Isolated From Jumping Spider Ground Extract

The radionuclide isotopes (¹³⁴Cs and ¹³⁷Cs) of Cesium (Cs), an alkali metal, are attracting attention as major causes of radioactive contamination. Although Cs⁺ is harmful to the growth of plants and bacteria, alkaliphilic bacterium Microbacterium sp. TS-1, isolated from a jumping spider, showed growth even in the presence of 1.2 M CsCl. The maximum concentration of Cs⁺ that microorganisms can withstand has been reported to be 700 mM till date, suggesting that the strain TS-1 is resistant to a high concentration of Cs ions. Multiple reports of cesium ion-resistant bacteria have been reported, but the detailed mechanism has not yet been elucidated. We obtained Cs ion-sensitive mutants and their revertant mutants from strain TS-1 and identified a Cs ion resistance-related gene, MTS1_00475, by performing SNP analysis of the whole-genome sequence data. When exposed to more than 200 mM Cs⁺ concentration, the intracellular Cs⁺ concentration was constantly lowered by MTS1_00475, which encodes the novel low-affinity Cs⁺/H⁺ antiporter. This study is the first to clarify the mechanism of cesium resistance in unexplained cesium-resistant microorganisms. By clarifying the new cesium resistance mechanism, it can be expected to be used as a bioremediation tool for treating radioactive Cs⁺ contaminated water.

The Dynamic Ion Motive Force Powering the Bacterial Flagellar Motor

The bacterial flagellar motor (BFM) is a rotary molecular motor embedded in the cell membrane of numerous bacteria. It turns a flagellum which acts as a propeller, enabling bacterial motility and chemotaxis. The BFM is rotated by stator units, inner membrane protein complexes that stochastically associate to and dissociate from individual motors at a rate which depends on the mechanical and electrochemical environment. Stator units consume the ion motive force (IMF), the electrochemical gradient across the inner membrane that results from cellular respiration, converting the electrochemical energy of translocated ions into mechanical energy, imparted to the rotor. Here, we review some of the main results that form the base of our current understanding of the relationship between the IMF and the functioning of the flagellar motor. We examine a series of studies that establish a linear proportionality between IMF and motor speed, and we discuss more recent evidence that the stator units sense the IMF, altering their rates of dynamic assembly. This, in turn, raises the question of to what degree the classical dependence of motor speed on IMF is due to stator dynamics vs. the rate of ion flow through the stators. Finally, while long assumed to be static and homogeneous, there is mounting evidence that the IMF is dynamic, and that its fluctuations control important phenomena such as cell-to-cell signaling and mechanotransduction. Within the growing toolbox of single cell bacterial electrophysiology, one of the best tools to probe IMF fluctuations may, ironically, be the motor that consumes it. Perfecting our incomplete understanding of how the BFM employs the energy of ion flow will help decipher the dynamical behavior of the bacterial IMF.

Ancestral Sequence Reconstructions of MotB Are Proton-Motile and Require MotA for Motility

The bacterial flagellar motor (BFM) is a nanomachine that rotates the flagellum to propel many known bacteria. The BFM is powered by ion transit across the cell membrane through the stator complex, a membrane protein. Different bacteria use various ions to run their BFM, but the majority of BFMs are powered by either proton (H+) or sodium (Na+) ions. The transmembrane (TM) domain of the B-subunit of the stator complex is crucial for ion selectivity, as it forms the ion channel in complex with TM3 and TM4 of the A-subunit. In this study, we reconstructed and engineered thirteen ancestral sequences of the stator B-subunit to evaluate the functional properties and ionic power source of the stator proteins at reconstruction nodes to evaluate the potential of ancestral sequence reconstruction (ASR) methods for stator engineering and to test specific motifs previously hypothesized to be involved in ion-selectivity. We found that all thirteen of our reconstructed ancient B-subunit proteins could assemble into functional stator complexes in combination with the contemporary Escherichia coli MotA-subunit to restore motility in stator deleted E. coli strains. The flagellar rotation of the thirteen ancestral MotBs was found to be Na+ independent which suggested that the F30/Y30 residue was not significantly correlated with sodium/proton phenotype, in contrast to what we had reported previously. Additionally, four among the thirteen reconstructed B-subunits were compatible with the A-subunit of Aquifex aeolicus and able to function in a sodium-independent manner. Overall, this work demonstrates the use of ancestral reconstruction to generate novel stators and quantify which residues are correlated with which ionic power source.

Coupling Ion Specificity of the Flagellar Stator Proteins MotA1/MotB1 of Paenibacillus sp. TCA20

The bacterial flagellar motor is a reversible rotary molecular nanomachine, which couples ion flux across the cytoplasmic membrane to torque generation. It comprises a rotor and multiple stator complexes, and each stator complex functions as an ion channel and determines the ion specificity of the motor. Although coupling ions for the motor rotation were presumed to be only monovalent cations, such as H⁺ and Na⁺, the stator complex MotA1/MotB1 of Paenibacillus sp. TCA20 (MotA1^TCA/MotB1^TCA) was reported to use divalent cations as coupling ions, such as Ca²⁺ and Mg²⁺. In this study, we initially aimed to measure the motor torque generated by MotA1^TCA/MotB1^TCA under the control of divalent cation motive force; however, we identified that the coupling ion of MotA1^TCAMotB1^TCA is very likely to be a monovalent ion. We engineered a series of functional chimeric stator proteins between MotB1^TCA and Escherichia coli MotB. E. coli ΔmotAB cells expressing MotA1^TCA and the chimeric MotB presented significant motility in the absence of divalent cations. Moreover, we confirmed that MotA1^TCA/MotB1^TCA in Bacillus subtilis ΔmotABΔmotPS cells generates torque without divalent cations. Based on two independent experimental results, we conclude that the MotA1^TCA/MotB1^TCA complex directly converts the energy released from monovalent cation flux to motor rotation.

MotP Subunit is Critical for Ion Selectivity and Evolution of a K+-Coupled Flagellar Motor

The bacterial flagellar motor is a sophisticated nanomachine embedded in the cell envelope. The flagellar motor is driven by an electrochemical gradient of cations such as H⁺, Na⁺, and K⁺ through ion channels in stator complexes embedded in the cell membrane. The flagellum is believed to rotate as a result of electrostatic interaction forces between the stator and the rotor. In bacteria of the genus Bacillus and related species, the single transmembrane segment of MotB-type subunit protein (MotB and MotS) is critical for the selection of the H⁺ and Na⁺ coupling ions. Here, we constructed and characterized several hybrid stators combined with single Na⁺-coupled and dual Na⁺- and K⁺-coupled stator subunits, and we report that the MotP subunit is critical for the selection of K⁺. This result suggested that the K⁺ selectivity of the MotP/MotS complexes evolved from the single Na⁺-coupled stator MotP/MotS complexes. This finding will promote the understanding of the evolution of flagellar motors and the molecular mechanisms of coupling ion selectivity.

Draft Genome Sequence of Calcium-Dependent Novosphingobium sp. Strain TCA1, Isolated from a Hot Spring Containing a High Concentration of Calcium Ions

Calcium-dependent Novosphingobium sp. strain TCA1 was newly isolated from a water sample from a hot spring containing a high concentration of calcium ions. Here, we report the draft genome sequence of this bacterium, which may be the basis for research on calcium ion homeostasis.

Evolution of the Stator Elements of Rotary Prokaryote Motors

The bacterial flagellar motor is driven by an ion flux that is converted to torque by motor-attendant complexes known as stators. The dynamics of stator assembly around the motor in response to external stimuli have been the subject of much recent research, but less is known about the evolutionary origins of stator complexes and how they select for specific ions. Here, we review the latest structural and biochemical data for the stator complexes and compare these with other ion transporters and microbial motors to examine possible evolutionary origins of the stator complex.

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.

Essential ion binding residues for Na+ flow in stator complex of the Vibrio flagellar motor

The bacterial flagellar motor is a unique supramolecular complex which converts ion flow into rotational force. Many biological devices mainly use two types of ions, proton and sodium ion. This is probably because of the fact that life originated in seawater, which is rich in protons and sodium ions. The polar flagellar motor in Vibrio is coupled with sodium ion and the energy converting unit of the motor is composed of two membrane proteins, PomA and PomB. It has been shown that the ion binding residue essential for ion transduction is the conserved aspartic acid residue (PomB-D24) in the PomB transmembrane region. To reveal the mechanism of ion selectivity, we identified essential residues, PomA-T158 and PomA-T186, other than PomB-D24, in the Na⁺-driven flagellar motor. It has been shown that the side chain of threonine contacts Na⁺ in Na⁺-coupled transporters. We monitored the Na⁺-binding specific structural changes using ATR-FTIR spectroscopy. The signals were abolished in PomA-T158A and -T186A, as well as in PomB-D24N. Molecular dynamics simulations further confirmed the strong binding of Na⁺ to D24 and showed that T158A and T186A hindered the Na⁺ binding and transportation. The data indicate that two threonine residues (PomA-T158 and PomA-T186), together with PomB-D24, are important for Na⁺ conduction in the Vibrio flagellar motor. The results contribute to clarify the mechanism of ion recognition and conversion of ion flow into mechanical force.

Functional Regulators of Bacterial Flagella

Bacteria move by a variety of mechanisms, but the best understood types of motility are powered by flagella (72). Flagella are complex machines embedded in the cell envelope that rotate a long extracellular helical filament like a propeller to push cells through the environment. The flagellum is one of relatively few biological machines that experience continuous 360° rotation, and it is driven by one of the most powerful motors, relative to its size, on earth. The rotational force (torque) generated at the base of the flagellum is essential for motility, niche colonization, and pathogenesis. This review describes regulatory proteins that control motility at the level of torque generation.

Flagellum-mediated motility in Pelotomaculum thermopropionicum SI

The basic functions of a propionate-oxidizing bacterium Pelotomaculum thermopropionicum flagellum, such as motility and chemotaxis, have not been studied. To investigate its motility, we compared with that of Syntrophobacter fumaroxidans, an aflagellar propionate-oxidizing bacterium, in soft agar medium. P. thermopropionicum cells spread, while S. fumaroxidans cells moved downward slightly, indicating flagellum-dependent motility in P. thermopropionicum SI. The motility of P. thermopropionicum was inhibited by the addition of carbonyl cyanide m-chlorophenyl hydrazone, a proton uncoupler, which is consistent with the fact that stator protein, MotB of P. thermopropionicum, shared sequence homology with proton-type stators. In addition, 5-N-ethyl-N-isopropyl amiloride, an Na⁺ channel blocker, showed no inhibitory effect on the motility. Furthermore, motAB of P. thermopropionicum complemented the defective swimming ability of Escherichia coli ∆motAB. These results suggest that the motility of P. thermopropionicum SI depends on the proton-type flagellar motor.

Ion Selectivity of the Flagellar Motors Derived from the Alkaliphilic Bacillus and Paenibacillus Species

Many bacteria can swim using their flagella, which are filamentous organelles that extend from the cell surface. The flagellar motor is energized by either a proton (H⁺) or sodium ion (Na⁺) as the motive force. MotAB-type stators use protons, whereas MotPS- and PomAB-type stators use Na⁺ as the coupling ions. Recently, alkaliphilic Bacillus alcalophilus was shown to use potassium ions (K⁺) and rubidium ions (Rb⁺) for flagellar rotation, and the flagellar motor from Paenibacillus sp. TCA-20 uses divalent cations such as magnesium ions (Mg²⁺), calcium ions (Ca²⁺), and strontium ions (Sr²⁺) for coupling. In this chapter, we focus on how to identify the coupling ions for flagellar rotation of alkaliphilic Bacillus and Paenibacillus species.

Nonconventional cation-coupled flagellar motors derived from the alkaliphilic Bacillus and Paenibacillus species

Prior to 2008, all previously studied conventional bacterial flagellar motors appeared to utilize either H⁺ or Na⁺ as coupling ions. Membrane-embedded stator complexes support conversion of energy using transmembrane electrochemical ion gradients. The main H⁺-coupled stators, known as MotAB, differ from Na⁺-coupled stators, PomAB of marine bacteria, and MotPS of alkaliphilic Bacillus. However, in 2008, a MotAB-type flagellar motor of alkaliphilic Bacillus clausii KSM-K16 was revealed as an exception with the first dual-function motor. This bacterium was identified as the first bacterium with a single stator-rotor that can utilize both H⁺ and Na⁺ for ion-coupling at different pH ranges. Subsequently, another exception, a MotPS-type flagellar motor of alkaliphilic Bacillus alcalophilus AV1934, was reported to utilize Na⁺ plus K⁺ and Rb⁺ as coupling ions for flagellar rotation. In addition, the alkaline-tolerant bacterium Paenibacillus sp. TCA20, which can utilize divalent cations such as Ca²⁺, Mg²⁺, and Sr²⁺, was recently isolated from a hot spring in Japan, which contains a high Ca²⁺ concentration. These findings show that bacterial flagellar motors isolated from unique environments utilize unexpected coupling ions. This suggests that bacteria that grow in different extreme environments adapt to local conditions and evolve their motility machinery.

Genetic Analysis of Collective Motility of Paenibacillus sp. NAIST15-1

Bacteria have developed various motility mechanisms to adapt to a variety of solid surfaces. A rhizosphere isolate, Paenibacillus sp. NAIST15-1, exhibited unusual motility behavior. When spotted onto 1.5% agar media, Paenibacillus sp. formed many colonies, each of which moved around actively at a speed of 3.6 μm/sec. As their density increased, each moving colony began to spiral, finally forming a static round colony. Despite its unusual motility behavior, draft genome sequencing revealed that both the composition and organization of flagellar genes in Paenibacillus sp. were very similar to those in Bacillus subtilis. Disruption of flagellar genes and flagellar stator operons resulted in loss of motility. Paenibacillus sp. showed increased transcription of flagellar genes and hyperflagellation on hard agar media. Thus, increased flagella and their rotation drive Paenibacillus sp. motility. We also identified a large extracellular protein, CmoA, which is conserved only in several Paenibacillus and related species. A cmoA mutant could neither form moving colonies nor move on hard agar media; however, motility was restored by exogenous CmoA. CmoA was located around cells and enveloped cell clusters. Comparison of cellular behavior between the wild type and cmoA mutant indicated that extracellular CmoA is involved in drawing water out of agar media and/or smoothing the cell surface interface. This function of CmoA probably enables Paenibacillus sp. to move on hard agar media.