CiteULike: kdesmond's bacteria_flagella_project

Torque Generated by the Flagellar Motor of Escherichia coli while Driven Backward

Richard Berry — 2008-05-04T21:32:13-00:00

Biophys. J., Vol. 76, No. 1. (1 January 1999), pp. 580-587.

The technique of electrorotation was used to apply torque to cells of the bacterium Escherichia coli tethered to glass coverslips by single flagella. Cells were made to rotate backward, that is, in the direction opposite to the rotation driven by the flagellar motor itself. The torque generated by the motor under these conditions was estimated using an analysis that explicitly considers the angular dependence of both the viscous drag coefficient of the cell and the torque produced by electrorotation. Motor torque varied approximately linearly with speed up to over 100 Hz in either direction, placing constraints on mechanisms for torque generation in which rates of proton transfer for backward rotation are limiting. These results, interpreted in the context of a simple three-state kinetic model, suggest that the rate-limiting step in the torque-generating cycle is a powerstroke in which motor rotation and dissipation of the energy available from proton transit occur synchronously.

The steady-state counterclockwise/clockwise ratio of bacterial flagellar motors is regulated by protonmotive force.

S Khan — 2007-04-18T13:34:20-00:00

J Mol Biol, Vol. 138, No. 3. (15 April 1980), pp. 563-597.

Proton chemical potential, proton electrical potential and bacterial motility.

S Khan — 2008-05-02T18:28:34-00:00

Journal of molecular biology, Vol. 138, No. 3. (15 April 1980), pp. 599-614.

The proton flux through the bacterial flagellar motor.

M Meister — 2008-05-02T18:25:04-00:00

Cell, Vol. 49, No. 5. (5 June 1987), pp. 643-650.

Bacterial flagella are driven by a rotary motor that utilizes the free energy stored in the electrochemical proton gradient across the cytoplasmic membrane to do mechanical work. The flux of protons coupled to motor rotation was measured in Streptococcus and found to be directly proportional to motor speed. This supports the hypothesis that the movement of protons through the motor is tightly coupled to the rotation of its flagellar filament. Under this assumption the efficiency of energy conversion is close to unity at the low speeds encountered in tethered cells but only a few percent at the high speeds encountered in swimming cells. This difference appears to be due to dissipation by processes internal to the motor. The efficiency at high speeds exhibits a steep temperature dependence and a sizable deuterium solvent isotope effect.

Solvent-Isotope and pH Effects on Flagellar Rotation in Escherichia coli

Xiaobing Chen — 2008-05-02T18:23:23-00:00

Biophys. J., Vol. 78, No. 5. (1 May 2000), pp. 2280-2284.

We studied changes in speed of the flagellar rotary motor of Escherichia coli when tethered cells or cells carrying small latex spheres on flagellar stubs were shifted from H2O to D2O or subjected to changes in external pH. In the high-torque, low-speed regime, solvent isotope effects were found to be small; in the low-torque, high-speed regime, they were large. The boundaries between these regimes were close to those found earlier in measurements of the torque-speed relationship of the flagellar rotary motor (Berg and Turner, 1993, Biophys. J. 65:2201-2216; Chen and Berg, 2000, Biophys. J., 78:1036-1041). This observation provides direct evidence that the decline in torque at high speed is due primarily to limits in rates of proton transfer. However, variations of speed (and torque) with shifts of external pH (from 4.7 to 8.8) were small for both regimes. Therefore, rates of proton transfer are not very dependent on external pH.

Successive incorporation of force-generating units in the bacterial rotary motor

Steven Block — 2008-05-02T18:22:41-00:00

Nature, Vol. 309, No. 5967. (31 May 1984), pp. 470-472.

Restoration of torque in defective flagellar motors

DF Blair — 2008-05-02T18:21:55-00:00

Science, Vol. 242, No. 4886. (23 December 1988), pp. 1678-1681.

Paralyzed motors of motA and motB point and deletion mutants of Escherichia coli were repaired by synthesis of wild-type protein. As found earlier with a point mutant of motB, torque was restored in a series of equally spaced steps. The size of the steps was the same for both MotA and MotB. Motors with one torque generator spent more time spinning counterclockwise than did motors with two or more generators. In deletion mutants, stepwise decreases in torque, rare in point mutants, were common. Several cells stopped accelerating after eight steps, suggesting that the maximum complement of torque generators is eight. Each generator appears to contain both MotA and MotB. 10.1126/science.2849208

A Protonmotive Force Drives Bacterial Flagella

Michael Manson — 2008-05-02T18:21:07-00:00

Proceedings of the National Academy of Sciences, Vol. 74, No. 7. (15 July 1977), pp. 3060-3064.

Streptococcus strain V4051 is motile in the presence of glucose. The cells move steadily along smooth paths (run), jump about briefly with little net displacement (twiddle), and then run in new directions. They stop swimming when deprived of glucose. These cells become motile when an electrical potential or a pH gradient is imposed across the membrane. Starved cells suspended in a potassium-free medium respond to the addition of valinomycin by a brief period of vigorous twiddling. They also twiddle, although less vigorously, when the external pH is lowered. Valinomycin-induced twiddling occurs in the absence of external alkali or alkaline earth cations and without significant net synthesis of ATP. When a chemoattractant is added to cells swimming in the presence of glucose, twiddles are transiently suppressed, and the cells run for a time. Similarly, when starved cells are suspended in a potassium-free medium containing both valinomycin and an attractant, many cells initially run rather than twiddle. We conclude that the flagella are driven by a protonmotive force. 10.1073/pnas.74.7.3060

Minimal requirements for rotation of bacterial flagella.

S Ravid — 2008-05-02T18:20:17-00:00

J. Bacteriol., Vol. 158, No. 3. (1 June 1984), pp. 1208-1210.

An in vitro system of cell envelopes from Salmonella typhimurium with functional flagella was used to determine the minimal requirements for flagellar rotation. Rotation in the absence of cytoplasmic constituents could be driven either by respiration or by an artificially imposed chemical gradient of protons. No specific ionic requirements other than protons (or hydroxyls) were found for the motor function.

Chemomechanical Coupling without ATP: The Source of Energy for Motility and Chemotaxis in Bacteria

Steven Larsen — 2008-05-02T18:18:49-00:00

Proceedings of the National Academy of Sciences, Vol. 71, No. 4. (15 April 1974), pp. 1239-1243.

The source of energy for bacterial motility is the intermediate in oxidative phosphorylation, not ATP directly. For chemotaxis, however, there is an additional requirement, presumably ATP. These conclusions are based on the following findings. (i) Unlike their parents, mutants of Escherichia coli and Salmonella typhimurium that are blocked in the conversion of ATP to the intermediate of oxidative phosphorylation failed to swim anaerobically, even when they produced ATP. When respiration was restored to the mutants, motility was simultaneously restored. (ii) Carbonylcyanide m-chlorophenyl-hydrazone, which uncouples oxidative phosphorylation, completely inhibited motility even though ATP remained present. (iii) Arsenate did not inhibit motility in the presence of an oxidizable substrate, though it did reduce ATP levels to less than 0.3%. (iv) Arsenate completely inhibited chemotaxis under conditions where motility was normal. 10.1073/pnas.71.4.1239

Constraints on Models for the Flagellar Rotary Motor

Howard Berg — 2008-05-02T18:17:48-00:00

Philosophical Transactions: Biological Sciences, Vol. 355, No. 1396. (2000), pp. 491-501.

Most bacteria that swim are propelled by flagellar filaments, each driven at its base by a rotary motor embedded in the cell wall and cytoplasmic membrane. A motor is about 45 nm in diameter and made up of about 20 different kinds of parts. It is assembled from the inside out. It is powered by a proton (or in some species, a sodium-ion) flux. It steps at least 400 times per revolution. At low speeds and high torques, about 1000 protons are required per revolution, speed is proportional to protonmotive force, and torque varies little with temperature or hydrogen isotope. At high speeds and low torques, torque increases with temperature and is sensitive to hydrogen isotope. At room temperature, torque varies remarkably little with speed from about -100 Hz (the present limit of measurement) to about 200 Hz, and then it declines rapidly, reaching zero at about 300 Hz. These are facts that motor models should explain. None of the existing models for the flagellar rotary motor completely do so.

Torque-speed relationship of the flagellar rotary motor of Escherichia coli.

X Chen — 2005-12-31T02:16:45-00:00

Biophys J, Vol. 78, No. 2. (February 2000), pp. 1036-1041.

The output of a rotary motor is characterized by its torque and speed. We measured the torque-speed relationship of the flagellar rotary motor of Escherichia coli by a new method. Small latex spheres were attached to flagellar stubs on cells fixed to the surface of a glass slide. The angular speeds of the spheres were monitored in a weak optical trap by back-focal-plane interferometry in solutions containing different concentrations of the viscous agent Ficoll. Plots of relative torque (viscosity x speed) versus speed were obtained over a wide dynamic range (up to speeds of approximately 300 Hz) at three different temperatures, 22.7, 17.7, and 15.8 degrees C. Results obtained earlier by electrorotation (, Biophys. J. 65:2201-2216) were confirmed. The motor operates in two dynamic regimes. At 23 degrees C, the torque is approximately constant up to a knee speed of nearly 200 Hz, and then it falls rapidly with speed to a zero-torque speed of approximately 350 Hz. In the low-speed regime, torque is insensitive to changes in temperature. In the high-speed regime, it decreases markedly at lower temperature. These results are consistent with models in which torque is generated by a powerstroke mechanism (, Biophys. J. 76:580-587).

Torque and switching in the bacterial flagellar motor. An electrostatic model.

RM Berry — 2008-05-02T18:15:51-00:00

Biophys. J., Vol. 64, No. 4. (1 April 1993), pp. 961-973.

A model is presented for the rotary motor that drives bacterial flagella, using the electrochemical gradient of protons across the cytoplasmic membrane. The model unifies several concepts present in previous models. Torque is generated by proton-conducting particles around the perimeter of the rotor at the base of the flagellum. Protons in channels formed by these particles interact electrostatically with tilted lines of charges on the rotor, providing "loose coupling" between proton flux and rotation of the flagellum. Computer simulations of the model correctly predict the experimentally observed dynamic properties of the motor. Unlike previous models, the motor presented here may rotate either way for a given direction of the protonmotive force. The direction of rotation only depends on the level of occupancy of the proton channels. This suggests a novel and simple mechanism for the switching between clockwise and counterclockwise rotation that is the basis of bacterial chemotaxis.

Protein turbines. I: The bacterial flagellar motor.

TC Elston — 2008-05-02T18:14:57-00:00

Biophys. J., Vol. 73, No. 2. (1 August 1997), pp. 703-721.

The bacterial flagellar motor is driven by a flux of ions between the cytoplasm and the periplasmic lumen. Here we show how an electrostatic mechanism can convert this ion flux into a rotary torque. We demonstrate that, with reasonable parameters, the model can reproduce many of the experimental measurements.

The rotary motor of bacterial flagella.

HC Berg — 2005-12-31T02:14:38-00:00

Annu Rev Biochem, Vol. 72 (2003), pp. 19-54.

Flagellated bacteria, such as Escherichia coli, swim by rotating thin helical filaments, each driven at its base by a reversible rotary motor, powered by an ion flux. A motor is about 45 nm in diameter and is assembled from about 20 different kinds of parts. It develops maximum torque at stall but can spin several hundred Hz. Its direction of rotation is controlled by a sensory system that enables cells to accumulate in regions deemed more favorable. We know a great deal about motor structure, genetics, assembly, and function, but we do not really understand how it works. We need more crystal structures. All of this is reviewed, but the emphasis is on function.

Torque generated by the bacterial flagellar motor close to stall.

RM Berry — 2006-01-12T00:39:55-00:00

Biophys J, Vol. 71, No. 6. (December 1996), pp. 3501-3510.

In earlier work in which electrorotation was used to apply external torque to tethered cells of the bacterium Escherichia coli, it was found that the torque required to force flagellar motors backward was considerably larger than the torque required to stop them. That is, there appeared to be substantial barrier to backward rotation. Here, we show that in most, possibly all, cases this barrier is an artifact due to angular variation of the torque applied by electrorotation, of the motor torque, or both; the motor torque appears to be independent to speed or to vary linearly with speed up to speeds of tens of Hertz, in either direction. However, motors often break catastrophically when driven backward, so backward rotation is not equivalent to forward rotation. Also, cells can rotate backward while stalled, either in randomly timed jumps of 180 degrees or very slowly and smoothly. When cells rotate slowly and smoothly backward, the motor takes several seconds to recover after electrorotation is stopped, suggesting that some form of reversible damage has occurred. These findings do not affect the interpretation of electrorotation experiments in which motors are driven rapidly forward.

Torque-speed relationship of the bacterial flagellar motor.

J Xing — 2007-05-03T22:24:38-00:00

Proc Natl Acad Sci U S A, Vol. 103, No. 5. (31 January 2006), pp. 1260-1265.

Many swimming bacteria are propelled by flagellar filaments driven by a rotary motor. Each of these tiny motors can generate an impressive torque. The motor torque vs. speed relationship is considered one of the most important measurable characteristics of the motor and therefore is a major criterion for judging models proposed for the working mechanism. Here we give an explicit explanation for this torque-speed curve. The same physics also can explain certain puzzling properties of other motors.