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Biophys J, February 2000, p. 1036-1041, Vol. 78, No. 2
Department of Molecular and Cellular Biology, Harvard University, Cambridge, Massachusetts 02138, and the Rowland Institute for Science, Cambridge, Massachusetts 02142 USA
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 × speed) versus speed were obtained over a wide dynamic range (up to
speeds of ~300 Hz) at three different temperatures, 22.7, 17.7, and
15.8°C. Results obtained earlier by electrorotation (Berg and Turner, 1993, Biophys. J. 65:2201-2216) were confirmed. The
motor operates in two dynamic regimes. At 23°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 ~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 (Berry and Berg, 1999, Biophys.
J. 76:580-587).
Biophys J, February 2000, p. 1036-1041, Vol. 78, No. 2
© 2000 by the Biophysical Society 0006-3495/00/02/1036/06 $2.00
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