The bacterial flagellar motor is a rotary molecular machine that rotates the helical filaments that propel many species of swimming bacteria1,2. The rotor is a set of rings up to 45 nm in diameter in the cytoplasmic membrane3; the stator contains about ten torque-generating units anchored to the cell wall at the perimeter of the rotor4,5. The free-energy source for the motor is an inward-directed electrochemical gradient of ions across the cytoplasmic membrane, the protonmotive force or sodium-motive force for H+-driven and Na+-driven motors, respectively. Here we demonstrate a stepping motion of a Na+-driven chimaeric flagellar motor in Escherichia coli6 at low sodium-motive force and with controlled expression of a small number of torque-generating units. We observe 26 steps per revolution, which is consistent with the periodicity of the ring of FliG protein, the proposed site of torque generation on the rotor7,8. Backwards steps despite the absence of the flagellar switching protein CheY indicate a small change in free energy per step, similar to that of a single ion transit.
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We thank H. Berg and K. Fahrner for the gift of strain HCB1271. The research of R.B., M.L. and A.R. was supported by combined UK research councils through an Interdisciplinary Research Collaboration in Bionanotechnology, that of A.I., M.H. and T.Y. by Grants-in-Aid from the Ministry of Education, Science, Sports, Culture and Technology of Japan, that of M.H. and T.Y. by Soft Nano-Machine Project of JST, and that of Y.S. by JSPS Research Fellowships for Young Scientists. Author Contributions BFP experiments were performed by Y.S. and A.R., fluorescence experiments by A.R. and M.L., experimental design was by R.B., A.I., A.R. and Y.S., data analysis by R.B., Y.S. and A.R., and strain construction by Y.S., T.Y. and M.H. Y.S. and A.R. contributed equally to this work.
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
Effect of Na+ concentration on stator number, and initial stator number estimates for step experiments. (DOC 1175 kb)
Further examples of stepping rotation. (DOC 1831 kb)
A 200 nm fluorescent bead rotating at ˜2 Hz, slowed down 40 times. Stepping rotation is visible. (MPG 130 kb)
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Sowa, Y., Rowe, A., Leake, M. et al. Direct observation of steps in rotation of the bacterial flagellar motor. Nature 437, 916–919 (2005). https://doi.org/10.1038/nature04003
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