June 24, 2008 | David F. Coppedge

Bacterial Flagellar Motor Has a Protein Clutch

The bacterial flagellum, the whiplike outboard motor that has become an icon of intelligent design, has another artificial-looking part: a clutch.  Science reported this in “machine language” as follows:1

The bacterial flagellum, powered by a motor that generates 1400 pN-nm of torque, can rotate at a frequency of greater than 100 Hz.  EpsE [the clutch protein] disabled this powerful biological motor when associated with a flagellar basal body and, in a manner similar to that of a clutch, disengaged the drive train from the power sourcee (fig.  S5B).  Clutch control of flagellar function has distinct advantages over transcriptional control of flagellar gene expression for regulating motility.  Some bacteria, such as E. coli and B. subtilis, have many flagella per cell.  The flagellum is an elaborate, durable, energetically expensive, molecular machine and simply turning off de novo flagellum synthesis does not necessarily arrest motility.  Once flagellar gene expression is inactivated, multiple rounds of cell division may be required to segregate preexisting flagella to extinction in daughter cells.  In contrast, the clutch requires the synthesis of only a single protein to inhibit motility.  Furthermore, if biofilm formation is prematurely aborted, flagella once disabled by the clutch might be reactivated, allowing cells to bypass fresh investment in flagellar synthesis.  Whereas flagellum expression and assembly are complex and slow, clutch control is simple, rapid, and potentially reversible.

The clutch thus puts the flagellum in neutral and lets the motor idle without having to be shut down.  Among the co-authors of the paper was Howard Berg of Harvard, who has spent many years studying the molecular motor.  The paper did not attempt to explain how a clutch might evolve by natural selection.
    For popular reports on this finding, with illustrations of how the clutch works, visit NSF News, Nano.org, Photonics.com, PhysOrg and Science DailyARN discussed the paper from an intelligent design perspective.


1.  Blair, Turner, Winkelman, Berg and Kearns, “A Molecular Clutch Disables Flagella in the Bacillus subtilis Biofilm,” Science, 20 June 2008: Vol. 320. no. 5883, pp. 1636 – 1638, DOI: 10.1126/science.1157877.

This is another of many instances of one of the best-established laws in nature: evolutionary storytelling is inversely proportional to observational detail.  A good scientific law needs a popular name.  Write in with your suggestion.

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