The measurements were made by adsorbing 0.356В Ојm diameter latex spheres to sticky-filament stubs ( 8) and monitoring rotation rates in motility medium containing different concentrations of the viscous agent Ficoll (0вЂ“15%) and by attaching 60В nm diameter gold spheres to hooks of strains that lack flagellar filaments ( 9) and monitoring rotation rates in motility medium. For comparison, we also measured the torqueвЂ“speed relationship with the same strain lacking the plasmid, which is locked in CCW rotation. Introduction of a plasmid that encodes wild-type CheY that can be induced to high levels with isopropyl ОІ-D-thiogalactoside (IPTG) enables CW rotation.

This strain is deleted for the genes that encode the response regulator, CheY, and its phosphatase, CheZ, as well as the adaptation enzymes CheR and CheB. Here, we measured the torqueвЂ“speed relationship for an E.

But why a linear torqueвЂ“speed relationship might be optimum for the latter purpose we do not know. We speculate that CCW rotation might be optimized for runs, with higher speeds increasing the ability of cells to sense spatial gradients, whereas CW rotation might be optimized for tumbles, where the object is to change cell trajectories. We obtain similar results for wild-type cells by reexamining data collected in previous work. We find that the torque decreases linearly with speed, a result remarkably different from that for CCW rotation. Here, we measure the torqueвЂ“speed relationship for cells that express large amounts of CheY and only spin their motors CW. This result is consistent with a вЂњpower-strokeвЂќ mechanism for torque generation. In this case, the torque declines slightly up to an intermediate speed called the вЂњknee speedвЂќ after which it falls rapidly to zero. Previous measurements of the torqueвЂ“speed relationship have been made with cells lacking the response regulator CheY that spin their motors exclusively CCW. The relationship between torque and speed is one of the most important measurable characteristics of the motor, used to distinguish specific mechanisms of motor rotation. Rotation in either direction has been thought to be symmetric and exhibit the same torques and speeds. Cells of Escherichia coli are able to swim up gradients of chemical attractants by modulating the direction of rotation of their flagellar motors, which spin alternately clockwise (CW) and counterclockwise (CCW).

Clockwise rotation