Adaptation by target remodelling

Abstract

Bacteria direct their movement in response to certain chemicals by controlling the rotation of whip-like appendages called flagella. The sensitivity of the response can be adjusted at the signal's target, the flagellar motor. See Letter p.233 Chemotaxis, the process used by bacteria to move in response to a chemical stimulus, is driven by a flagellar rotary motor that is exquisitely sensitive to intracellular levels of the signalling protein CheY-P within a narrow operating range. However, there is a large cell-to-cell variation in steady-state CheY-P concentration, prompting speculation as to whether Escherichia coli could possibly control its steady-state CheY-P concentration with sufficient precision to engage the flagellar motor when required. Howard Berg and colleagues have found an answer to this paradox. The bacterium seems to avoid the problem in a counter-intuitive way, by constantly adjusting the number of CheY-P target proteins in the motor. Such adaptive remodelling at the output end of a control process goes against the principles of good engineering practice, yet is likely to be common to many biological machines.