  Flagellated bacteria, such as Escherichia coli, perform directed motion in
gradients of concentration of attractants and repellents in a process called
chemotaxis. The E. coli chemotaxis signaling pathway is a model for signal
transduction, but it has unique features. We demonstrate that the need for fast
signaling necessitates high abundances of the proteins involved in this
pathway. We show that further constraints on the abundances of chemotaxis
proteins arise from the requirements of self-assembly, both of flagellar motors
and of chemoreceptor arrays. All these constraints are specific to chemotaxis,
and published data confirm that chemotaxis proteins tend to be more highly
expressed than their homologs in other pathways. Employing a chemotaxis pathway
model, we show that the gain of the pathway at the level of the response
regulator CheY increases with overall chemotaxis protein abundances. This may
explain why, at least in one E. coli strain, the abundance of all chemotaxis
proteins is higher in media with lower nutrient content. We also demonstrate
that the E. coli chemotaxis pathway is particularly robust to abundance
variations of the motor protein FliM.
