Signal processing times in bacterial chemotaxis

Abstract

The bacterium Escherichia coli responds to changes in the concentrations of various chemicals in its environment¹. A cell swims along a smooth trajectory (runs), moves erratically for a brief time (tumbles) and then runs again, choosing a new direction at random². If a run happens to carry the cell up a gradient of an attractant (such as aspartate, serine and certain sugars), the occupancy of the appropriate chemoreceptor increases with time^3,4 and a signal is sent to the flagellar motors that increases their counterclockwise bias⁵. On the average, this extends favourable runs and the cell moves up the gradient. The receptors for aspartate and serine^6–8 are proteins found in the cytoplasmic membrane, known as methyl-accepting chemotaxis proteins^9,10, and are the products of the tar and tsr genes¹¹. A cell can adapt to sustained changes of receptor occupancy by carboxymethylating these proteins¹²; it is not known, however, how these proteins signal the flagellar motors or how the signal controls the direction of flagellar rotation. Products of several che genes involved in signalling and adaptation have been identified^13,14, but with the exception of a methyltransferase¹⁵ (the cheR product) and a demethylase¹⁶ (the cheB product), their functions are largely unknown. In an attempt to learn more about the events that trigger a chemotactic response, we have now exposed cells to rapid changes in the concentration of attractants and repellents and measured the time required for flagellar reversal. In wild-type cells and in cells containing a cheR–cheB deletion, the response latency is ∼0.2 s. In cheZ mutants, it is much longer.