| Description |
Bacteria alter their locomotive behavior upon detecting changes in their chemical environment. In the absence of a chemical gradient, cells randomly interrupt periods of smooth swimming with chaotic tumbling events in a search for nutrients. Upon encountering an attractant or repellent gradient, the cells increase or decrease the probability of tumbling in order to pursue a favorable direction (1). This chemotactic response is controlled by a set of proteins that detect stimuli, convey signals to the bacterium's flagella and mediate the flagellar switching response to effect directed cell movement. Bacteria such as Escherichia coli are propelled by flagellar motors that rotate the helical filaments in either a counterclockwise (CCW) or clockwise (CW) direction. During CCW rotation, the flagella twist into a helical bundle to propel the bacterium forward. During CW rotation, the flagellar bundle flies apart, causing the cell to tumble chaotically (Figure 1a). Changes in flagellar rotation are mediated by the flagellar switch proteins, FliG, FliM and FliN, that are located at the base of each flagellum. A cytoplasmic protein called CheY has been implicated in the regulation of this flagellar switching response. This soluble protein transmits a signal to the flagella upon ligand binding to the chemoreceptors of the cell to allow the bacterium to respond to the environmental stimuli. Specifically, CheY modulates a switch in flagellar rotation to clockwise, which generates abrupt turns in cellular movement. Some form of communication must be occurring between the Fli proteins and CheY in order to trigger the switching response (Figure 2). Many of the molecular mechanisms responsible for the chemotactic response can be revealed by analyzing mutants that are defective in chemotactic ability. Functional interactions between proteins in the chemotactic pathway can be detected through genetic methods that involve the analysis of such mutants. This work employs genetic techniques to evaluate the interaction between CheY and the flagellar switch proteins and provides further evidence for the proposal that the CheY protein communicates with the fli gene products through direct contact (2). |
