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Unformatted text preview: Receptor sensitivity in bacterial chemotaxis Victor Sourjik and Howard C. Berg* Department of Molecular and Cellular Biology, Harvard University, Cambridge, MA 02138; and Rowland Institute for Science, Cambridge, MA 02142 Contributed by Howard C. Berg, November 2, 2001 Chemoreceptors in Escherichia coli are coupled to the flagella by a labile phosphorylated intermediate, CheY P. Its activity can be inferred from the rotational bias of flagellar motors, but motor response is stochastic and limited to a narrow physiological range. Here we use fluorescence resonance energy transfer to monitor interactions of CheY P with its phosphatase, CheZ, that reveal changes in the activity of the receptor kinase, CheA, resulting from the addition of attractants or repellents. Analyses of cheR and or cheB mutants, defective in receptor methylation demethylation, show that response sensitivity depends on the activity of CheB and the level of receptor modification. In cheRcheB mutants, the concentration of attractant that generates a half-maximal re- sponse is equal to the dissociation constant of the receptor. In wild-type cells, it is 35 times smaller. This amplification, together with the ultrasensitivity of the flagellar motor, explains previous observations of high chemotactic gain. Escherichia coli fluorescence fluorescence resonance energy transfer B acteria move up spatial gradients of chemical attractants in a biased random walk, now running smoothly, now tumbling randomly, extending runs that carry them in a favorable direction (1). The central point of regulation in the chemotaxis signal transduction pathway is the level of phosphorylation of the diffusible signaling protein CheY (2). CheY is phosphorylated by the kinase CheA that is coupled to specific receptors (3). CheY P binds to FliM, a component of the switch complex, at the cytoplasmic face of the flagellar motor and modulates the direction of motor rotation (46). The phosphatase CheZ binds to CheY P and accelerates its dephosphorylation (7, 8). The receptor is thought to exist in two states, active (activating CheA) and inactive (inhibiting CheA) (9). Attractant binding lowers the probability of the active state, decreasing the level of CheY P. Attractant removal raises the probability of the active state, increasing the level of CheY P. Adaptation is by receptor methylation by CheR and demethylation by CheB (1012). Receptor methylation compensates for attractant binding, with the modified receptor more likely to be in the active state. Thus, CheR promotes adaptation to increasing levels of attractants, and CheB promotes adaptation to decreasing levels of attract- ants. Two major chemoreceptors exist in Escherichia coli : the aspartate receptor, Tar, and serine receptor, Tsr. Tar has four methylation sites, which are glutamates; however, two of these are expressed as glutamines, deamidated by CheB. The glu- tamines are thought to be physiologically equivalent to methyl- ated glutamates (13).ated glutamates (13)....
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