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A stochastic version of the Barkai-Leibler model of chemotaxis receptors in{\it E. coli} is studied here to elucidate the effects of intrinsic networknoise in their conformational dynamics. It was originally proposed to explainthe robust and near-perfect adaptation of {\it E. coli} observed across a widerange of spatially uniform attractant/repellent (ligand) concentrations. Areceptor is either active or inactive and can stochastically switch between thetwo states. Enzyme CheR methylates inactive receptors while CheB demethylatesactive ones and the probability for it to be active depends on its level ofmethylation and ligandation. A simple version of the model with two methylationsites per receptor (M=2) shows zero-order ultrasensitivity (ZOU) akin to theclassical 2-state model of covalent modification studied by Goldbeter andKoshland (GK). For extremely small and large ligand concentrations, the systemreduces to two 2-state GK modules. A quantitative measure of the spontaneousfluctuations in activity (variance) estimated mathematically under linear noiseapproximation (LNA) is found to peak near the ZOU transition. The variance is aweak, non-monotonic and decreasing functions of ligand and receptorconcentrations. Gillespie simulations for M=2 show excellent agreement withanalytical results obtained under LNA. Numerical results for M=2, 3 and 4 showZOU in mean activity; the variance is found to be smaller for larger M. Themagnitude of receptor noise deduced from available experimental data isconsistent with our predictions. A simple analysis of the downstream signalingpathway shows that this noise is large enough to have a beneficial effect onthe motility of the organism. The response of mean receptor activity to smalltime-dependent changes in the external ligand concentration, computed withinlinear response theory, is found to have a bilobe form.

Ultrasensitivity and Fluctuations in the Barkai-Leibler Model of Chemotaxis Receptors in {\it Escherichia coli}