Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session V7: Noise in Biological Systems |
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Sponsoring Units: GSNP DBP Chair: Alex van Oudenaarden, Massachusetts Institute of Technology Room: Baltimore Convention Center 307 |
Thursday, March 16, 2006 11:15AM - 11:51AM |
V7.00001: Fluctuation-adaptation relation in bacterial chemotaxis Invited Speaker: I will present experiments and computer simulations demonstrating, at the single cell level, that the expression of key chemotaxis proteins is fine-tuned to values that maximize fluctuations and the chemotactic response. [Preview Abstract] |
Thursday, March 16, 2006 11:51AM - 12:27PM |
V7.00002: Noise effects in bacterial motor switch Invited Speaker: The clockwise (CW) or counter clockwise (CCW) spinning of bacterial flagellar motors is controlled by the concentration of a phosphorylated protein CheY-P. In this talk, we represent the stochastic switching behavior of a bacterial flagellar motor by a dynamical two-state (CW and CCW) model, with the energy levels of the two states fluctuating in time according to the variation of the CheY-P concentration in the cell. We show that with a generic normal distribution and a modest amplitude for CheY-P concentration fluctuations, the dynamical two-state model is capable of generating a power-law distribution (as opposed to an exponential Poisson-like distribution) for the durations of the CCW states, in agreement with recent experimental observations of Korobkova et al (Nature, 428, 574(2004)). In addition, we show that the power spectrum for the flagellar motor switching time series is not determined solely by the power-law duration distribution, but also by the temporal correlation between the duration times of different CCW intervals. We point out the intrinsic connection between anomalously large fluctuations of the motor output and the overall high gain of the bacterial chemotaxis system. Suggestions for experimental verification of the dynamical two-state model will also be discussed. [Preview Abstract] |
Thursday, March 16, 2006 12:27PM - 1:03PM |
V7.00003: Noise limitations on E. Coli cell division accuracy Invited Speaker: Accurate cell division in E. Coli requires the Min family of proteins. Remarkable, MinD and MinE exhibit spatial oscillations, resulting in a minimum of MinD near the center of the cell. This minimum is thought to be the signal for assembly of the cell division machinery. Although deterministic models reproduce many observed features, a stochastic treatment is needed to test the extent to which finite particle number noise limits the accuracy of this determination. This talk describes a stochastic dynamics based simulation of this system; our results are then compared to the measured cell division accuracy. [Preview Abstract] |
Thursday, March 16, 2006 1:03PM - 1:39PM |
V7.00004: Physical Limits to Biochemical Signaling Invited Speaker: Many crucial biological processes operate with surprisingly small numbers of molecules, where the impact of noise associated with these small numbers is potentially significant. Over twenty--five years ago, Berg and Purcell [1] showed that bacterial chemotaxis, where a single celled organism must respond to small changes in concentration of chemicals outside the cell, is limited directly by molecule counting noise, and that aspects of the bacteria's behavioral and computational strategies must be chosen to minimize the effects of this noise. We revisit and generalize their arguments to estimate the physical limits to biochemical signaling, where a ligand interacts with its receptor or cluster of receptors within the cell, and argue that recent experiments are consistent with performance approaching these limits [2]. \newline \newline [1] H. C. Berg and E. M. Purcell, {\em Biophysical Journal} {\bf 20}, 193 (1977). \newline [2] W. Bialek and S. Setayeshgar, {\em Proc. Natl. Acad. Sci.} {\bf 102}, 10040 (2005). [Preview Abstract] |
Thursday, March 16, 2006 1:39PM - 2:15PM |
V7.00005: Gene expression noise and robustness of signaling in bacterial chemotaxis Invited Speaker: Stochastic variations in protein levels are one of the major sources of noise affecting biological networks. Since networks involved in gene regulation and signal transduction must have a defined input-output relation, they can be expected to have undergone evolution for inherent robustness against such perturbations. Chemotaxis of a model bacterium \textit{Escherichia coli} -- a mechanism that allows motile cells to follow chemical gradients in the environment -- has one of the most thoroughly studied signaling networks in biology. Combining theoretical and experimental analysis, we investigated robustness of this network to intercellular variations in expression levels of chemotaxis proteins, or gene expression noise. The single-cell levels of different chemotaxis proteins showed strong co-variation, which implies that stochastic variations in transcriptional control are the main source of the noise. We demonstrated that the pathway is indeed robust to such kind of perturbations by testing the effect of concerted overexpression of all chemotaxis proteins on the pathway output. Using computer simulations and theoretical analysis, we determined the network design features responsible for robustness and showed that the experimentally established network in \textit{Escherichia coli} has the smallest topology that is sufficiently robust to allow a majority of the individuals in a population to maintain a correct pathway output. [Preview Abstract] |
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