POSTERS - BLAST X - University of Utah
POSTERS - BLAST X - University of Utah
POSTERS - BLAST X - University of Utah
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<strong>BLAST</strong> X Thurs. Evening Session<br />
PROBING ADAPTATION KINETICS IN VIVO BY FLUORESCENCE RESONANCE ENERGY<br />
TRANSFER<br />
Thomas S. Shimizu 1 , Yuhai Tu 2 and Howard C. Berg 1<br />
1 Department <strong>of</strong> Molecular & Cellular Biology, Harvard <strong>University</strong>, Cambridge, MA 02138.<br />
2 T. J. Watson Research Center, IBM, Yorktown Heights, NY 10598.<br />
Bacteria sense spatial gradients by taking time derivatives <strong>of</strong> ligand concentrations<br />
measured during runs <strong>of</strong> a random walk 1 . The remarkable sensitivity to shallow gradients in<br />
Escherichia coli has been explained mainly by cooperativity between receptors and<br />
ultrasensitivity <strong>of</strong> the flagellar motor. We have revisited the experimental findings <strong>of</strong> Block,<br />
Segall and Berg 2 , where the chemotactic response <strong>of</strong> tethered cells to time-varying stimuli were<br />
characterized quantitatively. A simple theoretical model 3 that combines robust adaptation 4 with<br />
an allosteric model <strong>of</strong> receptor cooperativity 5-7 can explain the general features <strong>of</strong> responses to<br />
temporal ramps and oscillatory stimuli.<br />
A notable feature <strong>of</strong> this model is that the steady-state amplitude <strong>of</strong> responses to<br />
exponential ramps do not depend on the degree <strong>of</strong> receptor cooperativity (the parameter N <strong>of</strong><br />
an MWC-type allosteric model 5 ). The time required to reach this steady state, however, depends<br />
inversely on N, so cooperativity speeds up computation <strong>of</strong> the derivative signal, but does not<br />
determine its amplitude. The latter is instead determined by the adaptation kinetics, and this<br />
relation allows us to infer quantitative characteristics <strong>of</strong> adaptation in vivo from measured rampresponse<br />
data.<br />
Here we present novel experiments in which the chemotactic responses <strong>of</strong> E. coli<br />
populations during time-varying stimuli are monitored by fluorescence resonance energy<br />
transfer 8 (FRET). This approach is far more efficient than the earlier experiments <strong>of</strong> Block et al. 2 ,<br />
in which the chemotactic responses <strong>of</strong> individual cells were characterized through the stochastic<br />
output <strong>of</strong> the motor. We find that the sensitivity <strong>of</strong> E. coli to gradients depends strongly on<br />
temperature, and using our model framework, we analyze how ultrasensitvity in the adaptation<br />
system 9 contributes to gradient sensitivity in vivo.<br />
REFERENCES<br />
1. Berg, H. C. & Brown, D. A. Chemotaxis in Escherichia coli analysed by three-dimensional<br />
tracking. Nature 239, 500-4 (1972).<br />
2. Block, S. M., Segall, J. E. & Berg, H. C. Adaptation kinetics in bacterial chemotaxis. J Bacteriol<br />
154, 312-23 (1983).<br />
3. Tu, Y., Shimizu, T. S. & Berg, H. C. Modeling the chemotactic response <strong>of</strong> Escherichia coli to<br />
time-varying stimuli. Proc Natl Acad Sci U S A 105, 14855-60 (2008).<br />
4. Barkai, N. & Leibler, S. Robustness in simple biochemical networks. Nature 387, 913-7 (1997).<br />
5. Monod, J., Wyman, J. & Changeux, J. P. On the Nature <strong>of</strong> Allosteric Transitions: A Plausible<br />
Model. J Mol Biol 12, 88-118 (1965).<br />
6. Mello, B. A. & Tu, Y. An allosteric model for heterogeneous receptor complexes: Understanding<br />
bacterial chemotaxis responses to multiple stimuli. Proc Natl Acad Sci U S A 102, 17354-9<br />
(2005).<br />
7. Keymer, J. E., Endres, R. G., Skoge, M., Meir, Y. & Wingreen, N. S. Chemosensing in<br />
Escherichia coli: two regimes <strong>of</strong> two-state receptors. Proc Natl Acad Sci U S A 103, 1786-91<br />
(2006).<br />
8. Sourjik, V., Vaknin, A., Shimizu, T. S. & Berg, H. C. In Vivo Measurement by FRET <strong>of</strong> Pathway<br />
Activity in Bacterial Chemotaxis. Methods Enzymol 423, 363-91 (2007).<br />
9. Emonet, T. & Cluzel, P. Relationship between cellular response and behavioral variability in<br />
bacterial chemotaxis. Proc Natl Acad Sci U S A 105, 3304-9 (2008).<br />
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