POSTERS - BLAST X - University of Utah

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