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Science 334 (6054): 366-369

Copyright © 2011 by the American Association for the Advancement of Science

Stochastic Pulse Regulation in Bacterial Stress Response

James C. W. Locke,* Jonathan W. Young,* Michelle Fontes, María Jesús Hernández Jiménez, Michael B. Elowitz{dagger}

Abstract: Gene regulatory circuits can use dynamic, and even stochastic, strategies to respond to environmental conditions. We examined activation of the general stress response mediated by the alternative sigma factor, {sigma}B, in individual Bacillus subtilis cells. We observed that energy stress activates {sigma}B in discrete stochastic pulses, with increasing levels of stress leading to higher pulse frequencies. By perturbing and rewiring the endogenous system, we found that this behavior results from three key features of the {sigma}B circuit: an ultrasensitive phosphorylation switch; stochasticity ("noise"), which activates that switch; and a mixed (positive and negative) transcriptional feedback, which can both amplify a pulse and switch it off. Together, these results show how prokaryotes encode signals using stochastic pulse frequency modulation through a compact regulatory architecture.

Howard Hughes Medical Institute, Division of Biology and Bioengineering, Broad Center, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA.

* These authors contributed equally to this work.

{dagger} To whom correspondence should be addressed. E-mail: melowitz{at}

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