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Published January 2012 | Published + Supplemental Material
Journal Article Open

Pulsed Feedback Defers Cellular Differentiation


Environmental signals induce diverse cellular differentiation programs. In certain systems, cells defer differentiation for extended time periods after the signal appears, proliferating through multiple rounds of cell division before committing to a new fate. How can cells set a deferral time much longer than the cell cycle? Here we study Bacillus subtilis cells that respond to sudden nutrient limitation with multiple rounds of growth and division before differentiating into spores. A well-characterized genetic circuit controls the concentration and phosphorylation of the master regulator Spo0A, which rises to a critical concentration to initiate sporulation. However, it remains unclear how this circuit enables cells to defer sporulation for multiple cell cycles. Using quantitative time-lapse fluorescence microscopy of Spo0A dynamics in individual cells, we observed pulses of Spo0A phosphorylation at a characteristic cell cycle phase. Pulse amplitudes grew systematically and cell-autonomously over multiple cell cycles leading up to sporulation. This pulse growth required a key positive feedback loop involving the sporulation kinases, without which the deferral of sporulation became ultrasensitive to kinase expression. Thus, deferral is controlled by a pulsed positive feedback loop in which kinase expression is activated by pulses of Spo0A phosphorylation. This pulsed positive feedback architecture provides a more robust mechanism for setting deferral times than constitutive kinase expression. Finally, using mathematical modeling, we show how pulsing and time delays together enable "polyphasic" positive feedback, in which different parts of a feedback loop are active at different times. Polyphasic feedback can enable more accurate tuning of long deferral times. Together, these results suggest that Bacillus subtilis uses a pulsed positive feedback loop to implement a "timer" that operates over timescales much longer than a cell cycle.

Additional Information

© 2012 Levine et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Received July 8, 2011; Accepted December 19, 2011; Published January 31, 2012. This work was supported by US NIH grant R01GM079771, NSF grant 0644463, and the Packard Foundation. JL was supported in part by a Rosen Fellowship. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. We thank Shaunak Sen, Avigdor Eldar, Jon Young, James Locke, and Jordi Garcia-Ojalvo for help and critical feedback during the course of this work, and members of the Elowitz Lab for general discussions. We also thank the Burkholder, Fujita, Grossman, Hoch, Lazazzera, Losick, and Rudner labs for the generous gift of B. subtilis strains. Author Contributions: The author(s) have made the following declarations about their contributions: Conceived and designed the experiments: JHL MBE JD. Performed the experiments: JHL MEF. Analyzed the data: JHL MBE. Contributed reagents/materials/analysis tools: JHL MEF MBE. Wrote the paper: JHL MBE.

Attached Files

Published - Levine2012p17380Plos_Biol.pdf

Supplemental Material - journal.pbio.1001252.s001.pdf

Supplemental Material - journal.pbio.1001252.s002.pdf

Supplemental Material - journal.pbio.1001252.s003.pdf

Supplemental Material - journal.pbio.1001252.s004.pdf

Supplemental Material - journal.pbio.1001252.s005.pdf

Supplemental Material - journal.pbio.1001252.s006.pdf

Supplemental Material - journal.pbio.1001252.s007.pdf

Supplemental Material - journal.pbio.1001252.s008.pdf

Supplemental Material - journal.pbio.1001252.s009.pdf

Supplemental Material - journal.pbio.1001252.s010.pdf

Supplemental Material - journal.pbio.1001252.s011.pdf

Supplemental Material - journal.pbio.1001252.s012.pdf

Supplemental Material - journal.pbio.1001252.s013.pdf

Supplemental Material - journal.pbio.1001252.s014.doc


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August 19, 2023
October 24, 2023