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Emergent bistability by a growth-modulating positive feedback circuit

Abstract

Synthetic gene circuits are often engineered by considering the host cell as an invariable 'chassis'. Circuit activation, however, may modulate host physiology, which in turn can substantially impact circuit behavior. We illustrate this point by a simple circuit consisting of mutant T7 RNA polymerase (T7 RNAP*) that activates its own expression in the bacterium Escherichia coli. Although activation by the T7 RNAP* is noncooperative, the circuit caused bistable gene expression. This counterintuitive observation can be explained by growth retardation caused by circuit activation, which resulted in nonlinear dilution of T7 RNAP* in individual bacteria. Predictions made by models accounting for such effects were verified by further experimental measurements. Our results reveal a new mechanism of generating bistability and underscore the need to account for host physiology modulation when engineering gene circuits.

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Figure 1: Bistability induced by a T7 RNAP* positive feedback circuit.
Figure 2: Circuit activation reduces bacterial growth rates.
Figure 3: Interplay between growth modulation and the positive feedback loop can lead to bistability.
Figure 4: Modulation of circuit dynamics by initial culture density.
Figure 5: Modulation of circuit dynamics.

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Acknowledgements

We thank M. Salehi, G. Yao, J. Wong, H. Song, T.J. Lee, Q. Wang, J. Niemi, I. Molineux, M. Wall and W. Studier for discussions or comments; M. Cook for assistance with flow cytometry; W. Thompson, E. Soderblom and L. Dubois for assistance with mass spectrometry; M. Elowitz (California Institute of Technology), R. Weiss (Princeton University) and Y. Yokobayashi (University of California, Davis) for plasmids and bacterial strains; and T. Hwa for discussions and for sharing unpublished results. This work was partially supported by the US National Science Foundation (BES-0625213), the US National Institutes of Health (1P50GM081883), a DuPont Young Professorship (L.Y.), a David and Lucile Packard Fellowship (L.Y.) and a Medtronic Fellowship (C.T.).

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Authors and Affiliations

Authors

Contributions

C.T. conceived research, designed and performed both modeling and experimental analyses, interpreted results and wrote the manuscript. P.M. purified and analyzed T7 RNAP* and assisted in manuscript revisions. L.Y. conceived research, assisted in research design and data interpretation and wrote the manuscript. All authors approved the manuscript.

Corresponding author

Correspondence to Lingchong You.

Supplementary information

Supplementary Text and Figures

Supplementary Methods and Supplementary Results (PDF 1919 kb)

Supplementary Video 1

A time lapse movie corresponding to colony 1 in Fig. 1b, for 220 minutes of growth. (MOV 393 kb)

Supplementary Video 2

A time lapse movie of another colony, for 335 minutes of growth under the same condition as in Supplementary Video 1. (MOV 221 kb)

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Tan, C., Marguet, P. & You, L. Emergent bistability by a growth-modulating positive feedback circuit. Nat Chem Biol 5, 842–848 (2009). https://doi.org/10.1038/nchembio.218

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