Emergent bistability by a growth-modulating positive feedback circuit

Abstract

A synthetic gene circuit is often engineered by considering the host cell as an invariable "chassis". Circuit activation, however, may modulate host physiology, which in turn can drastically 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 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 novel mechanism of generating bistability and underscore the need to account for host physiology modulation when engineering gene circuits.

A bistable genetic switch based on designable DNA-binding domains

“…The design of a bistable switch based on linear response elements has been experimentally demonstrated in a few special cases, such as using a T7 RNA polymerase that activates its own expression. In that case, the retardation of cell growth due to the circuit activation caused nonlinear dilution of the T7 RNA polymerase in individual bacteria, which, combined with the feedback, resulted in bistability 38 . A bistable switch with noncooperative elements has also been demonstrated based on sequestration and positive feedback using sigma and anti-sigma factors 39 .…”

Transcription factor levels enable metabolic diversification of single cells of environmental bacteria

“…This coupling has been shown to underlie phenotypic variability through bistability in bacteria evolving antibiotic resistance (Deris et al, 2013) as well as in stem cell differentiation (Kueh et al, 2013). The bistable behaviour observed in these examples and others (Sureka et al, 2008;Tan et al, 2009;Ghosh et al, 2011) is of deterministic type, due to a growth mediated positive feedback in the expression of certain genes that stabilizes particular expression states. Here we inspect a different effect mediated by the growth status of the cell: noise-induced bimodality in a bacterial metabolic response.…”

Modularity, context-dependence, and insulation in engineered biological circuits

“…Changes on cell growth and gene expression levels have very subtle consequences on the operation of gene circuits. Growth rate change during circuit operation leads to variations in protein dilution rates, which, in turn, bring about unexpected phenotypes such as bistability [54]. Further computational studies have found that, because modules share limited amounts of transcriptional/translational resources, the expression levels of proteins in seemingly unconnected modules become surprisingly coupled [55,56] and the dynamical behavior of simple activation cascades becomes unpredictable [57].…”