Gene regulation: Towards a circuit engineering discipline

In a summary of the general conclusions of a 1961 Cold Spring Harbor Symposium, Monod and Jacob comment that “it is obvious from the analysis of these [bacterial genetic regulatory] mechanisms that their known elements could be connected into a wide variety of ‘circuits’ endowed with any desired degree of stability” [1]. It is remarkable that now, nearly 40 years later, with near universal acceptance of the validity of Monod and Jacob’s vision, the practice of genetic circuit engineering is still in its infancy. Two papers published recently in Nature [2,3] provide a snapshot of the current state of the discipline. Elowitz and Leibler [2] describe a genetic circuit engineered into Escherichia coli cells that oscillates asynchronously with regard to the cell-division cycle. Gardner et al. [3] describe a toggle-switch circuit that can be switched between two stable states by transient external signals. In both studies, the circuits’ qualitative performance is consistent with the predictions of relatively simple differential equation models that characterize the dynamics of production, degradation and genetic regulation. The central feature of the toggle switch is a bistable, twopromoter configuration, involving constitutively active and repressible promoters (Figure 1a, P1 and P2), that each control transcription of a repressor of the other when activated. It is intuitively clear that there are likely to be two stable situations, P1 ON or P2 ON, given comparable promoter kinetics. Gardner et al. [3] demonstrate how this inherently bistable configuration can be engineered so that external signals can flip the system from one state to the other. In one design, for example, P2 is an IPTGinducible lac promoter, R1 is a thermally unstable CI repressor, and the state of the switch is sensed by addition of a reporter gene coding for green fluorescent protein (GFP) directly downstream of the cI gene (Figure 1b). Gardner et al. [3] found that all cells in colonies grown for six hours in the presence of IPTG under conditions for CI stability remained in the P2 ON state after removal of the IPTG signal. Transiently raising the temperature so that CI was unstable for seven hours caused the cell population to switch completely to the alternative P1 ON state.