ABSTRACT

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Synthetic biology aims to engineer novel cellular functions by assembling wellcharacterised molecular parts (i.e. nucleic acids and proteins) into biological ‘devices’ that exhibit predictable behaviour. For instance, a genetic toggle switch that responds to transient input signals, similar to how a machine might respond to an ‘on/off’ switch, would allow long-term maintenance of protein expression without the need for constant drug administration. This goal was rst demonstrated by seminal work in bacteria where a simple bi-stable switch was built using two mutually repressive genes. Each gene encoded a transcriptional repressor of the other and each repressor was blocked by a chemical input. This system could be switched between two stable states, that is, ‘gene 1 on, gene 2 off’ versus ‘gene 1 off, gene 2 on’. Gardner et al. (2000) used mathematical modelling to predict that such bi-stability is feasible for any set of promoters or repressors as long as they full certain conditions, such as balanced promoter strengths. Kramer et al. (2004) demonstrated that such genetic toggle switches could also be established in Chinese hamster ovary (CHO) cells. Designing synthetic circuits to operate reliably in the context of differentiating and morphologically complex cells still presents unique challenges and opportunities for progress in mammalian synthetic biology.