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One of the ideas behind synthetic biology is to engineer artificial systems in biological cells. This can be achieved by using well-characterized and simplified parts that can be found in nature. We will be using parts found in different systems in nature to design a bistable switch, an idea stolen from the world of electronics. A simplified version of our bistable switch is illustrated in Figure 1.

Figure 1: Simplified version of our bistable swicth

This simplified version of our switch, the repressor-repressor switch is analogous SR flip-flop of digital electronics. The SR (set-reset) flip-flop is constructed from a pair of cross-linked NAND or NOR logic gates. An SR flip-flop circuit built with NOR logic gates is illustrated in Figure 2.

Figure 2: SR flip-flop

The circuit, built with the pair of cross-linked NOR logic gates, functions such that output from a gate will only be true if no input is given as demonstrated by the truth table in Table 1. The biological interpretation of this logical operation shows is that the repression by a protein can be prevented either by an inducer (or anti-repressor protein as in our case) (Input 1/2) or by repression of its synthesis by another repressor-protein (Output 2/1). The elements in the circuit do not possess memory in them, but by cross-linking the elements, the circuit is capable of “remembering” and holding its current state. This means that when neither Input 1 nor Input 2 (no inducers) is present, the circuit is bistable with either Output 2 or Output 1 being active (only one of the repressor-proteins are expressed), respectively.