Fig. 1: Conceptual design of the synthetic cascading bistable switches (Syn-CBS) circuit.

a Diagram of the Syn-CBS circuit, in which two self-activation modules mutually activate each other. The araC self-activation in Module 1 (M1), regulated by L-ara, is designed to achieve one bistable switch. The luxR self-activation in Module 2 (M2), regulated by C6, is designed to achieve another bistable switch. b, c Phase plane analysis shows the two different expected cell fate transition paths depending on the strength of the links between the two switches. b A weak M1-to-M2 link and a strong M2-to-M1 link lead to a cell fate transition from a RFP-low/GFP-low state (black circle), to a RFP-low/GFP-high state (green circle), and then to a RFP-high/GFP-high state (yellow circle). c A strong M1-to-M2 link and a weak M2-to-M1 link lead to a cell fate transition from a RFP-low/GFP-low state (black circle), to a RFP-high/GFP-low state (red circle), and then to a RFP-high/GFP-high state (yellow circle). The nullclines of M1 and M2 are shown in green and red, respectively. The vector field of the system is represented by small arrows, where the color is proportional to the field strength. The three cell fates are indicated by filled circles at the intersections of the two nullclines.