Shown in (A) is a relatively simple circuit identified by OptCircuit. Interestingly the circuit is reminiscent of the genetic toggle switch. Similar to the behavior of the toggle switch, in presence of aTc, the activity of tetR protein is suppressed. This enables the expression of lacI as shown in B. Finally, as shown in (C) in presence of IPTG, activity of lacI is suppressed which enables the expression tetR from the P
promoter. The triangles with open circles at the vertices represent the promoter elements.