Abstract
Analysis of the system design principles of signaling systems requires model systems where all components and regulatory interactions are known. Components of the Lac and Ntr systems were used to construct genetic circuits that display toggle switch or oscillatory behavior. Both devices contain an “activator module” consisting of a modified glnA promoter with lac operators, driving the expression of the activator, NRI. Since NRI activates the glnA promoter, this creates an autoactivated circuit repressible by LacI. The oscillator contains a “repressor module” consisting of the NRI-activated glnK promoter driving LacI expression. This circuitry produced synchronous damped oscillations in turbidostat cultures, with periods much longer than the cell cycle. For the toggle switch, LacI was provided constitutively; the level of active repressor was controlled by using a lacY mutant and varying the concentration of IPTG. This circuitry provided nearly discontinuous expression of activator.
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