  Genetically encoded regulatory circuits control biological function. A major
focus of systems biology is to understand these circuits by establishing the
relationship between specific structures and functions. Of special interest are
multifunctional circuits that are capable of performing distinct behaviors
without changing their topology. A particularly simple example of such a system
is the AC-DC circuit. Found in multiple regulatory processes, this circuit
consists of three genes connected in a combination of a toggle switch and a
repressilator. Using dynamical system theory we analyze the available dynamical
regimes to show that the AC-DC can exhibit both oscillations and bistability.
We found that both dynamical regimes can coexist robustly in the same region of
parameter space, generating novel emergent behaviors not available to the
individual subnetwork components. We demonstrate that the AC-DC circuit
provides a mechanism to rapidly switch between oscillations and steady
expression and, in the presence of noise, the multi-functionality of the
circuit offers the possibility to control the coherence of oscillations.
Additionally, we provide evidence that the availability of a bistable
oscillatory regime allows the AC-DC circuit to behave as an excitable system
capable of stochastic pulses and spatial signal propagation. Taken together
these results reveal how a system as simple as the AC-DC circuit can produce
multiple complex dynamical behaviors in the same parameter region and is well
suited for the construction of multifunctional synthetic genetic circuits.
Likewise, the analysis reveals the potential of this circuit to facilitate the
evolution of distinct patterning mechanisms.
