Figure 1.

General class of two-gene circuits with one activator and one repressor. The species represent activator mRNA, X₁; nascent activator protein, X_A; mature activator protein, X₂; repressor mRNA, X₃; nascent repressor protein, X_R; and mature repressor protein, X₄. Barbed arrows represent stimulatory influences; blunt arrows represent inhibitory influences. Arrows ending on the shaft of other arrows represent influence on a given process; horizontal arrows represent mass flow. The alternative modes of transcription control are shown inside the large dashed boxes. The alternatives include two dual, one single and one constitutive mode of transcription control. The π and δ are binary indices that define the mode of transcriptional control. The primary mode of transcriptional control involves an activator (π=1) or a repressor (π=0). The transcriptional control involves dual (δ=1) or single (δ=0) regulators. The combination δ₁=0 and π₁=1 (or δ₃=0 and π₃=0) indicates a constitutive mode of transcription control for the activator (or repressor). For example, the relaxation oscillator design is represented by π₁=1, δ₁=1, π₃=1 and δ₃=0. Note: the single modes of transcriptional control of the activator (Box 1) and repressor (Box 2) are different.