Figure 1.

Schematic diagrams of the idealized (a) activator-titration circuit (ATC) and (b) repressor-titration circuit (RTC). Protein X is a transcription factor and Y is an inhibitor that can irreversibly associate with X to form an inactive complex. In the ATC, X is an activator that can sequentially bind multiple DNA sites in the promoter of gene Y and increase transcription of the inhibitor. In the RTC, X is a repressor that can bind its own promoter and repress transcription. (Online version in colour.)