Figure 1.

Three architectures and two modes for the molecular control of transcription manifested by two-gene circuits with an extra first-order process involved in the maturation of the functional form for each regulator. Architectures: (a) Negative feedback loop, (b) positive and negative feedback loops, and (c) positive and nested negative feedback loops. Combinations of two modes of molecular control: (d) activator-primary and (e) repressor-primary. The symbols are the following: NA, nucleic acid precursors; mRNA, messenger RNA; A, activator; R, repressor.