Figure 3.

Epistatic patterns underlie local regulatory architectures. Three alternative regulatory modules involving a pair of gene products (X and Y) are shown in (A), where X represses the activator Y (1), X and Y cooperate to activate gene expression (2), and X and Y act in parallel/redundantly (3). In the simplified language of Boolean logic, these are indicated by ‘AND’ and ‘OR’ relationships. Examples of these behaviors involving regulation of nitrogen catabolism (B) and galactose metabolism (C) are shown. See text for a detailed interpretation of the individual interactions.