The role of effectors in positive regulation. The binding of effectors to activators can (a) inhibit or (b) enhance activation. In (a), the activator binds in the absence of the effector and transcription proceeds; when the signal is present, the activator dissociates and transcription is inhibited. In (b), the activator binds in the presence of the signal to stimulate transcription. The activator dissociates and transcription ceases only when the signal is removed (not shown).