The role of effectors in negative regulation. The binding of signal molecules (known as effectors) to repressors can (a) relieve or (b) enhance repression. In (a), the repressor binds DNA in the absence of the effector; the external signal causes dissociation of the repressor to permit transcription. In (b), the repressor binds DNA in the presence of the signal, shutting down transcription. The repressor dissociates and transcription ensues only when the signal is removed (not shown).