It is distinguished between regulator genes (ochre) and structural genes (light blue). A group of structural genes (an operon) is preceded by a promoter (green: starting site of the DNA-dependent RNA polymerase) and an operator (o: binding site of a regulator [activator or repressor]). At the top of the picture: substrate-induced control of transcription. The repressor encoded by the regulator gene binds to o and inhibits thus the transcription of the structural genes. It can be inactivated by a substrate, so that the transcription of the structural genes (newly synthesized RNA: violet line) is possible. The enzymes encoded by the structural genes are involved in the breakdown of the mentioned substrate (Jacob-Monod-model). In the middle: An inactive activator is encoded by the regulator gene. The rate of transcription of the structural genes is low. The activator itself is activated by a substrate. In its active state does it bind the promoter and increases thus the activity of the DNA-dependent RNA-polymerase, the rate of transcription is enhanced. At the bottom: The regulator gene encodes an active inducer, the transcription rate of the structural genes is high. The inducer is inactivated by binding to a substrate molecule.