Other DNA sequences in and near the operon control the operon s expression. The presence or absence of the enzyme s substrate turns on or off the controls. When the lac operon is turned off, a regulator gene is transcribed and translated into a repressor protein. The repressor protein binds with the operator region of the operon, repressing the transcription of the genes further along the operon 6. This permits the expression of the remainder of the operon.
Expression continues until the substrate is used up.
Then the repressor is free to repress the operator, and the operon turns off as above . The lac operon is repressed when lactose is absent and transcribed when lactose is present. Another operon, the trp operon, is transcribed when tryptophan is absent and repressed when tryptophan is present. The enzymes expressed by trp help synthesize tryptophan 9. A third type of operon uses activators rather than repressors.