Beta-galactosidase synthesis is an example of induction. A small molecule, a co-inducer , induces enzyme synthesis. Further information Repression systems, in which the presence of a small molecule, a co-repressor , turns gene expression off, also exist. Inhibition is in part due to occlusion of the promoter . In some other systems, examples of positive regulation, protein binding stimulates transcription.
Positive regulation is thought to act by altering DNA conformation.
The stimulated sensor activates the DNA-binding protein. The expression complex controlled by a single operator is called an operon . Multiple operons controlled by the same repressor or activator are called a regulon . Quiz yourself on lac operon genetics .