Induction of Lac Operon
Lac operon is a negatively controlled inducible operon; the lacZ, lacY, and lacA genes are only expressed in the presence of lactose. The lac regulator gene (I gene) encodes a repressor that is 360 amino acids long. It is an active repressor with four subunits (homo-tetramer).
The lac repressor binds to two operators—either O1 and O2 or O1 and O3—simultaneously and bends the DNA into a hairpin or a loop, respectively preventing binding of DNA pol to the promoter.
In the absence of an inducer (allolactose), the repressor binds to the lac operator, preventing RNA polymerase from catalyzing the transcription of three structural genes. A few molecules of lacZ, lacY, and lacA gene products are synthesized in the uninduced state providing a low background level of enzyme activity. The background activity is essential for induction of the lac operon because the inducer of the operon, all-lactose is derived from lactose in reaction catalyzed β-galactosidease. Once formed, allolactose is bound by the repressor, causing the release of the repressor from the operator. In this way, allolactose induces the transcription of the lacZ, lacY and lacA genes.
