It is mentioned that the turning on or off of structural genes is mediated by a repressor protein encoded by another gene known as the regulator gene.
Two types of gene regulations are characterized based on the mode of action of the repressor protein: (a) negative regulation and (b) positive regulation.
(a) Negative Regulation:
Turning off of the structural gene expression by a repressor protein is known as negative regulation. In an inducible system, the repressor protein alone binds to the operator and inhibits transcription in the absence of an inducer.
However, in the presence of an inducer, the repressor protein binds to it and undergoes a conformational change. The resultant repressor-inducer complex can not bind to the operator ancKhereby, initiates transcription. (Example: lac-operon)
(b) POSITIVE REGULATION:
Turning on of the structural gene expression by the active repressor protein is known as positive regulation.
In a repressible system, the repressor protein alone can not bind to the operator and thus, facilitates the binding of the RNA polymerase and transcription. The repressor-co-repressor complex undergoes a conformational change in the structure. This complex can bind to the site and inhibit transcription.