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.
ADVERTISEMENTS:
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.