The synthesis of an enzyme in response to a substance (substrate) is called turning on or induction. A structural gene is induced to transcribe an mRNA and then the mRNA translates a protein.
Genes, whose expressions are regulated in this manner, are called inducible genes and their products, if enzymes, are inducible enzymes. The substrate is known as an inducer.
Generally speaking, enzymes of the catabolic pathways are inducible).Inducible Operon model, (a) The repressor alone forms an activated dimer, which binds to the operator inhibiting the RNA polymerase binding to the promoter, thereby inhibiting the expression (transcription) of the structural genes, (b) The repressor, in the presence of an inducer (lactose) forms repressor-inducer complex.
This complex forms an inactivated dimer, which does not bind to the operator. The RNA polymerase binds to the promoter, thereby bringing about the expression of the structural genes.
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Conversely, when the expression of a gene is turned off in response to a substance, the process is called repression. The genes, whose expression is regulated in this manner, are called repressible genes and the enzymes, repressible enzymes. The repressing substance is known as the co-repressor. The enzymes of anabolic pathways are repressible. Induction and repression occur at the level of transcription.
Repressible Operon model, (a) the repressor, in the presence of a co-repressor (tryptophan) forms a repressor-corepressor complex.
This complex forms an activated dimer, which binds to the promoter, thereby inhibiting the expression of the structural genes; (b) The repressor alone forms an inactivated dimer, which does not bind to the promoter, thereby bringing about the expression of the structural genes.