Explain how lactose acts as an inducer? | Jacob-Monod Hypothesis Questions & Answers

Inducer is a chemical (most often a substrate or any other metabolite) which inactivates the repressor of an operon. The inducer binds with the repressor, forming a repressor-inducer complex. This complex prevents the repressor from binding with the operator gene of the operon. This frees the operator gene so that the RNA polymerase can move from the promoter to the structural genes. The structural genes are depressed and are then transcribed, forming a piece of polycistronic mRNA. The latter is translated by ribosomes and tRNAs into enzymes (Polypeptides). The entire process is called enzyme induction. The enzyme produced in response to the presence of an inducer is called inducible enzyme.

When lactose is added to a culture of E. coli, a few lactose molecules are carried into the cell by the enzyme lactose permease, as a small amount of this enzyme is present in the cell even when the operon is not working. These few lactose molecules are converted into an active form which acts as an inducer and binds with the repressor, a product of the regulator gene. The repressor-inducer complex fails to join with the operator gene, which is turned on. The three structural genes are expressed as three enzymes to metabolise lactose to galactose and glucose. Lactose, thus, acts as the inducer for its own breakdown.

When the concentration of inducer falls, having been metabolised by the enzymes, the operator is blocked again by the repressor so that the structural genes are repressed once again. This is an example of a negative feedback mechanism.

The inducible operons generally function in catabolic pathways.

2 Answers