Question

Explain role of inducer in lac operon with the help of diagram. What is switch on and off position in lac operon?

Answer 1

Lac operon consists of regulator gene, promoter gene, operational gene and structural gene. The E. coli bacterium carries numerous genes and these genes turn on and off as per requirement. When these genes are turned on, they undergo enzymes which metabolise the new substrate. This phenomenon is known as induction and small molecules eliciting this induction is referred to as inducers.

In this lac operon, the presence of lactose acts as an inducer. The lac operon contains a promoter, an operator and three closely related structural genes, z, y, a coding for enzymes β-galactosidase, β-galactoside permease and β- galactoside transacetylase respectively. β-galactoside permease pumps lactose into the cells whereas β-galactosidase catalyses the conversion of lactose into glucose and galactose. These genes are not expressed in the absence of lactose.

The promoter (P) for the operon is the site at which RNA polymerase binds to initiate transcription of the structural genes. The operator (O) is the site at which the protein repressor – the product of regulator gene binds. In the presence of a regulator protein, the RNA polymerase is prevented from attaching to the promoter. A regulator gene is a DNA segment independent of an operon and it synthesizes a repressor protein. This protein combines with an operator and makes it inactive. This prevents RNA polymerase from binding to the adjoining promoter (P) and from initiating transcription of the structural gene. Therefore, RNA polymerase is required to negotiate the operator before transcription can occur.

The repressor binds to the operator in the absence of a metabolite (effector molecule – lactose). When an inducer or effector molecule-lactose is added to the system, it binds to the repressor to form a complex which is unable to bind the operator. The RNA polymerase enzyme now becomes free to bind with a promoter (P) and so the operator is switched on. This initiates the transcription of structural genes, producing the three polypeptides. These enzymes bring about the metabolism of lactose into glucose and galactose.