Question

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

Answer 1

Hint: A disaccharide that comprises glucose and galactose serves as inducer in lac operon. Switching on and off in lac operon depends on the presence of substrate, secondary messenger, and repressor.

Complete answer:
Lac operon consists of the gene regulator, gene promoter, operating gene and structural gene. It is the E.Coli bacterium that carries various genes, and these genes are turned on and off as needed. When these genes are switched on, they produce enzymes that metabolize the new substrate. This phenomenon is known as induction, and small molecules that cause this induction are referred to as inducers. The presence of lactose in this lac operon serves as an inducer.

The lac operon consists of a promoter, an operator and three closely related structural genes, z, y, a. The structural genes z, y and a, produce coding needed for processing of enzymes such as β-galactosidase, β-galactoside permease and β-galactoside transacetylase, respectively. The main action of β-galactoside permease is that it pumps lactose into the cells. The enzyme β-galactosidase catalyzes the conversion of lactose into glucose and galactose. When lactose is not present, these genes do not express themselves.

The operon promoter (P) is the site where RNA polymerase binds to initiate transcription of structural genes. The operator (O) is the site where the protein repressor which is the product of the regulator gene binds.

In the presence of a regulatory protein, RNA polymerase is prevented from binding to the promoter. The regulator gene is an operon-independent segment of DNA and synthesizes the repressor protein. This protein interacts with the operator and leaves it inactive. This prevents RNA polymerase from binding to the adjacent promoter (P) and from initiating the transcription of the structural gene. RNA polymerase is therefore necessary to negotiate with the operator prior to the transcript.

In the absence of a metabolite (the effector molecule – lactose), the repressor binds to the operator site. When an inducer or effector molecule-lactose is added to the device, it binds to the repressor to form a complex that the operator cannot bind. The RNA polymerase enzyme is now free to bind to the promoter (P) so that the operator is turned on. This initiates the transcription of the structural genes, which contain the three polypeptides. These enzymes enable the conversion of lactose to glucose and galactose.

Therefore, the switch on position in lac operon is the position when the operon is synthesizing all the lac genes in order to carry out the breakdown of lactose into glucose and galactose. On the other hand, the lac operon gets switched off when there is a lot of glucose in the cell and lactose is not present. The absence of lactose causes the binding of the repressor to halt, which in turn is involved in switching off the operon and no more enzymes could be synthesized.

Note: In the absence of inducer lactose, the regulatory gene, R produces a repressor protein that binds to the operator site and prevents transcription of structural genes.
When inducer lactose is added into the medium, it binds to the repressor and inhibits its binding to the operator. The operator then induces the RNA polymerase to bind to the promoter site and transcribe mRNA via the processing of structural genes and enzymes.