Define an operon. Giving an example, explain an Inducible operon.

Operon:

Operon is a cluster of genes which have a common promoter, they are transcribed as a single mRNA. Examples of operon include lac operon and trp operon.

lac operon:

lac operon is an inducible operon present in Escherichia coli.

● It encodes the genes involved in the catabolism of lactose.

● It is induced by the sugar lactose or allolactose.

● It has three structural genes like lac z, lac y and lac a and three regulatory genes like promoter, operator and repressor.

Role of structural genes in lac operon:

The lac operon consists of the control elements and three structural genes

● lac z : It codes for β - galactosidase enzyme and hydrolyzes lactose into glucose and galactose.

● lac y: It codes for permease enzyme and it increases the permeability of the cell to lactose.

● lac a: It codes for transacetylase enzymes and it transfers acetyl group to $\beta$ - galactosidase.

Operator gene:

It is present adjacent to the structural genes and directly controls the synthesis of mRNA, over the structural genes. It interacts with the repressor protein and prevents the transcription of structural genes.

Promoter gene:

Promoter gene is the site for initial binding of RNA polymerase.

Regulator gene:

Regulator gene codes for repressor protein. It produces a repressor that binds to the operator gene and stops the working of the operator.

Repressor:

Repressor is a protein produced by the regulator gene and it binds to the operator gene, so that the transcription of mRNA structural genes stops.

Inducer:

Inducer is a chemical like hormone or metabolite which after coming in contact with the repressor forms an inducer repressor complex. This complex cannot bind with the operator gene, which is thus switched on.

Switch on mechanism of lac operon:

Switch on mechanism of lac operon is as follows:

When lactose is present in the growth medium, it enters into the cell by the action of permease. The low level of expression of lac operon is necessary for admitting the lactose into the cells.

The lactose acts as an inducer and binds to repressor protein. It inactivates the repressor and allows the RNA polymerase to bind on the promoter region. Now the lac operon is switched on. This is positive regulation.



Switch off mechanism of lac operon:

Switch off mechanism of lac operon is as follows:

Normally, the repressor protein binds to the operator region, blocking the RNA polymerase and preventing the transcription. Now the lac operon is switched off. This is negative regulation.