Inducible and repressible operon

Inducible and repressible operon

An operon is a cluster of genes on the chromosome that is controlled by a single promoter.

They are two types of operons according to the functions they perform: Inducible operons and repressible operons.

Major difference:  Inducible operon is regulated by a substrate present in the metabolic pathway while repressible operon is regulated by the presence of a metabolic end product known as a co-repressor.

Inducible operon

Definition: Structural genes in the operon is normally ‘Off’ but can be turned on by an inducer.

  • In inducible operons, the genes are kept switched off until a specific metabolite inactivates the repressor.
  • The inducible operons function in catabolic pathways.
  • The nutrients utilized in the pathway activate enzyme synthesis.
  • An inducible operon is switched on by an inducer. An inducer functions by converting the repressor protein into an inactive form.

Example: Lac operon is an inducible operon

Allolactose acts the inducer molecule that binds to the repressor protein called as lac repressor produced by the lacl gene (the regulatory gene of lac operon) and switches on the operon to transcribe the gene.

How the inducible operon is regulated?

  • The inducible operon is regulated in the presence of a chemical substance known as inducer. An inducer is usually the precursor/substrate for a metabolic pathway. In the presence of precursor, the operon is said to be switched on.
  • When no precursor is available, it would be wasteful for the cell to synthesize the enzymes needed to metabolize the precursor. So the operon remains switched off with the help of an active repressor.
  • As soon as precursor becomes available, some of it binds to the repressor, rendering the repressor inactive so that it unable to bind to the operator site.
  • Now RNA polymerase can bind to the promoter and transcribe the structural genes. The resulting mRNA is then translated into enzymes which convert substrate into product.
Figure 1: Regulation of an inducible operon

Repressible operon

Definition: Structural genes in the operon is normally ‘On’ but can be turned off by a co repressor

  • In repressible operons, genes are kept switched on until the repressor is activated by a specific metabolite
  • The repressible operons function in anabolic pathways.
  • The production is switched off by the end products of the pathway which repress enzyme synthesis.
  • The free repressor (aporepressor) cannot bind to the operator. Only the repressor/effector molecule (co-repressor) complex is active in binding to the operator.

Example: Trp operon is a repressible operon.

The amino acid tryptophan can be synthesized by bacterial cells. If a sufficient supply of tryptophan is present in the environment or culture medium, then there is no reason for the organism to expend energy in synthesizing the enzymes necessary for tryptophan production.

A mechanism has therefore evolved whereby tryptophan plays a role in repressing the transcription of mRNA needed for producing tryptophan-synthesizing enzymes.

 How the repressible operon is regulated?

  • The repressible operon is regulated in the presence of a chemical substance known as co-repressor. A co-repressor is always an end product of a metabolic pathway. In the presence of a co-repressor, the operon is said to be switched off.
  • The concentration of the co-repressor is directly proportional to the regulation of transcription within the cell. With the increment of the co-repressor concentration, apo-repressor and co-repressor complex is formed. (The apo repressor is a protein and is coded by the regulator gene present in the operon). This complex binds to the operator region and stops the transcription of structural genes.
  • During low level of co-repressor concentrations, the joining of apo-repressor and operator gene is prevented. This enables the continuation of the formation of co-repressor. As a result its concentration again increases.
  • Then the apo-repressor and co-repressor complex combines with the operator gene and turns off the gene expression. This prevents the process of transcription and thereby stops the synthesis of enzymes.
Figure 2: Regulation of a repressible operon

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