Lac-operon is a concept which involves regulation of protein synthesis or the synthesis of an enzyme by the set of genes. This concept can be well studied by the gene expression in prokaryotes like E.coli and other bacteria.

The concept of Lac-operon was first explained by Jacob and Monad in E.coli. Lac- operon simply refers to the utilization of lactose (as sole carbon source) by the DNA of prokaryotic organisms for further transcription to RNA and translation to proteins.

Content: Lac-Operon

  1. Operon
  2. Components of Lac-Operon
  3. Control of Gene Expression in prokaryotes
  4. Inducer


It is a system of DNA that contains a sequence of genetic code or carries a sequence of genes that code the m-RNA that directs the synthesis of enzymes for the protein synthesis. An operon is a coordinated system where all genes coordinate to mediate the regulation of gene expression.

Components of Lac-Operon

An operon majorly consists of two elements or genes:

Regulatory Elements

It includes the following regions:
regulatory elements

Promotor Region: It codes the “Lac-P gene. It lies between the regulator and the operator. RNA-polymerase binds to this site, as a promoter is a region for initiation of transcription. It is 100 base pairs long. It consists of palindromic sequences. This site promotes and controls the transcription of structural genes or m-RNA. The promoter site is regulated by the regulatory genes of the repressor.

Operator Region: It codes the “Lac-O gene. It lies between a promoter and the structural gene (Lac-Z). It contains an operator switch, which decides whether transcription should take place or not. The regulatory gene binds to the operator.

Regulator Region: It codes for regulator gene (Lac-I) that controls the activity of promotor and an operator gene. This regulatory gene produces regulatory proteins known as “Repressor proteins” which can bind to the promoter and operator.

Structural Elements

These are the regions of DNA which contain genes that codes for protein synthesis, which are of three kinds:
structural elements

Lac-Z: Encodes for the enzyme beta-galactosidase.
Function: Beta-galactosidase brings about the hydrolysis of lactose into galactose and glucose subunits.

Lac-Y: Encodes for the enzyme lactose permease.
Function: Lactose permease brings lactose into the cell.

Lac-A: Encodes for the enzyme thiogalactoside transacetylase.
Function: Thiogalactoside transacetylase function is not very clear but it assists the activity of an enzyme beta-galactosidase.

These three i.e. Lac Z, Y and A genes are present adjacent to each other. Therefore, all the elements like promotor, operator, repressor and structural genes together form a unit refers to “Operon”.


Control of Gene Expression in prokaryotes

In prokaryotes, the control mechanism is controlled by two ways:

  • Positive control
  • Negative control

Positive Control of Lac-Operon

It also refers to a Positive inducible system. It includes the following steps:

  1. The regulatory gene is expressed by the repressor.
  2. After expression of a regulatory gene, the repressor proteins produces.
  3. Repressor protein has binding sites for the operator and the inducer i.e. lactose.
  4. Therefore, when lactose is present as an inducer it binds with the repressor protein and forms “R+I complex”.
  5. After the binding of inducer to the repressor, it blocks the binding of the repressor to the operator.
  6. As the repressor protein does not block the operator, the RNA polymerase binds to the promotor and moves further to transcribes mRNA.

This concept is known as “switch on” of Lac-operon (by the presence of inducer).
positive control

Negative Control of Lac-Operon

This also refers to Negative control of repressor system. It includes the following steps:

  1. First, the regulatory gene is expressed by the repressor.
  2. After expression of a regulatory gene, the repressor proteins produces.
  3. In the absence of inducer or lactose the, repressor protein directly binds to an operator.
  4. This blocks the movement of RNA polymerase and its attachment to the promoter.
  5. At last, inhibits the mRNA transcription.

This concept is known as “switch off” of Lac-operon (by the absence of inducer).
negative control


It also refers as “Antirepressor“. It suppresses the activity and binding of the repressor protein to the operator and makes it “inactive repressor” from the active repressor. In Lac-operon, lactose or allolactose use as an inducer. Allolactose forms by the enzyme beta-galactosidase as a result of isomerization of lactose i.e. galactose links to the C6  instead of  C. It is the kind of complementary substance that participates to switch on and off the operon system.

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