Lac Operon - Regulation of Gene Expression

Gene expression is basically the synthesis of the polypeptide chain encoded by that particular gene. Therefore, we can say that the expression of the gene can be quantified in terms of the protein synthesis by the genes. Thus, gene regulation can be defined as any kind of alteration in the gene to give rise to a different expression which might result in a change in the synthesized amino acid sequence. Let’s learn about the regulation of gene expression in prokaryotes with the example of Lac Operon.

Regulation of Gene Expression

We can infer that the gene regulation can take place at various steps of gene expression which includes the following:

  • Replication level – Any error in copying the DNA may result in an altered expression.
  • Transcriptional level – During transcription, any error in the polymerization may again lead to a change in expression of the gene.
  • Post-transcriptional level – During the post-transcriptional modification i.e., RNA splicing, there may be some changes.
  • Translational level – During translation, if there is an error in the attachment of mRNA to the tRNA molecules, there may arise some changes.

The regulation of the expression of the gene can be explained with the example of an enzyme, say peroxidase in bacteria. This enzyme catalyzes the hydrolysis of hydrogen peroxide to water and oxygen.  Now, if the bacteria are moved to another environment where they fail to manufacture hydrogen peroxide, the enzyme no longer plays its role. In such a situation, the bacteria will not synthesize the enzyme anymore. Thus, we can say that the environmental, metabolic and physiological condition regulate the expression of genes.

Gene Regulation in Prokaryotes

Gene regulation in prokaryotes is most extensively observed at the initiation of transcription. Thus, the gene expression during transcription initiation is affected by regulation. The regulation usually takes place in the expression of the RNA polymerase at the promoter site. This affects the accessory proteins which bind to the recognition sites. These accessory proteins can regulate the promoter site in two ways:

  • Positive regulation by activators
  • Negative regulation by repressors

In Operons, the operator is situated right next to the promoter where the regulator binds to control its entire functioning.

Lac Operon Model

Regulation in prokaryotes can be explained with the help of Lac Operon model. Here the alteration in physiological and environmental conditions leading to alteration in expression in prokaryotes can be observed. It was observed by Jacob and Monod. The lac operon consists of:

  • A regulatory gene i – It codes for the repressor protein.
  • z gene – It codes for beta-galactosidase which catalyzes the hydrolysis of lactose into glucose and galactose.
  • y gene – It codes for permease which regulates the lactose permeability in the cell.
  • a gene – It codes for transacetylase which assists the enzyme beta-galactosidase.

Lac operon

Hence, all these genes help in the lactose metabolism. In lac operon, lactose acts as an inducer. If lactose is provided in the medium for the bacteria, the regulatory gene is activated. The inducer will bind to the repressor protein and render it inactive which allows transcription of the operon. Thus, lac operon is negatively regulated in this case.

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Practise This Question

In regulation of gene expression in prokaryotes
I.  Lactose acts as the suppressor for gene expression.
II.  Tryptophan acts as the inducer for gene expression.
III.  Regulator gene is the one that produces the repressor molecule.
Which of the statements given above are correct?