• Regulation of gene expression in eukaryotes takes place in different level:
  • Transcriptional level (formation of primary transcript)
  • Processing level (regulation of splicing)
  • Transport of mRNA from nucleus to the cytoplasm.
  • Translational level.
  • In prokaryotes control of rate of transcriptional initiation is the predominant site for control of gene expression.
  • The activity of RNA polymerase at the promoter is regulated by accessory proteins, which affects its ability to recognize the start site.
  • The regulatory proteins can acts both positively (activators) or negatively (repressor)
  • The regulatory proteins interact with specific region of DNA called operator, which regulate the accessibility of RNA polymerase to promoter.


Lac operon:

  • Francois Jacob and Jacque Monod first to describe a transcriptionally regulated system of gene expression.
  • A polycistronic structural gene is regulated by common promoter and regulatory genes. Such regulation system is common in bacteria and is called operon.
  • Lac operon consists of:-
    • One regulator gene ( i-gene)
    • Three structural genes (z,y,a)
    • Operator. (binding site of repressor protein)
    • Promoter.(binding site of the RNA polymerase)
  • The i-gene codes for repressor of the lac operon.
  • The structural gene consist  of three gene (z, y and a)
    • ‘z’-gene codes for beta-galactosidase, which hydrolyze lactose into Galactose and glucose.
    • ‘y’ –gene codes for permease, which increases the permeability of bacterial cell to lactose.
    • ‘a’-gene codes for transacetylase.

  • All three genes are required for the metabolism of lactose in bacteria.
  • Inducer: lactose is the substrate for β- galactosidase and it regulates the switching on and off of the lac operon. Hence it is called inducer.
  • In the absence of glucose, if lactose is added in the growth medium of the bacteria, the lactose is transported into the cell by permease.
  • Very low level of expression of lac operon has to be present in the cell all the time; otherwise lactose cannot enter the cell.

Mechanism of regulation of lac operon:

  • The repressor protein is synthesized from i-gene (all time constitutively)
  • In the absence of the inducer i.e. lactose the active repressor binds to the operator and prevents RNA polymerase from transcribing the structural gene
  • In the presence of the inducer such as lactose or allolactose, the repressor is inactivated by interaction with inducer.
  • This allows RNA polymerase access to the promoter and transcription proceeds.
  • The regulation of lac operon by repressor is referred to as negative regulation.



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