Operon Model of Gene Regulation in Prokaryotes

In this operon model of gene regulation in prokaryotes post we have briefly explained about operon model of gene regulation, components of operon model of gene regulation (The structural genes, the operator gene, the promoter gene, and the repressor gene).

Operon Model of Gene Regulation in Prokaryotes

In 1960, a short report in the Proceedings of the French Academy of Sciences proposed the name “operon.” The so-called broad theory of the operon was derived from this study. According to this hypothesis, operons govern all genes through a single feedback regulatory mechanism called repression.

Later research revealed that gene regulation is a far more complex process. Indeed, there is no such thing as a universal regulatory mechanism since there are so many, and they differ from operon to operon. Despite its alterations, the creation of the operon concept is regarded as a watershed moment in molecular biology.



Operon Model of Gene Regulation in Prokaryotes

The structural genes

The structural genes form a single long polycistronic m RNA molecule and the number of structural genes corresponds to the number of proteins. Each structural gene is controlled independently and transcribe mRNA molecule separately, this, depends on substrate to be utilized.

Example: In lac operon three structural genes (Z, Y, A) are associated with lactose utilization. Beta-galactosidase is the product of lac Z that cleaves beta (1-4) linkage of lactose & releases the free monosaccharides. The enzyme permease (a product of lacy) facilitates the lactose the entry inside the bacterium. The enzyme transacylase is a product of lac A where no definite role has been assigned.

The lac operon consists of a promoter (p) operator (o) together with structural genes. The lac operon cannot function in the presence of sugars other than lactose.

The operator gene

The operator gene is presented adjacent to lac Z gene. The operator gene overlaps the promoter region. The lac repressor protein binds to the operator invitro & protect part of the promoter region from the digestion of DNase.

The repressor protein binds to the operator & forms an operator repressor complex which in turn physically blocks the transcription of Z, Y & A genes by preventing the release of RNA polymerase to begin transcription.

Operon Model of Gene Regulation

The promoter gene

The promoter gene is long nucleotide &continuous with the operator gene. The promoter gene lies between the operator &regulator gene, like operators the promoter region consists of palindromic sequences of nucleotides. These palindromic sequences are recognized by such proteins that have symmetrically arranged subunits.

This section of two fold symmetry is present on the CRP site(c-AMP receptor protein site that binds to a protein called CRP). The CRP is encoded by CRP gene, it has been shown experimentally that CRP gene binds to cAMP (c AMP found in e.coli & other organisms) molecule & form a cAMP CRP complex.

This complex is required for transcription because it binds to promoter& enhances the attachment of RNA polymerase to the promoter therefore it increases the transcription &translation process.

The repressor gene

Regulator gene determines the transcription of structural gene. It is of two types-active & inactive repressor. It codes for amino acids of a defined repressor protein. After synthesis, the repressor molecules are diffused from the ribosome & bind to the operator in the absence of an induces.

Finally the path of RNA polymerase is blocked & m RNA is not transcribed consequently; no protein synthesis occurs . This type of mechanism occurs in inducible system of active repressor.

Moreover when an inducer is present it binds to repressor proteins forms an inducer – repressor complex. Due to formation of complex the repressor undergoes changes in the confirmation of shape becomes inactive consequently the structural genes can synthesize the polycistronic mRNA and later synthesize enzyme.

In contrast in the reversible system the regulator gene synthesis repressor protein that is inactive & therefore fails to binds to operator, consequently ,proteins are synthesized by the structural genes.

However the repressor protein can be activated in the presence of an corepressor. The co-repressor together with repressor proteins forms the repressor-co repressor complex. This complex binds to operator gene & blocks the protein synthesis

Further Readings


  1. https://courses.lumenlearning.com/microbiology/gene-regulation-operon-theory/
  2. https://ib.bioninja.com.au/72-transcription-and-gene/operons.html
  3. https://open.umn.edu/opentextbooks/textbooks/biochemistry-free-for-all-ahern

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