Regulation of Gene Expression in Prokaryotes | B.Sc Notes
The regulation of expression of prokaryotic gene is determined by Operon model. This model was proposed by Jacob & Monad (1961) based on E. coli.
Operon is the unit of regulation of gene expression in prokaryotes.
Each operon consist of contiguous set of structural gene associated with the same metabolism and are linked by promoter site, operon site, regulatory protein binding site and these entire units are under the control of a single regulatory genes.
Structural Gene– These are those nucleotide sequences of DNA that will be converted into functional properties.
Promoter– These are the conserved nucleotide sequences associated with specific structural gene that provide site for binding of RNA polymerase to initiate the process of transcription of the contiguous structural gene.
Operator- These are the nucleotide sequences that partially overlap with the promoter sequences and provides the binding site of the regulatory gene product either repressor or activator.
Regulator Gene– These are those nucleotide sequence that codes for specific regulatory protein that bind specifically with the regulatory protein binding site of the operator and allosterically change.
Negative regulation of expression of Lac Operon
- It is the operon i.e. associated with the utilization of lactose sugar in E. coli.
- It is an example of catabolic expression of gene.
- It is an example of inducible system because with an increasing concentration of metabolic lactose in the medium, the expression of lac operon get induce.
Lac operon consists of three structural genes:
- Lac Z- It codes for enzyme β-galactosidase which catalyze the conversion of lactose into glucose and galactose.
Lactose → β-galactosidase→→ Glucose + Galactose
- Lac Y- It codes for enzyme β-galactosidase permease. It increases the permeability of lactose inside the cell of coli.
- Lac A- It codes for enzyme thiogalactosidase. This enzyme breakdown the thiogalactosidase, a toxic compound that enters the bacterial cell along with galactose.
Lac operon is an example of negative control system because the product of regulatory gene is an active repressor that complimentarily bind with operator shielding of the promoter and inhibiting the expression of structural gene lac Z, Y, A.
Positive regulation of expression of Lac operon
Two types of cases are present:
- Lactose present, glucose absent (cAMP level high), abundant lac mRNA synthesized.
- Lactose present, glucose present (cAMP level low), little lac mRNA synthesized.
When the glucose is present in the medium, E. coli preferably utilizes glucose, as a result lac operon fails to express, it is called glucose effect or catabolite repressor.
When the level of glucose declines in the medium, enzyme adenyl cyclase get activated which catalyze the conversion of ATP into cAMP.
There occurs a positive allosteric binding protein called catabolic activator protein which has ability to bind on the CRP site (Catabolite Receptor binding Protein) on the promoter.
However, in the absence of cAMP, an activator CAP fails to binds with the CRP site.
When the glucose level declines cAMP concentration increases, cAMP binds with the CAP-Camp complex forms which acts as a positive regulator.
CAP-cAMP complex binds on the CRP sites on the promoter and enhance the binding ability of RNA polymerase on the promoter site and facilitate the expression of lac Z, Y and A as a result catabolic enzymes are produces that catabolizes the lactose for the generation of the energy.
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