Regulation of gene expression
It is the metabolic, physiological or environmental conditions that regulate the expression of genes.
Housekeeping or constitutive genes: genes whose products are continuously required by the cell hence they continuously transcribe and translate.
Non-constitutive genes: genes whose products are not continuously required by the cell & if they continuously transcribe and translate there will be a lot of wastage.
- Regulation of expression of such genes is a must.
Genes can be regulated at
- Transcriptional level.
- During processing i.e., splicing.
- Transport of mRNA from the nucleus to the cytoplasm.
- Translational level.
Genes are regulated at the transcriptional level.
There are 2 systems of regulations:
- An inducible system and a repressible system.
- In the inducible system, an addition of a substance induces transcription of certain genes. It produces catabolic enzymes. It is always OFF. E.g., lac operon.
- In the repressible system, an addition of a substance stops transcription of certain genes. It produces anabolic enzymes. It is always ON. E.g., Tryptophan operon.
An operon consists of different sets of genes which together regulate gene expression.
- A Structural gene; that transcribes to form mRNA, which translates to form protein
- A promoter gene; to which RNA polymerase binds.
- A regulator gene; which synthesizes a protein called repressor that can bind to the operator.
- An operator gene; when repressor binds to the operator RNA polymerase cannot bind to the promoter and when repressor does not bind to operator RNA polymerase binds to the promoter.
Each operon has its specific operator and specific repressor.
The lac operon
Given by Francis Jacob and Jacque Monod in E.coli.
- It is an inducible system and the inducer is lactose/allolactose, the substrate for β-galactosidases. It regulates the switching on and off of the operon.
- The structural gene is polycistronic as it has three genes (z, y & a).
- z codes for β-galactosidases which catalyze the hydrolysis of lactose (a disaccharide) into glucose and galactose.
- y codes for β-galactoside permease, a transport protein that pumps lactose into the cell.
- a codes for β-galactoside transacetylase, which transfers an acetyl group to galactose.
- Only z & y are required for lactose catabolism.
- The regulator gene i.e., ‘i’ codes for the repressor of lac.
- The operator gene i.e., ‘o’.
- The promoter gene i.e., ‘p’.
- When lactose is provided in the growth medium of the bacteria, permease helps to transport it into the cell.
- A very low expression of lac operon is present all the time in the cell, otherwise, lactose cannot enter the cell.
Lactose induces the operon in the following manner:
- In the absence of lactose (the inducer),
- The repressor formed by the regulator gene is an active repressor, it binds to the operator
- Due to which RNA polymerase cannot bind to the promoter preventing transcription.
- In the presence of lactose (the inducer),
- The inducer inactivates the repressor so it cannot bind to the operator.
- Hence RNA polymerase binds to the promoter causing transcription.
- Regulation of lac operon by repressor is a negative regulation.
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