Gene expression and Gene regulation
We know that in a living cell a gene expresses itself and as a result, either a structural protein or an active protein e.g. enzyme or RNA is produced.
All these are required for different metabolic activities.
However the gene expression can be controlled or regulated at various levels such as transcriptional or post transcriptional or translational level.
The transcriptional regulation of gene expression.
Usually, small extra cellular or intracellular metabolites trigger either initiation or inhibition of gene expression.
The clusters of genes with related functions are called operons.
They usually transcribe single mRNA molecule.
In E. coli 260 genes are grouped in 75 different operons.
Structure of the operon:
Each operon is a unit of gene expression and regulation which includes the structural genes and their control elements i.e. promoters and operators.
(i) The structural genes code or proteins. rRNA and tRNA required by the cell.
(ii) Promoters are the signal sequences in DNA that stat RNA synthesis. These are the sites where the RNA polymerases are bound during transcription.
(iii) The operators are present between the promoters and structural genes. The repressor protein binds to the operator region or the operon.
Regulatory genes are responsible for formation of repressors which interact with operators.
The Lac (Lactose) Operon:
The metabolism of lactose in cells requires three enzymes
1. Permease: is needed for entry of lactose in the cell.
2. galactosidase: brings about hydrolysis of lactose into glucose and galactose
3. transacetylase - transfers acetyl group from acetyl Co 'A' to beta-galactoside.
The lac operon has promoter sites (p), regulatory site (i) and operator site (o).
Besides that has three structural genes namely z, y and a.
"z" gene codes for beta-galactosidase.
'y' gene codes for permease and
'a' gene for transacetylase.
Francois Jacob and J. Monod, proposed the classical model of Lac operon which can explain properly gene expression and regulation in E. coli.
In lac operon a polycistronic structural gene is regulated by common promoter and regulatory genes.
Normal Energy source: Glucose Present
When the cell is using its normal energy source glucose, the 'i' gene transcribes a repressor mRNA.af ter translation of which, a repressor protein is produced.
It binds the operator region of the operon and prevents the RNA polymerase from transcribing the operon.
As a result beta-galactosidase is not produced.
In absence of glucose, if lactose is available as energy source
The lactose enters the cell as a result of the activity of permease enzyme.
It acts as inducer and interacts with the repressor to inactivate it.
As the repressor is inactivated, the RNA polymerase can bind itself to the operator site and transcribe the operon to produce lac mRNA
It leads to formation of all the required three enzymes needed for lactose metabolism.
This regulation of lac operon by the repressor is an example of negative control of transcription initiation
