REGULATION OF GENE EXPRESSION
Gene expression is the
Mechanism at the molecular level by which a gene is able to express itself in
the phenotype of an organism. Gene regulation is the mechanism of switching off
and on of the genes depending upon the requirement of the cells and the
state of development.
There are two types of
gene regulations positive and negative.
In negative gene
regulation, the genes continue to express their effect till their activity is
suppressed. This type of gene regulation is also called repressible
regulation. The repression is due to a product of regulatory genes.
Positive gene regulation
is the one in which the genes remain non expressed unless they are induced to
do it. It is there for inducible regulation. Here a product removes a
biochemical that keeps the genes in non Expressed state.
In prokaryotes the gene
expression is controlled by the initiation of transcription.
In eukaryotes the
regulation can be exerted at four levels:
·
Transcription level
·
mRNA processing level
·
Transport of mRNA from nucleus to cytoplasm and
·
Translational level
Gene regulation in prokaryotes
The regulation of gene
expression in bacteria is carried out through Operon System.
Operon model was first
given by Francis Jacob and Jacques Monod in E.coli.
An Operon is a part of
genetic material which act as a single regulated unit. All the genes
controlling one metabolic pathway constitute an operon.
A few examples are
lac-operon, val-operon, his-operon etc in E.Coli.
An operon consists of
the following components:
Structural gene - they
transcribe the mRNA for the amino acid sequence of proteins.
Promoter gene - the
promoter gene is a sequence of DNA, where the RNA polymerase binds and
initiates transcription.
Operator - the operator is a sequence of bases on DNA adjacent to the promoter.
The accessibility of
promoter gene for RNA polymerase is regulated by protein called repressor.
Regulator gene - this codes for the repressor protein that binds to the operator
and switches off the transcription unit.
So the regulatory gene
is also represented as ‘i’ gene (inhibitory gene).
Inducer - the substance that prevents the repressor from binding to the
operator is called an inducer, it keeps the switch on and transcription
continues.
Operons can be of two
types inducible and repressible
Inducible Operon
An Inducible Operon
System is a regulated unit of genetic material which is switched on In response
to the presence of a chemical (inducer) it is usually found in catabolic
Pathways.
Lac – operon in Escherichia coli
It is an inducible
operon, where lactose is the inducer; it is the substrate for the enzyme
β-galactosidase.
The components of
lac-operon and their functions are as follows:
Structural genes
There are three
structural genes (z,y,a) which transcribe a polycistronic mRNA.
Gene z codes
β-galactosidase that catalyses the hydrolysis of lactose into galactose and
glucose.
Gene y codes for
permease, which increases the permeability of the cell to β-galactosides.
Gene a codes for
transacetylase, that catalyses the transacetylation of lactose into its active
form.
Promoter
It is a sequence of
bases nearer to the structural genes; it is the site where RNA polymerase binds
for transcription.
Operator
It is the sequence of
bases of DNA near the promoter, where the repressor always binds.
It functions as a switch
for the operon.
Repressor
It is a protein coded by
i gene, synthesized all the time constitutively.
It binds to the operator
and prevents the RNA polymerase from transcribing.
Inducer
Lactose is the inducer
that inactivates the repressor and prevents it from binding to the operaror.
This allows an access
for the RNA polymerase to the promoter and transcription.
Thus the substrate,
lactose regulates the lac operon
Repressible Operon
Repression is blocking
of the operator gene of Operon through a complex repressor that is formed by
union of apo-repressor formed by regulator gene and Co-repressor, a product of
anabolic pathway.
Repressible Operon System
normally remains active but can be switched off when the cell does not require
the metabolite.eg: Tryptophan operon system.
In the absence of co-repressor ( tryptophan ).
Regulator gene à Inactive
repressor (aporepressor) Ã No
blocking of operator gene à Transcription
by structural genes à Enzyme
synthesis
In the presence of co-repressor (tryptophan)
Regulator gene à Inactive
repressor (aporepressor) + Corepressor
Active repressor à Binds to
and blocks operator gene à No mRNA
transcription by structural genes.
Five structural genes
are not expressed (trpE, trpD, trpC, trpB, trpA) hence no enzyme are formed.
