2. Regulation of gene expression
• Gene expression is controlled at:
• A. At transcription level.
• B. At post transcription level.
3. A. Transcriptional control of gene
expression:
• Each differentiated cell has the capacity to
synthesize certain types of protein in suitable
amounts through differential regulation of
gene expression.
• Regulation of gene expression can be
illustrated by the Lac-Operon model in E. Coli
as the simplest model for regulation of gene
expression since regulation of other genes in
E. Coli or eukaryotes are far more complex.
4. Cont------
• Operon is a coordinated unit of gene
expression (Lac = lactose-metabolizing
enzymes, Oper = operation, and On = is on).
• Lac-Operon is formed of two genes with their
regulatory sequences. One gene is
constitutively regulated regulatory gene, the
Lac i-gene, i.e., Lac-gene inhibitory gene that
gives a monocistronic mRNA translated into
the repressor protein.
5. Cont------
• The other gene is structural, Lac-gene formed
of three units (Z, Y and A) and gives a
polycistronic mRNA translated into three
proteins (-galactosidase, lactose permease
and acetylase).
• E. Coli can metabolize glucose, glycerol,
lactose or galactose as a source of energy with
preference to glucose.
6. Cont------
• For the integrated metabolism of lactose the
bacterium requires lactose permease that
permits entry of lactose into the cells
• -galactosidase that hydrolyzes lactose into
glucose and galactose.
• They are co-regulated since they are produced as
one polycistronic mRNA along with a third
enzyme of unknown function, i.e., acetylase(it
may acetylate specific proteins and facilitate gene
induction).
7. Cont------
• Since Lac-gene is an inducible gene, in
presence of glucose the bacterium contains a
few molecules of these enzymes per cell.
• But when the media lacks glucose, the
bacterium expresses this gene at a very high
rate (as much as 100-fold the uninduced rate)
producing large amount of these enzymes per
cell to increase lactose metabolism.
8. lacI
lacZ lacY lacA
CAP binding site Operator (lac repressor binding site)
Promoter (RNA polymerase binding site)
lac Operon
DNA
Promoter
inhibitory gene
promoter operator Structural protein (enzymes - B-galactosidase, Lactose permease and
acetylase)
control genes
Catabolite activator protein
9. Cont------
• Jacob and Monod elucidated the mechanism
of the coordinated regulation of the
expression (induction/repression) of that
Operon in 1961.
• As above, the Lac-gene has a promoter
sequence downstream of which there is the
Operator sequence at which Lac repressor
binds to hinder the binding of RNA
polymerase to the promoter,
10. Cont------
• and upstream of which there is the CAP sequence
at which the catabolite-activated DNA-binding
protein (CAP, cAMP-dependent) binds to activate
binding of RNA polymerase.
• The regulatory Lac repressor DNA-binding
protein is the product of Lac repressor gene (Lac-
I gene) which has the promoter as the only
regulatory sequence and is a regulatory gene
with a constitutive constant rate of transcription
and hence protein synthesis.
Alo-lactose binds to repressor protein to inhibit repression
11. The two states of Lac-Operon
(Repression and induction):
1. Repression:
• When E. Coli is grown in presence of glucose,
transcription of the Lac-gene is repressed.
• Repression is mediated by Lac repressor, which
binds as a tetramer to the operator sequence
preventing the binding of RNA polymerase to the
promoter and prevents the transcription of the
Lac-gene.
• The repressor is a negative regulator, and its
sequence is a silencer on expression of the
Operon
12.
13. 2.Induction (or derepression):
• When glucose is absent, the Lac-gene is
induced, i.e., expression rate of the Operon is
increased.
• Starvation of the bacterium of glucose leads
to increase of cAMP that binds and activates
the DNA binding of the Catabolite gene
Activator Protein (CAP).
14. Cont------
• The CAP binds to the CAP binding sequence
of DNA up-stream the promoter to facilitate
binding of RNA polymerase to the promoter to
induce transcription of the Lac-gene at a low
level.
• Therefore, cAMP-activated CAP is a positive
regulator, and its sequence is an enhancer.
15. Cont------
• The produced small amount of permease
facilitates entrance of lactose into the cell.
• Lactose (an inducer) has high affinity to bind
the repressor causing a change in its
conformation. This change prevents the
repressor from binding to the operator.
• Therefore, lactose acts as an inducer and the
operator becomes free for higher rate of
transcription by RNA polymerase.
16.
17. B. Post-Transcriptional control of gene
expression:
• Gene expression is also regulated at the levels
of RNA processing (e.g., capping, tailing,
alternative splicing and editing), RNA
transport, mRNA half-life and rate of
translation (e.g., proteins utilized in iron
absorption, transport and storage).
