This slide share is useful for undergraduate medical students

1. REGULATION OF GENE EXPRESSION
Dr. K. Santha

2. • Total no: genes:
prokaryotes - 4000
eukaryotes - 35000
• Gene expression
Multistep process that results in the
production of functional gene product
• Importance
=>Cellular differentiation
=>Morphogenesis
=>Adaptability of organism

3. TYPES OF GENES
• Constitutive genes (House keeping genes):
Expressed at a reasonably constant rate
Not subjected to regulation
eg: enzymes of glycolysis
• Regulated genes
expressed only under certain conditions
expressed in all cells / subset of cells
eg: expression of insulin gene in pancreas

4. TYPES OF GENES
• Inducible gene:
expression increases in response to inducer
or activator
eg: bile acid induces ALP
• Gratuitous inducers
Compounds structurally similar to substrates
may act as inducers
eg: IPTG (isopropylthiogalactoside) – lactose
analog

5. • Repression
• inhibition of gene expression by repressor eg:
inhibition of ALA synthase by heme
• Catabolite repression: Catabolite of a
molecule inhibit gene expression

6. TYPES OF GENE EXPRESSION
REGULATION
• Positive regulation
increased gene expression mediated by
positive regulator / enhancer / activator 
• Negative regulation
reduced gene expression mediated by
negative regulator / silencer / repressor

7. REGULATION OF GENE EXPRESSION
Gene expression can be modulated by
Control of transcription
Post transcriptional modifications
Gene amplification
Gene rearrangements
Control of translation
Protein modification / stabilization
Gene regulation is influenced by hormones,
heavy metals and chemicals

8. REGULATION OF GENE EXPRESSION IN
PROKARYOTES
• Regulation occurs at transcription
• Jacob And Monod described operon model
• Genes involved in a metabolic pathway are often
found sequentially grouped on the chromosome
along with cis-acting regulatory elements.
• Cistron - Smallest unit of gene expression
• Polycistronic mRNA: A single mRNA that encodes
more than one separately translated protein.
• It is referred to as an operon

9. LAC OPERON
• Structural gene
lac Z : β- galactosidase (lactase)
lac Y : galactoside permease
lac A : thiogalactoside transacetylase
All these proteins are produced when lactose is
available to the cell but glucose is not.
• Inhibitor gene
A regulatory gene - the lac l gene code for the
repressor protein
• Promoter site – RNA polymerase binds
• Operator site and CAP site – regulatory proteins
bind.

10. • When glucose is the only sugar available –
lac operon is repressed and reduced
synthesis of lactose metabolizing enzymes
• Presence of lactose – lac operon is induced
and increased synthesis of lactose
metabolizing enzymes
• When both glucose and lactose are available –
reduced synthesis of enzymes even if lactose
is present in high concentration.

11. LAC OPERON
LACI P O LacZ LacY LacA
CAP
binding
site
CAP – bound by a complex of cAMP and
catabolite gene activator protein.

13. REPRESSION OF LAC OPERON
(presence of glucose)
• Binding of the repressor interferes with the
progress of RNAP and blocks the transcription of
the structural genes.
• Repression is mediated by the binding of
repressor protein via a helix turn helix motif to
the operator site. This is an example of negative
regulation.

14. DEREPRESSION OF LAC OPERON
(PRESENCE OF LACTOSE)
A small amount of lactose is converted to an
isomer allolactose which binds to the repressor
protein producing conformational changes in
repressor so that repressor can not bind to
operator site .
In absence of glucose, adenylyl cyclase is active
cAMP-CAP complex binds to the CAP binding site
causing RNAP to initiate transcription of structural
genes. This is an example of positive regulation.

15. Both glucose and lactose available
• Adenylyl cyclase is deactivated in the presence
of glucose - catabolite repression.
• No c AMP –CAP complex forms and CAP
binding site is empty.
• RNAP unable to initiate transcription even if
lactose is present at high concentration.

16. EUKARYOTIC GENE EXPRESSION
• Eukaryote genome is complex.
• DNA is extensively folded and packed into
chromatin (protein – DNA complex)
Differs from prokaryotic gene regulation
• Genes are not organized into operon
• Separation of transcription and translation
• RNA processing : capping at 5’ ends,
addition of poly A tail at 3’ end, splicing

17. REGULATION OF EUKARYOTIC GENE EXPRESSION
• Regulation at transcription level
Chromatin remodeling
Enhancers
Trans acting molecules – DNA binding proteins
function as transcriptional factors
Cis-acting regulatory elements
eg: Hormone response elements
• Regulation by post transcriptional process
alternative mRNA splicing
mRNA editing
• Gene rearrangement
• Gene amplification

18. Trans-acting molecules
• DNA binding proteins are trans-acting
molecules that function as transcriptional
activators or specific transcriptional factors.
• Two domains : DNA binding domain and
transcription activation domain(TAD)
• When TAD binds coactivators (eg: histone
transacetylase) and facilitate the formation of
transcription initiation complex at the
promoter and thus activate transcription.

19. Cis acting regulatory elements
• Hormone –response elements (HREs) are
cis-acting elements that bind trans acting protein
factors and regulate gene expression in response to
hormone signals.
Regulatoy signals mediated by intracellular receptors
(Steroid Hormones)
Ligand-receptor complex binds via Zinc finger motif to
nuclear DNA at HRE - coordinate expression of group
of target genes even when these genes are present in
different chromosome.

20. • Regulatory signals mediated by cell surface
receptors for insulin, epinephrine and
glucagon.
• Act through G-protein coupled plasma
membrane receptors
• Increase in cAMP activates CREB which binds
via leucine Zipper to a cis-acting element
cAMP response element ( CRE) resulting in
transcription of target genes

21. Regulation Through Chromatin Remodeling
• Regulation by chromatin remodeling is
actually a complex reaction in which large
numbers of different types of enzymes and
proteins are involved.
• Chromatin remodeling is facilitated by histone
acetylation / deacetylation.
• This type of gene regulation is called epigenetic
regulation

22. • When a transcriptional activation protein binds to a specific
enhancer, histone acetyltransferases also bind to them .
• This promotes sequential dissociation of the histones
and DNA from the binding site to the surrounding regions.
• As a result, the promoter is exposed and the RNA
polymerase complex easily binds to it.
• Conversely, if expression is to be suppressed, then histone
deacetylases remove the acetyl group and restore the
nucleosome structure.

23. • Chromatin remodeling
• Euchromatin and Heterochromatin are the two
structural forms of DNA in the genome found in
the nucleus.
• 90 % of human genome consists of
Euchromatin.
• Differential expression by having different
regions of chromatin for transcription in various
tissues
• Eg: β -globin gene cluster is in “active”
chromatin in reticulocytes but in “inactive”
chromatin in muscle cells

24. DNA elements
• DNA elements which facilitate / enhance the
initiation of transcription at the promoter are
termed enhancers
• Features : exert their effect on transcription even
when separated by tens of thousands of base pairs
from a promoter.
• They work when oriented in either direction and
they can work upstream (5’) or downstream (3’)
from the promoter.
• It is active only when it exists within the same DNA
molecule ( as cis to the promoter). Enhancer binding
proteins are responsible for this effect.

25. • The cis acting elements that decrease or
repress/silence expression of specific genes
are known as repressors.
Tissue specific expression may result from cis
acting regulatory elements.
Insulators serve as transcriptional boundary
elements prevent an enhancer from acting on
a promoter in another transcription domain.

26. DNA BINDING MOTIFS
• DNA BINDING MOTIFS
• Features :
• Mediate DNA-protein interactions
• Binds with high affinity to the specific site
• Maintained by hydrogen bonds, ionic
interactions and vander Waals forces
• Involved in providing: trans -activation
domains / ligand binding sites/ surfaces for
interaction with co- activators/ co-repressors

27. TYPES OF MOTIFS
• Binding motif Organism Regulatory
protein
• Helix-turn-Helix E.Coli Lac repressor, CAP
mammals Pit 1, Oct 1
• Zinc finger E.Coli TFIIIA, Gene 32 protein
mammals Steroid receptor
family
• Leucine Zipper Yeast GCN4
mammals c- Fos, c- jun, c-myc

28. • Proteins that regulate transcription have
several domains.
• DNA binding domain distinct from ligand
binding domain and several activation domains
• Transcription activation domain allows binding
of co-activators.
• Regulation is achieved by the formation of
protein- protein and protein-DNA interactions
controlling the assembly of the complex.

29. • GENE AMPLIFICATION
• Increases the number of genes available for
transcription
• Eg: resistance to methotrexate by cancer cells
cancer cells
increase the number of genes for DHFR
drug resistance

30. • GENE REARRANGEMENT
• Occurs during immunoglobulin synthesis
• IgG light chain mRNAs are encoded by
different segments that are tandemly
repeated in the germline.
• For a particular subset of IgG light chains
there are tandemly repeated
variable (VL) = 300 coding sequences
joining (JL) = 5 coding sequences
constant (CL) = 10 coding segments

31. • Thus a particular IgG light chain single VL, JL
and CL coding sequences must be recombined
to generate a single contiguous transcription
unit excluding the multiple nonutilized
segments.
• RNAP transcribe into single monocistronic
mRNA.

32. Alternative RNA splicing
• Alternative RNA splicing is a mechanism
that allows different protein products to
be produced from one gene when
different combinations of introns, and
sometimes exons are removed from the
transcript.
• This mechanism control the production of
different protein products in different cells
or at different stages of development.

33. • mRNA editing
• Posttrancriptional modification - change of
base in mRNA
• Eg: apo B 100 made in liver and small
intestine
• In intestine CAA to UAA results in shorter
protein - apoB48 ( Chylomicron)

34. • REGULATION OF MRNA STABILITY
• Ends of mRNA
• 5’ cap structure: prevents attack by 5’
exonuclease
• 3’ poly (A) tail: prevents attack by 3’
exonuclease
• Messenger RNA exist in the cytoplasm as
Ribonucleoprotein particles. Some of these
proteins may protect mRNA by nucleases
while others promote the nuclease attack in
certain conditions.

35. Control of translation initiation
• It occurs by phosphorylation of translational
initiation factor 2(eiF-2)
• In the phosphorylated state, eiF-2-GDP
• complex cannot be regenerated to form active
eiF-2-GTP complex
• The activity of eiF-4E is regulated by
phosphorylation either directly or through
inhibitor protein
• Insulin and mitogenic factors activate 4E through
• Phosphorylation of eiF-4E through MAP kinase
pathway

36. Control of translation initiation by
Repressor protein