gene regulation

1. Gene regulation in
prokaryotes and eukaryotes

2. INTRODUCTION
• Gene is the basic unit of genetic information.
• Gene is made up of DNA, double helix of two inter
wound polynucleotide.
• The biological information carried by a gene is contained
in its nucleotide sequence.
• This information is in essence a set of instruction for the
synthesis of an RNA molecules that may subsequently
direct the synthesis of an enzyme or other protein
molecule.
• This sum of the total process is called “Gene Expression”

3. • The expression of gene may be assayed in term of
RNA production, protein/enzyme activity or the
specific phenotype produced.
• Eg .prokaryotes produce only those enzymes that are
needed by them at particular time.
• No cell produces all the proteins that is capable of; it
produces only that group of proteins that are required
for its efficient function.

4. Gene Expression
• Gene expression is the activation of a gene that
results in a protein
GENE REGULATION
• The mechanism by which the expression of
different genes is controlled in different tissues
and/or at different times is called Gene
Regulation, Regulation of Gene Expression or
Regulation of Gene Action.

5. Levels of Regulation of Gene Action
The expression of a gene in Prokaryotes may be
subjected to regulation at one or more of the
following levels:
Gene Amplification, Destruction or Distribution
Transcription
Post- Transcription
Translation
Post-Translation

6. i. Gene Amplification, Destruction or Distribution: Regulation at this level
determines whether a gene is present in a cell, and if present the number of copies in which
it is present. In general, those genes whose products are required in large quantities are
present in multiple copies in the genome. In addition, additional copies of some genes are
also produced by replication. These remain in extra chromosomal state; the copies are
usually produced at specific times, are used for transcription, and are subsequently
digested, this phenomenon is called Amplification.
ii. Transcription: This mechanism of gene regulation determines whether a given gene will
be transcribed or not in a given cell or at a given time. This mode of replication is
universal, so much so that ordinarily the term gene regulation implies regulation of
transcription.
iii. Post-transcriptional Regulation: Controls after transcription determines if the
mRNA produced by a gene is available for translation.
iv. Translational Regulation: Translational regulation of gene action controls if an
mRNA that is suitable for translation will be translated or not. This mode of regulation is
based on ribosome, tRNA, mRNA, regulatory proteins and regulatory RNA.
v. Post-Translational Regulation: It governs activity of the protein products of genes; it
mainly involves protein modification, protein degradation and feed-back inhibition.

7. Cont…
• Genes that encode a product required in the maintenance of basic
cellular processes or cell architecture are called housekeeping genes
or constitutive genes.
• A constructive gene is an unregulated gene, whose expression is
uninterrupted, in contrast to the regulated expression of a gene.
• The studies of bacterial genetics indicate that all genes not only
specify the structure of an enzyme but some of them also regulate
the expression of other genes.
• These genes are called regulator genes.
• This concept of gene regulation has been studied by F. Jacob and J.
Monod in 1961 in E. coli, who proposed the operon concept.
• They were awarded Nobel prize for this discovery in 1965. The
operon model was developed working with lactose region (lac
region) of the human intestine bacteria E.coli.
• The gene regulation was studied for degradation of the sugar
lactose.

8. Cont…
According to the operon concept, gene regulation
in prokaryotes and bacteriophages involves:
• Structural genes
• The operator
• The promoter
• The regulator genes
• Repressor proteins
• Co repressor
• Inducer

9. Lac Operon of E. coli
• A genetic unit that consists of one or more “structural
genes” (cistrons that code for polypeptides) and an
adjacent “operator – promoter” region that controls
the transcriptional activity is called operon.
• Operator and promoter are up stream to the structural
genes.
• Thus an operon refers to a group of closely linked
genes which act together and code for various
enzymes required for a particular biochemical
pathway.

10. Cont…
• Lac operon consists of several components:
1. Structural genes
• The lactose operon of E. coli is composed of three structural genes
z, y and a the ‘z’ gene codes for an enzyme ß-galactosidase,
which converts lactose into glucose and galactose.
• The ‘y’ gene codes for an enzyme permease, which facilitates the
entry of lactose into the cell. The ‘a’ gene specifies the enzyme
thiogalactoside transacetylase, which transfers an acetyl group
from acetyl co-A to ß-galactoside.
• Hence all the three gene products in lac operon are required for the
metabolism of lactose.
• Such genes, which are sequential and transcribed as a single m-RNA
from a single promoter are called structural genes.
• The m-RNA synthesized is the polycistronic mRNA.
• Only the last cistron has the signals for the termination of
transcription.

11. • LAC OPERON
I P O Lac Z lacY Lac A
Repressor gene Promoter-operator Β-galactose gene Permease gene Transacetylase gene
Regulatory region Structural gene
Lac operon

12. Cont…
2. The operator region
• Operator lies immediately upstream to the
structural genes between the promoter and
structural genes.
• Operator is the target site for the attachment of
repressor protein produced by the regulator gene.
• Binding of repressor with operator prevents
initiation of transcription by RNA polymerase.
• When operator is free, the RNA polymerase can
bind to the promoter to initiate the mRNA
synthesis.

13. Cont…
3. The promoter region
• The actual site of transcription initiation is known
as promoter region.
• It also lies upstream to the structural genes next to
the operator region.
• mRNA transcription by the structural gene is
catalysed by an enzyme RNA polymerase.
• This enzyme first binds to the promoter region
and then moves along the operator region and
structural genes.

14. Cont…
4. Regulator gene
• Regulator gene (i) specifies a repressor protein, which
in the absence of the inducer (lactose), bound to the
operator (o), thereby inactivating the operator and
preventing transcription of the three structural genes
by RNA polymerase.
• In the presence of an inducer (lactose), the repressor
is inactivated by interaction with the inducer.
• This allows the RNA polymerase to bind to the
promoter allowing the transcription of the adjacent
structural genes.

15. Cont…
5. Repressor
• Repressor is a protein molecule specified by
the regulator gene.
• Repressor may be in active form or inactive
form.
• In the active form, repressor binds to the
operator region and prevents transcription.
• When the repressor is in inactivate form, the
transcription takes place.

16. Cont…
6. Co repressor
• Co repressor is perhaps a product of one of the
enzymes synthesized by structural genes.
• The co repressor makes the inactive repressor
active in a repressible system after combining
with the same.
• The repressor – co repressor complex can
block the operator gene and stop protein
synthesis by structural genes.

17. Cont…
7. Inducer
• The inducer binds to the repressor making it
inactive such that it cannot bind to the operator.
• RNA polymerase path way is cleared allowing the
expression of structural genes.
• A few molecules of lactose present in the
cytoplasm of E. coli are metabolized into
allolacatose, which is an isomer of lactose.
• Such molecules that induce the expression of any
operon by binding to the repressor are called
inducers and such operons are inducible operons.

18. Regulation of Lac Operon
• In an uninduced E. coli, repressor protein binds to the operator.
• Hence, expression of structural genes is not induced. E. coli initially
contains a few molecules of ß-galactosidase enzyme.
• A few molecules of lactose slowly diffuse into cytoplasm.
• ß-galactosidase present in cytoplasm metabolises lactose into
allolactose which acts as an inducer.
• In an induced E. coli, allolactose binds to repressor protein.
• The repressor protein is detached from the operator.
• RNA polymerase allows the transcription of structural genes to
synthesize a polycistronic mRNA.
• Permease synthesized from mRNA allows the rapid uptake of
lactose.
• Large number of ß-galactosidase molecules in the cytoplasm
metabolise lactose into galactose and glucose.

19. In the "repressed or uninduced" state, the repressor bound to
the operator

20. In the "induced" state, the lac repressor can not bound to the
operator site

21. Mechanism of Gene Regulation
• The mechanism of gene regulation is of two types, viz,
(1) Negative regulation
(2) Positive regulation
• Negative Control
• In the negative regulation, absence of a product enhances the
synthesis of enzyme and presence of the product decreases the
synthesis of enzyme.
• In the lac operon of E. coli. The synthesis of protein depends
whether the operator gene is blocked or free.
• When the operator gene is free, protein synthesis by structural genes
will take place.
• On the other hand, when the operator gene is blocked, the protein
synthesis is prevented.
• Thus, the on-off of protein synthesis is governed by the free and
occupied position of the operator gene.

22. Cont…
• In negative control, regulator protein acts as a inhibitor and prevents
protein synthesis.
• In lac operon of E.coli, there is negative control of gene regulation.
• In the negative control, the regulator protein is the repressor which
inhibits protein synthesis.
• In the inducible system, the effector molecule is the inducer.
• The inducer binds with repressor and inactivates it so that it cannot
bind with operator.
• Thus, inducer permits protein synthesis by inactivating the repressor.
• In the repressible system, the effector molecule is the co repressor.
• The co repressor on binding with in-active repressor makes it active
and inhibits protein synthesis, because when repressor becomes
active it will bind with operator and stop transcription.

23. Cont…
• Positive Control
• In positive regulation, presence of a product
will enhance the synthesis of enzyme.
• In other words, in positive control the regulator
protein acts as an activator and enhances the
protein synthesis.
• The arabinose operon of E.coli is an example
of positive gene regulation.

24. Positive and negative types of control can be of
two types :
• Inducible
• Repressible

25. Negative control
1. Inducible operon
• The regulator genes of such operons produce active
repressors that bind to the operator on their own.
• When these repressors interact with molecules called
inducer (effector) they become inactive.
• The inactive repressors unable to bind operator .
• Transcription of the operon begins.

26.  lac operon is a cluster of three genes coding for enzymes involved in the conversion of
disaccharide lactose to monosaccharides, glucose and galactose.
 These genes do not express all the time, but only in presence of lactose
First Operon to
be discovered
was lac Operon
Jacob & Monod

27. lac operon model for gene regulation
In presence of lactose, E. coli produces -
galactosidase to break down and utilize lactose
In the absence of lactose in the medium, E. coli
shuts of the expression of this enzyme
lactose -galactosidase
galactose
glucose

28. Lactose binding changes repressor property
1. When lactose, the inducer binds to the lac repressor protein
2. The binding of lactose changes the configuration of the lac repressor protein,
inactivates.
3. The inactivated repressor protein is unable to bind to the operator region.

29. RNA polymerase binds to promoter
4. Since lactose bound repressor cannot bind to the operator region, RNA polymerase can
now bind to the promoter region and transcribes the genes

30. Transcription occurs in presence of lactose
5. RNA polymerase transcribes the three genes (Z, Y and A) of the lac operon into RNA

31. Negative control
Repressible operon
• The repressors encoded by the regulator gene is inactive and
unable to bind operator.
• The operon is normally functional or depressed, when the
repressor interacts with the effector (co repressor), it
becomes active and binds operator DNA .
• Transcription of the operon stopped.

32. Eg. Tryptophan operon
• Consist of 5 structural genes -TrpE, TrpD, TrpB, TrpC, TrpA
• When repressor encoded by gene R is inactive; it can bind the
operator sequence as a result, Trp operon transcribed.
• When Trp accumulates in cell above threshold level it interacts
with inactive repressor , transcription is prevented .

33. Positive control
1. Inducible positive control
• The activator is in an inactive state, and can not bind
DNA.
• When an inducer molecule interacts with the activator, it
becomes active and binds DNA.
• Transcription takes place.

34. Repressible positive control
• The activators is by itself is active .
• Binds to the promoter and allows transcription
• The activators become inactive when it interacts with co
repressors
• Transcription does not takes place

35. REGULATION OF GENE
ACTIVITY IN EUKARYOTES

36. • Eukaryotes have involved a more complex system of gene regulation.
EUKARYOTIC GENE REGULATION : DIFFERENT FROM REGULATION
IN PROKARYOTES
• Eukaryotic cell contain a much greater amount of genetic information
then prokaryotic cells, and this DNA complexed with histones and other
proteins to form chromatin.
• Genetic information in eukaryotes is carried on many chromosomes,
and these chromosomes are enclosed within the double membrane
bound nucleus.
• Since the genetic information in eukaryotes is segregated from the
cytoplasm, transcription is specially and temporally separated from
translation-transcription occur in the nucleus and translation occur later
in the cytoplast. Because of this, attenuation control, a regulatory
mechanism in prokaryotes , is not possible.
• The transcripts of eukaryote genes are processed before transport to
the cytoplasm.

37. • Eukaryotes mRNA has a much longer half life than does the
prokaryotic mRNA. When prokaryotes want to stop making a
protein, they turn off transcription and the mRNA decays within
minutes.
• Because mRNA is much more stable, eukaryotes have a series of
translation control.
• More eukaryotes are multicellular with differentiated cell types.
• Gene regulation in eukaryotes
• Mechanism gene regulation is not well understood and it is more
complex
• General mode of gene regulation similar in both i.e prokaryotes
and eukaryotes
• Do not have operon

38. In Eukaryotes, gene action may be regulated at the level of :
 Activation of gene structure
 Transcription
 Translation
 Gene Replication
 After Transcription
 After Translation

40. Regulation of transcription
• Transcription initiation begins only after binding of transcription factors
to promoter DNA.
• Which enables RNA polymerase to bind the promoter.
• It is a positive regulation
• Examples
• GRE ( gluco-corticoid response element)
• BLE ( basal level element )
• MRE ( metal response element)
• TRE ( TPA response element)
Here the binding of regulatory transcription factors to any one of the
response element is able to activate transcription initiation.

41. Negative regulation
• Eg. Gene encoding histone H2B in sea urchin; expressed only during
spermatogenesis
• Promoter has two CAAT boxes
• The CAAT binding factor must bind these two boxes for transcription to
be initiated
• But in tissues other than testis eg. embryonic tissue. These are occupied
by CAAT displacement factor
• CAAT binding factor unable to bind the CAAT boxes
• Transcription does not take place

42. The Britten-Davidson Model of Regulation
• In 1969, Roy Britten and Eric Davidson proposed a theory to explain
gene regulation in the cells of higher organisms.
• It summarizes many of the observations and assumptions made about
regulation in higher organisms.
• For example, as cells undergo differentiation, it is apparent that
previously inactive sets of gene become activated.
• Such activation is sometimes associated with external signals such as
hormonal action or embryonic inductive events.
• The essence of the model is the simultaneous regulation of batteries of
genes during development.
• Britten and Davidson proposed that repetitive sequences serve as major
control units.

43. The basic components of Britten-Davidson model
• A series of batteries of genes is activated by the presence of some signal
molecule, such as hormone.
• The hormone interacts with a sensor gene.
• This event activates a contiguous integrator gene, which produces an
activator RNA molecule.
• It is this molecule which activates genes to produce materials essential
to the cell.
• Activator RNA interacts with receptor genes (comparable to operator
regions in bacteria) to activate transcription.
• The receptor genes control the transcription of adjacent producer genes,
which are comparable to the structural genes in bacteria.

44. THE COMPONENTS OF THE BRITTEN-DAVIDSON
MODEL OF GENE REGULATION IN EUKARYOTES

45. • The model proposes that each set of producer genes in a
given battery contains a common nucleotide sequence in
its adjacent receptor gene site.
• Thus a single activator molecule may activate numerous
non contiguous producer genes, called a battery.
• The basic model may be expanded into more complex
interrelationships.
• In figure, three different sensor/integrator gene sets are
shown in relationship to six receptor/structural gene sets.

46. THE BRITTEN-DAVIDSON MODEL OF EUKARYOTIC
GENE REGULATION AND THE COMPLEX INTERACTIONS
PROPOSED IN THE MODEL

47. • Britten and Davidson revised their theoretical model in 1979
• In the modified version, repetitive sequences still serve as the focal point
of genetic regulation in eukaryotes, but play quite different role in the
process
The newer model based on following observations and assumptions:
• The mRNA nucleotide sequences found in the cytoplasm, differ in
various cell and tissue types.
• That is, transcriptional products of structural genes found in the
cytoplasm are unique when different cell types are compared.
• However, if RNA found in the nuclei of these various cell types is
examined, the uniqueness disappears.
• Thus, it may be that genetic regulation does not occur at the
transcriptional level.
.

48. • The second important observation concerns the RNA
transcripts of repetitive DNA sequences, as found in the
nuclei of different cell types.
• It is proposed that this RNA, also part of the total hnRNA,
varies quantitatively and qualitatively between specialized
cell types.
• The final portion of the revised model incorporated the recent
discovery of intervening sequences.
• These sequences found within DNA and the original RNA
transcript of structural genes are subsequently excised during
maturation of mRNA.

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