can you exchange gene expression in eukaryotes.If not never mind,let it be next time. Dr. Anil Batta

2. 
It takes a lot of energy to make RNA and
protein.

Therefore some genes active all the time
because their products are in constant
demand.

Others are turned off most of the time and are
only switched on when their products are
needed.

3. 
The control of gene expression is much more
complex in eukaryotes than in prokaryotes.

Reasons being, Eukaryotes have:
• Compartmentalization of cells
• More extensive transcript processing
• Regulation from a distance
• Cell and tissue specific gene expression
• Larger Genome size
• Genes scattered about the genome

4. 
In prokaryotes, the control of transcriptional initiation is
the major point of regulation

In eukaryotes the regulation of gene expression is
controlled nearly equivalently from many different points:
•
•
•
•
•
•
•
•
•
Initiation of transcription (most important control)
Chromatin control
Epigenetic control
Transcript processing
Transcript stability
RNA transport
Protein stability
Protein transport
Post-Translational modifications

5. 
One way in which prokaryotes control gene
expression is to group functionally related
genes together so that they can be regulated
together.

This grouping is called an operon.

The clustered genes are transcribed together
from one promoter giving a polycistronic
messenger.

6. Gene Control in Prokaryotes

An operon can be defined as a cluster gene
that encode the proteins necessary to perform
coordinated function

Genes of the same operon have related
functions within the cell and are turned on
(expressed) and off together (suppressed).

The first operon discovered was the lac operon
so named because its products are involved in
lactose breakdown.

7. An operon consists of:
• a promoter (binding site for RNA polymerase)
• a repressor binding site called an operator that
overlaps the promoter.
• structural genes

8. Operator
 Repressor proteins encoded by repressor genes,
are synthesized to regulate gene expression.

They bind to the operator site to block
transcription by RNA polymerase.
Promoter
 The promoter sequences are recognized by RNA
polymerase.

When RNA polymerase binds to the promoter,
transcription occurs

9. Actvators

The activity of RNA polymerase is also
regulated by interaction with accessory
proteins called activators

The presence of the activator removes
repression and transcription occurs

10. 
Two major modes of transcriptional regulation function
in bacteria (E. coli) to control the expression of operons:
• repression and
• induction.

Both mechanisms involve repressor proteins.

Induction happens in operons that produce gene
products needed for the utilization of energy.

Repression regulates operons that produce gene
products necessary for the synthesis of small
biomolecules such as amino acids.

11. Inducible system
Negative control
the effector molecule interacts with the repressor
protein such that it cannot bind to the operator

With inducible systems, the binding of the effector
molecule to the repressor:
• greatly reduces the affinity of the repressor for the
operator
• the repressor is released and transcription proceeds.

12. 
In addition to negative control mediated by a
repressor, expression from an inducible operon is
also under positive control, mediated by an
activator

A classic example of an inducible (catabolitemediated) operon is the lac operon, responsible for
obtaining energy from galactosides such as lactose.

13. Repressible system
Negative control
the effector molecule interacts with the repressor
protein such that it can bind to the operator

With repressible systems, the binding of the effector
molecule to the repressor:
• greatly increases the affinity of repressor for the
operator
• the repressor binds and stops transcription.

For the trp operon , the addition of tryptophan (the
effector molecule) to the E. coli environment shuts off
the system because the repressors binds at the
operator.

14. 
In addition to negative control mediated by a
repressor, expression from a repressible operons is
attenuated by sequences within the transcribed
RNA.

A classic example of a repressible (and attenuated)
operon is the trp operon, responsible for the
biosynthesis of tryptophan.

15. Structure of the lac Operon

The lac operon three structural genes:
• Z
• y
• a

The z gene codes for β-galactosidase , responsible for
the hydrolysis of the disaccharide, lactose into its
monomeric units, galactose and glucose.

16. 
The y gene codes for permease, which increases
permeability of the cell to galactosides.

The a gene encodes a transacetylase.
In addition to the structural genes the lac operon also has
regulatory genes:
• Promoter: Binding site for RNA polymerase
• Operator: Binding site of repressor

19. 
The control of the lac operon occurs by both positive
and negative control mechanisms.
Negative control of the lac operon
What happens to lac operon when glucose is
present and lactose is absent?

During normal growth on a glucose-based medium
(lacking lactose), the lac repressor is bound to the
operator region of the lac operon, preventing
transcription.

20. What happens when glucose is absent
and lactose is present?

The few molecules of lac operon enzymes
present will produce a few molecules of
allolactose from lactose.

Allolactose is the inducer of the lac
operon.

The inducer binds to the repressor
causing a conformational shift that causes
the repressor to release the operator.

21. 
With the repressor removed, the RNA
polymerase can now bind the promoter
and transcribe the operon.

22. What happens when both glucose and lactose
levels are high?

Since the inducer is present, the lac operon will be
transcribed.

However the rate of transcription is very slow
(almost repressed) because glucose levels are high
and therefore cAMP levels are low.

23. 
The repression of the lac operon under these
conditions is termed catabolite repression and is as a
result of the low levels of cAMP that results from an
adequate glucose supply.

This repression is maintained until the glucose supply
is exhausted.
What happens when glucose levels start dropping in
the presence of lactose?

As the level of glucose in the medium falls, the level of
cAMP increases.

Simultaneously the inducer (allolactose) is also
binding to the lac repressor (since lactose is present).

24. 
The net result is an increase in transcription
from the operon.

The ability of cAMP to activate (increase)
expression from the lac operon results from an
interaction of cAMP with a protein termed CRP
(for cAMP receptor protein).

The protein is also called CAP (for catabolite
activator protein).

25. 
The cAMP-CAP complex binds to a region of the lac
operon just upstream of the promoter

The binding of the cAMP-CRP complex to the lac operon
stimulates RNA polymerase activity 20-to-50-fold.

(Repression of the lac operon is relieved in the
presence of glucose if excess cAMP is added.)

cAMP is therefore an activator of the lac operon.

This type of regulation by an activator is positive in
contrast to the negative control exerted by repressors.

27. trp operon

The trp operon encodes the genes for the synthesis of tryptophan.
As with all operons, the trp operon consists of the promoter, operator and
the structural genes.

It is also subject to negative control by a repressor

In this system, unlike the lac operon, the gene for the repressor is not
adjacent to the promoter, but rather is located in another part of the E. coli
genome.

Another difference is that the operator resides entirely within the
promoter

Unlike an inducible system, the repressible operon is usually turned on.


28. The operon consists of:

The operon consists of 5 structural genes that code for
the three enzymes required to convert chorismic acid
into tryptophan’

The operon also contains a gene coding for a short
oligopeptide (trpL) which functions in attenuation

Operator

promoter

29. Gene
Gene Function
P/O
promoter
trp L
leader
Promoter; operator sequence is found in the
Leader sequence; containing attenuator (A) sequence the
trp E
Gene for anthranilate synthetase subunit
trp D
Gene for anthranilate synthetase subunit
trp C
Gene for glycerolphosphate synthetase
trp B
trp A
Gene for tryptophan synthetase subunit
Gene for tryptophan synthetase subunit

30. Negative control of trp operon

The affinity of the trp repressor for binding the operator
region is enhanced when it binds tryptophan, blocking
further transcription of the operon and, as a result, the
synthesis of the three enzymes will decline.

hence tryptophan is a corepressor.

This means that when tryptophan is absent expression of
the trp operon occurs

the rate of expression of the trp operon is graded in
response to the level of tryptophan in the cell.

32. Attenuation of the trp operon

Expression of trp operon is reduced by the
addition of trytophan in trpR mutants.

Further research established that this second
level of tryptophan control involved two
components:
1. tRNA, specifically tryptophanyl-tRNA,
tRNATrp, i.e. tRNATrp charged with
tryptophan.
2. the trpL gene

33. 
The attenuator region is composed of sequences found
within the transcribed RNA of the operon

It is involved in controlling transcription from the
operon after RNA polymerase has initiated synthesis of
the proteins.

The leader sequences are located prior to the start of
the coding region for the first gene of the operon (the
trpE gene).

34. 
The leader sequence (trp L) contains tandem
tryptophan codons.
How does this affect transcription of the trp operon?

It contains two consecutive trp codons and therefore
serves to measure the tryptophan supply in the cell.

If the supply is good, then the tRNA will be charged
and the leader peptide will be translated without
problem.

If the supply is inadequate, then the tRNA will not be
charged, and translation will stall at the trp codons.

35. 
The trpL mRNA region can adopt a number of different
conformations. It contains several self-complementary
regions which can form a variety of stem-loop
structures

36. Different stem-loops can form:
• Depending on the level of tryptophan in the cell and
hence the level of charged trp-tRNAs
• Depending on the position of ribosomes on the
leader polypeptide and
• Depending on the rate at which they are translated

37. 
Recall that transcription and translation can occur
simultaneously in bacteria.

This means that the ribosome will attach to mRNA
and is able to influence the formation of secondary
structures by the mRNA.

38. 
In the case of the trpL mRNA, when the cellular levels of
tryptophan are high, the levels of the tryptophan tRNA are
also high.

Immediately after transcription, the ribosome follows
right behind RNA polymerase until it is halted by a stop
codon.

Translation is quick because of the high levels of
tryptophan tRNA.

This permits formation of the terminator stem-loop which
will cause RNA polymerase to dissociate (recall rho
independent termination of transcription in prokaryotes)

39. How is the terminator stem-loop formed?

Because of the quick translation of domain 1, domain 2
becomes associated with the ribosome complex.

Then domain 3 binds with domain 4, and transcription is
attenuated because of this stem loop formation.

The stem loop formed by binding of domains 3 and 4 is
found near a region rich in uracil and acts as the
transcriptional terminator loop (see transcription notes
from unit 3)

Consequently, RNA polymerase is dislodged from the
template.

40. trp Operon Transcription Under Low
Levels of Tryptophan

Under low cellular levels of tryptophan, the translation of the short
peptide on domain 1 is slow.

As a result domain 2 does not become associated with the ribosome.

Rather domain 2 of the leader mRNA associates with domain 3 of the
leader mRNA.

This step loop structure is the anti-terminator. Its formation prevents
formation of the terminator

41. 
This structure permits the continued transcription of the operon.
Then the trpE-A genes are translated, and the biosynthesis of
tryptophan occurs

Domain 4 is called the attenuator because its presence is
required to reduce (attenuate) mRNA transcription in the presence
of high levels of tryptophan.

Domain 1 is also an important component of the attenuation
process.

The section of the leader sequence encodes a 14 amino acid
peptide that has two tryptophan residues.

43. 
http://www.biocourse.com/ui/swf/iLabs/
lac_operon.swf
http://faculty.plattsburgh.edu/donald.slish/A

http://highered.mcgraw-hill.com/sites/dl/fre
