Chapter 14
Lecture Outline
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INTRODUCTION
n
n
The term gene regulation means that the level of
gene expression can vary under different conditions
Genes that are unregulated are termed constitutive
n
n
n
They have essentially constant levels of expression
Frequently, constitutive genes encode proteins that are
continuously necessary for the survival of the organism
The benefit of regulating genes is that encoded
proteins will be produced only when required
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14-2
INTRODUCTION
n
Gene regulation is important for cellular processes
such as
n
n
n
n
1. Metabolism
2. Response to environmental stress
3. Cell division
Regulation can occur at any of the points on the
pathway to gene expression
n
Refer to Figure 14.1
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14-3
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REGULATION OF!
GENE EXPRESSION
Transcription
Gene
Genetic regulatory proteins bind to"
the DNA and control the rate of"
transcription.
In attenuation, transcription terminates"
soon after it has begun due to the"
formation of a transcriptional terminator.
Translation
mRNA
Translational repressor proteins"
can bind to the mRNA and prevent"
translation from starting.
Riboswitches can produce an RNA"
conformation that prevents translation"
from starting.
Antisense RNA can bind to the mRNA"
and prevent translation from starting.
Protein
Posttranslation
In feedback inhibition, the product of a"
metabolic pathway inhibits the first"
enzyme in the pathway.
Covalent modifications to the"
structure of a protein can alter"
its function.
Functional protein
Figure 14.1
14-4
14.1 TRANSCRIPTIONAL
REGULATION
n
The most common way to regulate gene expression in
bacteria is by influencing the initiation of transcription
n
n
n
The rate of RNA synthesis can be increased or decreased
Transcriptional regulation involves the actions of two main
types of regulatory proteins
n  Repressors à Bind to DNA and inhibit transcription
n  Activators à Bind to DNA and increase transcription
Negative control refers to transcriptional regulation by
repressor proteins
n  Positive control refers to regulation by activator proteins
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14-5
n
Small effector molecules affect transcription regulation
n
n
These bind to regulatory proteins but not to DNA directly
In some cases, the presence of a small effector molecule
may increase transcription
n
n
These molecules are termed inducers
They function in two ways
n
n
n
n
Bind activators and cause them to bind to DNA
Bind repressors and prevent them from binding to DNA
Genes that are regulated in this manner are termed inducible
In other cases, the presence of a small effector molecule
may inhibit transcription
n
n
n
Corepressors bind to repressors and cause them to bind to DNA
Inhibitors bind to activators and prevent them from binding to DNA
Genes that are regulated in this manner are termed repressible
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14-6
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In the absence of the inducer, this"
repressor protein blocks transcription."Promoter
The presence of the inducer causes"
a conformational change that inhibits"
Repressor"
the ability of the repressor protein to "
protein
bind to the DNA. Transcription proceeds.
RNA"
polymerase
DNA
mRNA
No transcription
or
Transcription occurs
Inducer
Repressor"
protein
(a) Repressor protein, inducer molecule, inducible gene
This activator protein cannot bind"
to the DNA unless an inducer is"
present. When the inducer is bound"
to the activator protein, this enables"
the activator protein to bind to the"
DNA and activate transcription.
Inducer
No transcription
RNA"
polymerase
or
Activator"
protein
Transcription occurs
Activator"
protein
(b) Activator protein, inducer molecule, inducible gene
n
Regulatory proteins have
two binding sites
n
n
Figure 14.2
One for a small effector
molecule
The other for DNA
14-7
RNA"
polymerase
In the absence of a corepressor,"
this repressor protein will not bind"
to the DNA. Therefore, transcription"
can occur. When the corepressor is"
bound to the repressor protein, this"
causes a conformational change that"
allows the protein to bind to the DNA" Repressor"
protein
and inhibit transcription.
Transcription occurs
or
No transcription
Corepressor
Repressor protein
(c) Repressor protein, corepressor molecule, repressible gene
This activator protein will bind to"
the DNA without the aid of an effector"
molecule. The presence of an inhibitor"
causes a conformational change that"
releases the activator protein from the"
DNA. This inhibits transcription.
RNA"
polymerase
Transcription occurs
Activator"
protein
No transcription
or
Inhibitor
Activator"
protein
(d) Activator protein, inhibitor molecule, repressible gene
Figure 14.2
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14-8
The Phenomenon of Enzyme
Adaptation
n
At the turn of the 20th century, scientists made the
following observation
n
n
n
A particular enzyme appears in the cell only after the cell
has been exposed to the enzyme s substrate
This observation became known as enzyme adaptation
François Jacob and Jacques Monod at the Pasteur
Institute in Paris were interested in this phenomenon
n
They focused their attention on lactose metabolism in
E. coli
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14-9
The lac Operon
n
An operon is a regulatory unit consisting of a few
structural genes under the control of one promoter
n
n
n
An operon encodes a polycistronic mRNA that contains
the coding sequence for two or more structural genes
This allows a bacterium to coordinately regulate a group
of genes that encode proteins with a common functional
goal
An operon contains several important DNA
sequences
n
Promoter; terminator; structural genes; operator
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14-10
n
n
Figure 14.3a shows the organization and transcriptional
regulation of the lac operon genes
There are two distinct transcriptional units
n  1. The actual lac operon
n
a. DNA elements
n
n
n
n
Promoter à Binds RNA polymerase
Operator à Binds the lac repressor protein
CAP site à Binds the Catabolite Activator Protein (CAP)
b. Structural genes
n
n
n
lacZ à Encodes β-galactosidase
n  Enzymatically cleaves lactose and lactose analogues
n  Also converts lactose to allolactose (an isomer)
lacY à Encodes lactose permease
n  Membrane protein required for transport of lactose and analogues
lacA à Encodes galactoside transacetylase
n  Covalently modifies lactose and analogues
n  Its functional necessity remains unclear
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14-11
n
n
Figure 14.3a shows the organization and transcriptional
regulation of the lac operon genes
There are two distinct transcriptional units
n
2. The lacI gene
n
Not considered part of the lac operon
n
Has its own promoter, the i promoter
n
Constitutively expressed at fairly low levels
n
Encodes the lac repressor
n
The lac repressor protein functions as a tetramer
n
Only a small amount of protein is needed to repress the lac operon
n
There are usually ten homotetramer proteins per cell
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14-12
lac operon
Regulatory gene
i promoter
lacY
lacZ
lacI
CAP site
Encodes lactose"
permease
Encodes β-galactosidase
Operator site (lacO)
lac promoter (lacP)
E. coli!
chromosome
lacA
Encodes"
galactoside"
transacetylase
lac!
terminator
(a) Organization of DNA sequences in the lac region of the E. coli chromosome
Lactose
H+
Lactose permease
H
CH2OH
O
HO
Lactose
H
Cytoplasm
CH2OH
O OH
H
H
OH
H
H
OH
O
H
H
OH
H
H
O
β-galactosidase"
side reaction"
H
β-galactosidase"
H
CH2OH
H
CH2OH
O OH
HO
H
OH
H
H
OH
Galactose
H
O
HO
H
OH
H
H
CH2
O OH
H
H
H
OH
H
H
OH
HO
OH
Allolactose
H
O OH
H
+
O
CH2OH
HO
β-galactosidase"
H
OH
H
H
OH
H
Glucose
(b) Functions of lactose permease and β-galactosidase
Figure 14.3
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14-13
The lac Operon Is Regulated By
a Repressor Protein
n
The lac operon can be transcriptionally regulated
n
n
n
1. By a repressor protein
2. By an activator protein
The first method is an inducible, negative control
mechanism
n
n
It involves the lac repressor protein
The inducer is allolactose
n
n
It binds to the lac repressor and inactivates it
Refer to Figure 14.4
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14-14
RNA pol can t
transcribe
lacZ, lacY, and
lacA
lac operon
lac!
regulatory"
gene
Promoter
lacI
lacP
Operator
lacO
lacZ
lacY
lacA
Constitutive
expression
mRNA
lac repressor binds"
to the operator and"
inhibits transcription.
lac repressor"
(active)
(a) No lactose in the environment
Figure 14.4
The lac operon is
repressed
Therefore no allolactose
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14-15
RNA polymerase
lacI
lacZ
lacO
lacP
lacY
lacA
Transcription
mRNA
The lac operon is now
induced
Polycistronic"
mRNA
"
β-galactosidase
Allolactose
Lactose"
permease
Galactoside"
transacetylase
The binding of allolactose causes a"
conformational change that prevents"
the lac repressor from binding to the"
operator site.
Conformational"
change
(b) Lactose present
Figure 14.4
Some gets converted to allolactose
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14-16
Repressor does not completely
inhibit transcription
So very small amounts of the
lac operon proteins are made
Transacetylase
Lac repressor
β-galactosidase"
1. When lactose becomes"
available, a small amount of it"
is taken up and converted to"
allolactose by β-galactosidase."
The allolactose binds to the"
repressor, causing it to fall off"
the operator site.
Lac repressor
Lactose
r on
la
pero
Lactose permease
2. lac operon proteins"
are synthesized. This"
promotes the efficient"
metabolism of lactose.
4. Most proteins involved"
with lactose utilization"
are degraded.
on
ac
pero
Lac repressor
Figure 14.5
3. The lactose is depleted."
Allolactose levels decrease."
Allolactose is released from"
the repressor, allowing it to"
bind to the operator site.
The cycle of lac operon induction and repression
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14-17
Experiment 14A: The lacI Gene
Encodes a Repressor Protein
n
n
In the 1950s, Jacob and Monod, and their colleague
Arthur Pardee, had identified a few rare mutant
strains of bacteria with abnormal lactose adaptation
One type of mutant involved a defect in the lacI gene
n
n
n
It was designated lacI–
It resulted in the constitutive expression of the lac operon
even in the absence of lactose
The lacI– mutations mapped very close to the lac operon
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14-18
n
Jacob, Monod and Pardee proposed two different
functions for the lacI gene
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lac operon
lac operon
lacI
lacP
Encodes a"
repressor"
protein"
lacO
lacZ
lacY
lacA
The lacI– mutation eliminates the"
function of the lac repressor.
lacI
lacP
lacO
lacZ
lacY
lacA
The lacI– mutation results in the"
synthesis of an internal inducer.
The repressor protein"
is a diffusible protein.
(a) Correct explanation
(b) Internal inducer hypothesis
Figure 14.6
n
Jacob, Monod and Pardee applied a genetic
approach to distinguish between the two hypotheses
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14-19
n
n
n
They used bacterial conjugation methods to
introduce different portions of the lac operon into
different strains
They identified F factors (plasmids) that carried
portions of the lac operon
For example: Consider an F factor that carries the
lacI gene
n
Bacteria that receive this will have two copies of the lacI
gene
n
n
One on the chromosome and the other on the F factor
These are called merozygotes, or partial diploids
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14-20
n
n
Merozygotes were instrumental in allowing Jacob,
Monod, and Pardee to elucidate the function of the
lacI gene
There are two key points
n
1. The two lacI genes in a merozygote may be different
alleles
n
n
n
lacI– on the chromosome
lacI+ on the F factor
2. Genes on the F factor are not physically connected
to those on the bacterial chromosome
n
If hypothesis 1 is correct
n
n
The inducer protein produced from the chromosome can diffuse and
activate the lac operon on the F factor
If hypothesis 2 is correct
n
The repressor from the F factor can diffuse and turn off the lac
operon on the bacterial chromosome
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14-21
The Hypothesis
n
The lacI– mutation either
n
n
1. Results in the synthesis of an internal inducer
2. Eliminates the function of a lac repressor that can
diffuse throughout the cell
Testing the Hypothesis
n
Refer to Figure 14.7
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14-22
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Experimental level
Conceptual level
Mutant
P O Z+Y+ +
A
I–
1. Grow mutant strain and merozygote"
strain separately.
Mutant"
strain
+
P O Z Y+ + P O Z+
A +
–
I F′ Y+
I
A+
Merozygote"
strain
1.
P
I–
2.Divide each strain into two tubes.
Merozygote
O Z+ Y +
A+
– Lactose
Operon is turned on because"
no repressor is made.
2.
PO Z Y +
A+
+
I–
+ Lactose
Lactose
3. To one of the two tubes, add lactose.
3.
I–
1
2
3
4
– Lactose
The lacI+ gene on the F´ factor"
makes enough repressor to"
bind to both operator sites.
4. Incubate the cells long enough to allow"
lac operon induction.
5. Lyse the cells with a sonicator. This"
allows β-galactosidase to escape from"
the cells.
+ +
PO Z Y +
+
A P O Z Y+
I+
A+
F
4.
+ +
P O Z Y A+
I–
+ +
POZ Y +
I+ F A
+ Lactose
Lactose is taken up, is converted to
allolactose, and removes the
repressor.
Figure 14.7
14-23
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"
6. Add β-o-nitrophenylgalactoside"
(β-ONPG). This is a colorless"
compound. β-galactosidase will"
cleave the compound to produce"
galactose and o-nitrophenol (O-NP)."
O-nitrophenol has a yellow color."
The deeper the yellow color, the"
more β-galactosidase was produced.
β-ONPG
β-o-nitrophenyl-"
galactoside"
Galactose O-NP
+
1.
NO2
NO2
Broken cell
β-galactosidase"
+
2.
NO2
NO2
3.
7. Incubate the sonicated cells to allow
β-galactosidase time to cleave "
β-o-nitrophenylgalactoside.
"
NO2
1
2
3
4
4.
+
NO2
NO2
8. Measure the yellow color produced"
with a spectrophotometer. (See"
the Appendix for a description"
of spectrophotometry.)
Figure 14.7
14-24
The Data
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14-25
Interpreting the Data
Strain
Addition of lactose
Expected result because of
constitutive expression in the
lacI– strain
Amount of β-galactosidase
(percentage of parent strain)
Mutant
No
100%
Mutant
Yes
100%
Merozygote
No
<1%
Merozygote
Yes
220%
In the presence of lactose, both lac
operons are induced, yielding a
higher level of enzyme activity
In the absence of
lactose, both lac
operons are repressed
This result is consistent with hypothesis 2
The lacI gene codes for a repressor protein that can
diffuse throughout the cell and bind to any lac operon
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14-26
Interpreting the Data
n
The interaction between regulatory proteins and DNA
sequences have led to two definitions
n
1. Trans-effect
n
n
n
n
Genetic regulation that can occur even though DNA segments are
not physically adjacent
Mediated by genes that encode regulatory proteins
Example: The action of the lac repressor on the lac operon
2. Cis-effect or cis-acting element
n
n
n
A DNA sequence that must be adjacent to the gene(s) it regulates
Mediated by sequences that bind regulatory proteins
Example: The lac operator
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14-27
Interpreting the Data
n
n
Table 14.1 summarizes the effects of lacI gene
mutations versus lacO (operator) mutations in
merozygotes
Overall
n
n
A mutation in a trans-acting factor is complemented by
the introduction of a second gene with normal function
However, a mutation in a cis-acting element is not
affected by the introduction of another normal cis-acting
element
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14-28
14-29
The lac Operon Is Also Regulated
By an Activator Protein
n
n
The lac operon can be transcriptionally regulated in
a second way, known as catabolite repression
When exposed to both lactose and glucose
n
n
n
E. coli uses glucose first, and catabolite repression
prevents the use of lactose
When glucose is depleted, catabolite repression is
alleviated, and the lac operon is expressed
The sequential use of two sugars by a bacterium is
termed diauxic growth
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14-30
The lac Operon Is Also Regulated
By an Activator Protein
n
n
The small effector molecule in catabolite repression
is not glucose
This form of regulation involves a small molecule,
cyclic AMP (cAMP)
n
n
It is produced from ATP via the enzyme adenylyl cyclase
cAMP binds an activator protein known as the Catabolite
Activator Protein (CAP)
n
Also termed the cyclic AMP receptor protein (CRP)
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14-31
The lac Operon Is Also Regulated
By an Activator Protein
n
The cAMP-CAP complex is an example of
transcriptional regulation that is inducible and under
positive control
n
n
The cAMP-CAP complex binds to the CAP site near the
lac promoter and increases transcription
In the presence of glucose, the enzyme adenylyl
cyclase is inhibited
n
This decreases the levels of cAMP in the cell
n
Therefore, cAMP is no longer available to bind CAP
n  Transcription rate decreases
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14-32
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CAP site Promoter Operator
Allolactose
cAMP
High rate of transcription
CAP
Binding of RNA polymerase"
to promoter is enhanced"
by CAP binding.
Repressor"
(inactive)
(a) Lactose, no glucose (high cAMP)
CAP site Promoter Operator
Repressor
cAMP
CAP
Figure 14.8
Transcription is very low"
due to the binding of the"
repressor.
(b) No lactose or glucose (high cAMP)
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14-33
CAP site Promoter Operator
Allolactose
CAP
Repressor"
(inactive)
Transcription rate is low"
due to the lack of CAP"
binding.
(Inactive)
(c) Lactose and glucose (low cAMP)
CAP site Promoter Operator
CAP
(Inactive)
Transcription is very low due"
to the lack of CAP binding and"
the binding of the repressor.
(d) Glucose, no lactose (low cAMP)
Figure 14.8
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14-34
The trp Operon
n
The trp operon (pronounced trip ) is involved in
the biosynthesis of the amino acid tryptophan
n
n
The genes trpE, trpD, trpC, trpB and trpA encode
enzymes involved in tryptophan biosynthesis
The genes trpR and trpL are involved in regulation of the
trp operon
n
trpR à Encodes the trp repressor protein
n
n
Functions in repression
trpL à Encodes a short peptide called the Leader peptide
n
Functions in attenuation
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14-43
RNA pol can bind
to the promoter
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When tryptophan levels are low, tryptophan does not bind to the trp"
repressor protein, which prevents the repressor protein from binding"
to the operator site. Under these conditions, RNA polymerase can"
transcribe the operon, which leads to the expression of the trpE,!
trpD, trpC, trpB, and trpA genes. These genes encode enzymes"
involved in tryptophan biosynthesis.
trpR
trp operon
P
O trpL
trpE trpD trpC trpB trpA
Transcription
RNA polymerase
Inactive"
trp repressor
Cannot bind to
the operator site
(a) Low tryptophan levels, transcription of the entire trp operon!
occurs
Figure 14.13 Organization of the trp operon and regulation via the trp
repressor protein
14-44
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When tryptophan levels are high, tryptophan acts as a corepressor"
that binds to the trp repressor protein. The tryptophan-trp repressor"
complex then binds to the operator site to inhibit transcription.
trpR
P O
2 tryptophan"
molecules bind,"
which causes a"
conformational"
change.
trpL
trpE trpD trpC trpB
trpA
Corepressor–repressor"
binds to operator and"
blocks transcription.
Corepressor–repressor"
form active complex.
(b) High tryptophan levels, repression occurs
Figure 14.13 Organization of the trp operon and regulation via the trp
repressor protein
14-45
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Another mechanism of regulation is attenuation. When attenuation"
occurs, the RNA is transcribed only to the attenuator sequence, and"
then transcription is terminated.
Attenuator"
sequence
trpR
P O trpL
trpE trpD trpC trpB trpA
Transcription begins"
but stops at the"
attenuator sequence.
(c) High tryptophan levels, attenuation occurs
Another mechanism
of regulation
Figure 14.13 Organization of the trp operon and regulation via the trp
repressor protein
14-46
n
n
Attenuation can occur in bacteria because transcription and
translation are coupled
During attenuation, transcription actually begins but it is
terminated before the entire mRNA is made
n
n
n
n
A segment of DNA, termed the attenuator, is important in facilitating
this termination
In the case of the trp operon, transcription terminates shortly past the
trpL region (Figure 14.13c)
Thus attenuation inhibits the further production of tryptophan
The segment of trp operon immediately downstream from
the operator site plays a critical role in attenuation
n
The first gene in the trp operon is trpL
n
n
It encodes a short peptide termed the Leader peptide
Refer to Figure 14.14
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14-47
Region 2 is complementary to regions 1 and 3
Region 3 is complementary to regions 2 and 4
n  Therefore multiple stem-loops structures are possible
n
n
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Val
Met
Lys Ala Ile
The 3-4 stem loop is
followed by a sequence
of Uracils
Gly
Beginning of trpL!
coding sequence
Trp
5′
It acts as an intrinsic
(ρ-independent) terminator
Key:
Arg
C
Region 1
Region 2
Region 3
Region 4
Met Gln Thr
3′
U
G G
A
Stop
C
These two codons provide a way
to sense if there is sufficient
tryptophan for translation
Ser
Thr
U-rich"
attenuator
Beginning of trpE!
coding sequence
Figure 14.14 Sequence of the trpL mRNA produced during attenuation
14-48
n
n
n
The formation of the 3-4 stem-loop causes RNA pol
to pause, allowing the U-rich sequence to
dissociate from the DNA
Conditions that favor the formation of the 3-4
stem-loop rely on the translation of the trpL mRNA
There are three possible scenarios
n
1. No translation
2. Low levels of tryptophan
3. High levels of tryptophan
n
Refer to Figure 14.15
n
n
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14-49
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Most stable form of
mRNA occurs
When translation is not coupled with transcription, the most stable"
form of the mRNA occurs when region 1 hydrogen bonds to region 2"
and region 3 hydrogen bonds to region 4. A terminator stem-loop"
forms, and transcription will be terminated just past the trpL gene.
5′
RNA
RNA"
polymerase
U-rich"
attenuator
Trp codons
Transcription
terminates just
past the trpL
gene
3′
3−4 stem-loop"
(rho-independent"
terminator)
trpE
DNA
trpL
(a) No translation
Therefore no ribosome are
associated with the RNA
Figure 14.15 Possible stem-loop structures formed from trpL mRNA under
different conditions of translation
14-50
Region 1 is blocked
Coupled transcription and translation occur under conditions"
in which the tryptophan concentration is very low. The"
ribosome pauses at the Trp codons in the trpL gene because"
insufficient amounts of charged tRNATrp are present. This pause"
blocks region 1 of the mRNA, so region 2 can hydrogen bond only"
with region 3. When this happens, the 3−4 stem-loop cannot form."
Transcriptional termination does not occur, and RNA polymerase"
transcribes the rest of the operon.
3-4 stem-loop
does not form
Leader peptide
Trp codon
2
Ribosome stalls
5′
1
RNA pol transcribes
rest of operon
3
4
mRNA"
for trpE
trpL
Insufficient amounts
of tRNAtrp
DNA
trpE
(b) Low tryptophan levels, 2–3 stem-loop forms
Figure 14.15 Possible stem-loop structures formed from trpL mRNA under
different conditions of translation
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14-51
Sufficient amounts of tRNAtrp
Translation of the trpL mRNA
progresses until stop codon
3-4 stem-loop forms
Ribosome pauses on region 2
Region 2 cannot base pair
with any other region
Coupled transcription and translation occur under conditions"
in which a sufficient amount of tryptophan is present in the cell."
Translation of the trpL gene progresses to its stop codon, where"
the ribosome pauses. This blocks region 2 from hydrogen bonding"
with any region and thereby enables region 3 to hydrogen bond"
with region 4. This terminates transcription at the U-rich attenuator.
Transcription
terminates
Translation"
termination"
codon
U-rich"
attenuator
5′
1
2
3′
3
4
3-4
stem-loop
trpE
trpL
DNA
(c) High tryptophan levels, 3–4 stem-loop forms
Figure 14.15 Possible stem-loop structures formed from trpL mRNA under
different conditions of translation
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14-52
Inducible vs Repressible Regulation
n
The study of many operons revealed a general trend
concerning inducible versus repressible regulation
n
Operons involved in catabolism (ie. breakdown of a
substance) are typically inducible
n
n
The substance to be broken down (or a related compound) acts
as the inducer
Operons involved in anabolism (ie. biosynthesis of a
substance) are typically repressible
n
The inhibitor or corepressor is the small molecule that is the
product of the operon
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14-53
14.2 TRANSLATIONAL AND
POSTTRANSLATIONAL
REGULATION
n
Genetic regulation in bacteria is exercised
predominantly at the level of transcription
n
n
However, there are many examples of regulation that
occurs at a later stage in gene expression
For example, regulation of gene expression can be
n
n
Translational
Posttranslational
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14-54
Translational Regulation
n
n
n
For some bacterial genes, the translation of mRNA
is regulated by the binding of proteins
A translational regulatory protein recognizes
sequences within the mRNA
In most cases, these proteins act to inhibit
translation
n
These are known as translational repressors
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14-55
Translational Regulation
n
Translational repressors inhibit translation in one of
two ways
n
1. Binding next to the Shine-Dalgarno sequence and/or
the start codon
n
n
2. Binding outside the Shine-Dalgarno/start codon region
n
n
This will sterically hinder the ribosome from initiating translation
They stabilize an mRNA secondary structure that prevents initiation
Translational repression is also found in eukaryotes
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14-56
Posttranslational Regulation
n
n
n
A common mechanism to regulate the activity of
metabolic enzymes is feedback inhibition
The final product in a pathway often can inhibit the
an enzyme that acts early in the pathway
Refer to Figure 14.17
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14-59
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Catalytic site
n
Enzyme 1 is an allosteric enzyme,
which means it contains two
different binding sites
n
n
Catalytic site à binds substrate
Regulatory site à binds final
product of the pathway
Enzyme 1
Substrate
Regulatory"
site
Intermediate 1
Enzyme 2
n
n
n
If the concentration of the
final product becomes high
Feedback inhibition:!
If the concentration"
of the final product"
becomes high, it will"
bind to enzyme 1 and"
inhibit its ability to"
convert the substrate"
into intermediate 1.
It will bind to enzyme 1
Thereby inhibiting its ability to
convert substrate 1 into
intermediate 1
Intermediate 2
Enzyme 3
Final product
Figure 14.17
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14-60
Posttranslational Regulation
n
n
A second strategy to control the function of proteins
is by the covalent modification of their structure
Some modifications are irreversible
n
n
n
Proteolytic processing
Attachment of prosthetic groups, sugars, or lipids
Other modifications are reversible and transiently
affect protein function
n
n
n
Phosphorylation (–PO4)
Acetylation (–COCH3)
Methylation (–CH3)
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14-61
14.3 Riboswitches
n
n
First discovered in 2001
RNA exists in two different secondary
conformations
One is active
n  The other inhibits gene expression
n  Conversion between forms is due to the binding
of a small molecule
n  3-5% of all bacterial genes may be regulated by
riboswitches
n
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14-62