Control of Gene Expression
= Gene Regulation in
Prokaryotic cell
Objectives:





To understand the concept of the gene function control.
To describe the operon model of prokaryotic gene regulation.
To know the genetic sequence involved in the regulation
To discuss the evident role of genetic induction & repression.
To identify the level of regulation control in eukaryotic cells.
Gene Regulation
in Prokaryotes:

Gene regulation is Economic:
- E. Coli Contains constitutive genes encode enzymes that are
needed (e.g. enzymes of glycolysis).
- Activation other genes occurs only under special condition
(e.g. absence of glucose & presence of lactose in the media).
General levels of Gene Expression Control:
1) Transcriptional level;
- Positive control
( activation)
- Negative control
( repression)

2) Translation level;
- Increases or decreases the rate of translation
( rate of ribosomal function).
3) Post-translation level;
- activation or inhibition the function of
the enzymes.
( feedback inhibition mechanism).
Inducible and Repressible Operons: Two Types of
Negative Gene Regulation
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An inducible operon is one that is usually off; a
molecule called an inducer inactivates the
repressor and turns on transcription
The classic example of an inducible operon is the
lac operon, which contains genes coding for
enzymes in hydrolysis and metabolism of lactose
A repressible operon is one that is usually on;
binding of a repressor to the operator shuts off
transcription
The trp operon is a repressible operon
• Transcriptional level
1. LACTOSE OPERON
= System of Gene Complex
LAC OPERON Composition:
1) Structural genes:
-
-
Group of linked genes with related function
(Lactose operon contains 3 linked genes).
Form unit on bacterial DNA.
Coding for group of enzymes with related functions.
Their transcription results in single mRNA.
Translation results in separate 3 enzymes, because:
Each enzyme is marked by initiation and termination codons on
mRNA. 1) Permease
2) ß- galactosidase
3) Galactoside transacetylase
2) Operator:
- DNA –sequence
- Switch the transcription on or off
- Overlapping the promoter
3) Promoter:
- Binding site of RNA-polymerase
Composition of Operon
R
repressor gene
P
promoter
O
1
2
operator
structural linked genes
Carries binding site
of repressor protein
3
DNA strand
How works ?
The LAC OPERON
• Lac operon is inducible
system of genes
(= An inducible gene is not
transcribed unless a specific
inducer inactivates its
repressor).
- becomes active under certain
conditions such as absence of
glucose & presence of lactose.
It works to:
Transform lactose into glucose
to be used as a source of
energy in absence of glucose.
-Catabolism is the metabolic
pathway.
• Repressor protein
- Encoded by
a repressor gene
• Constitutive gene (its always on),
so it produces continuously small
amount of repressor -protein.
• Located upstream from the
promoter site.
-
Repressor protein binds to
operator, so it switches the
transcription off.
- Inactivated by inducer that
switches the transcription
(operator) on.
1)
repressor
R
P
O
blocked
Operator
unblocked
R
P
O
Binding of repressor to the operator
switches operator off. (negative Control)
1
2
3
NO Transcription of Lac operon genes
2) In presence of lactose, few molecules enter
the cell and act as inducer. (positive Control)
1
2
3
Transcription of Lac operon genes
Inducer
In presence of Lactose
and absence of glucose
repressor
(allolactose)
Small molecule
formed from
lactose
RNApolymerase
1
2
Single
mRNA
3
Translation to 3 separate
enzymes
Inactive repressor
- Permease
- ß- galactosidase
- Galactoside transacetylase
Can’t link with operator, so
the Lac operon genes
expression switches on
Glucose
Lactose
Converts into
Lac operon of E. Coli :
Catabolizes the disaccharide
lactose into glucose (in presence
of lactose & absence of glucose).
Permease
Transport Lac
across the Pl.m.
Lactose
Few molecules
enter the cell &
form allolactose
lactose
- ß- galactosidase
- Galactoside
transacetylase
allolactose
(Inducer)
- Galactose
- Glucose
E. Coli plasma
membrane
Binding of inducer to
repressor
Inactivate the repressor
by conformational change,
It becomes unable to recognize and bind the
operator.
RNA –polymerase binds to the
unblocked promoter
Switch the
transcription on
Inducer
+
repressor
a) High lactose, high glucose, low
cAMP (inactive operator) due to low
affinity of promoter to RNA-polymerase
inactive repressor
R
P
1
O
2
3
No transcription
RNApolymerase
CAP is
irrelevant
R
P
b) High lactose, low glucose, high
cAMP. Activation of promoter by CAPcAMP complex.
O
1
2
3
Transcription of Lac operon On
repressor
allolactose
CAP-cAMPcomplex
helps RNApolymerase to bind
promoter so it
activates gene
expression
RNApolymerase
1
In presence of Lactose
and absence or low of
glucose concentration
2
Single mRNA
Inactive repressor
3
Types of transcriptional control of
Lac Operon
1) Negative Control.
•- Inhibit the activity of Lac operon as
economical process in presence of
glucose.
•-The controlling -element is the
repressor protein that switches the
transcription off.
•- in presence of glucose:
bacteria produces
repressor
binds to operator
inactive
operator
turn transcription
off.
2) Positive Control.
•- activation of lactose catabolism.
•- Pomoter of Lac operon has low affinity
for RNA-polymerase, although the
repressor protein is inactive by
allolactose.
•- Activation of Lac operon;
- Takes place by CAP (catabolic
activator protein).
- CAP is inactive, becomes active as it
combines with cAMP (co-activator) to
form CAP- cAMP- complex.
- cAMP is regulated by glucose (it is
inversely proportional to glucose
concentration).
glucose
cAMP
trp operon
Promoter
Promoter
Genes of operon
trpR
DNA
Regulatory
gene
mRNA 5
trpE
trpA
Operator
Start codon Stop codon
3
RNA
polymerase
mRNA 5
E
Protein
trpB
trpC
trpD
Inactive
repressor
D
C
B
A
Polypeptides that make up
enzymes for tryptophan synthesis
Tryptophan absent, repressor inactive, operon on
Repressible system
Anabolic pathway (synthesis of amino acid tryptophan)
DNA
No RNA made
mRNA
Active
repressor
Protein
Tryptophan
(corepressor)
Tryptophan present, repressor active, operon off
Increase
co-repressor rep. complex
Switch operator
off
Turn transcription
off
Synthesis
of enzymes
Tryptophan
level
(tryptophan act as
co-repressor)
Stop enzyme
synthesis
Switch
transcription on
Turn operator on
Inactive
repressor
Decrease
Types of Transcriptional Control in Prokaryotes
NEGATIVE CONTROL
1) Inducible genes
- Represser protein alone
lactose repressor alone
- Represser protein + inducer
lactose repressor + allolactose
2) Repressible genes
- Repressor protein alone
Tryptophan repressor alone
- Repressor + corepressor
Tryptophan repr. + tryptophan
POSITIVE CONTROL
- Activator protein alone
CAP alone
- Activator protein + coactivator
CAP + cAMP
Active represser "turns off" regulated gene.
Lactose operon not transcribed.
Inactive repressor/inducer complex fails to
"turn off" regulated gene(s).
Lactose operon transcribed.
Inactive represser fails to "turn off" regulated
gene(s).
Typrtophan operon transcribed.
Active repressor-corepressor complex "turns
off" regulated gene(s).
Tryptophan operon not transcribed.
Activator alone cannot stimulate
transcription of regulated gene(s).
Transcription of lactose operon not stimulated
Functional activator-coactivator complex
stimulates transcription of regulated gene(s).
Transcription of lactose operon stimulated.