Gene regulation

Two types of genes:
1)Structural genes – encode specific
proteins
2)Regulatory genes – control the
level of activity of structural genes
ie. Control structural gene
expression.
Gene regulation in
prokaryotes
http://www.youtube.com/watch?v=oBwtxdI1zvk


In prokaryotes, operons control the rate of
transcription.
An operon is a group of genes that work
together and code for the enzymes
regulating a particular metabolic pathway.
OPERON
Regulator
gene
Produces the
repressor
Promoter
RNA
polymerase
binding site
Operator
Repressor
binding site
Structural
gene A
Structural
gene B
Structure of the operon
The operon in prokaryotes comprises a number of
different features:
1)
Structural genes: code for particular enzymes
in a metabolic pathway
2)
Promoter gene: recognition site for the RNA
polymerase to bind to.
3)
Operator gene: controls the production of
mRNA from structural genes.
INDUCTION (initiation)
If a substrate is uncommon the bacteria will not need the enzymes
most of the time. So the repressor is usually attached.
This prevents RNA polymerase from forming mRNA. Therefore: no
enzymes.
When the substrate molecule is present some of it acts as an
inducer; it binds to the repressor, changing its shape so it can’t
bind to the DNA.
See this movie on the
Transcription takes place.
Lac operon in E. coli
for more detail
Example: ON
Sugar lactose enters cell, genes coding for enzymes
to break sugar down turned on
Inducer
R
R
RR
P
O
SG1
SG2
REPRESSION
When a substrate is normally present the enzyme should be normally
operating. The only time this should stop is when the end product levels
build up too much.
The repressor cannot bind to the operator.
Some of the excess product acts as an effector, which helps the
repressor to bind.
Transcription is stopped.
See this movie on the
Tryp operon in E. coli
for more detail
R
RR
P
O
SG1
SG2
http://www.sumanasinc.com/webcontent/anisamples/majorsbiolo
gy/lacoperon.html
Lac Operon - induction:
Lac gene off
(normal state)
Repressor molecule
binds to operator and
prevents transcription by
RNA polymerase
Lactose presentacts as an inducer.
Lactose binds to
the repressor
protein.
Repressor can’t bind
to the operator.
Lactose all
used up.
RNA polymerase binds .
Lac gene on. Structural
proteins made.
Tryptophan operon - repression
Tryp gene on
(normal state)
Tryptophan accumulates in
excess. Some of it acts as an
effector and activates the
repressor molecule.
RNA polymerase
binds
Tryptophan doesn’t bind
to the repressor which
then can’t bind to the
operator.
Tryptophan levels in cells
decrease, no excess.
Effector and
repressor
molecule bind to
the operator gene
and prevent
transcription by
RNA polymerase.
Gene regulation in
prokaryotes - summary




Genes for a metabolic pathway are linked together
in operons with a common switch mechanism
(operator).
No introns – no RNA processing
The structural genes undergo transcription and
translation simultaneously.
Regulation occurs by switching all genes of a
pathway on or off.
Gene regulation in
eukaryotes




Genes for metabolic pathways in eukaryotic cells
are separated, not grouped as operons.
The genes for a metabolic pathway are switched on
separately.
Genes have introns that are removed in RNA
processing.
Eukaryotic genes have a relatively large number of
control elements.
Regulatory DNA regions


Eukaryotic genes have a promoter region
upstream of the coding region, where RNA
polymerase binds.
There are 2 two types of regulatory
sequences that effect transcription of the
structural gene:
1) enhancer
2) silencer
Enhancer sequences
These are non-protein-coding sections of DNA that
help regulate transcription by binding proteins
called transcription factors (turn on).
Silencer sequences
These are non-protein-coding sections of DNA that
help regulate transcription by binding proteins
called repressors (decrease).
Transcription factors
 Two types:
1) Activators – these are small proteins that
bind to enhancer sequences or RNA
polymerase. They cause an increase in
transcription.
2) Repressors – these are small proteins that
bind to silencer regulatory genes. They
cause a decrease in transcription.
Transcription factors
(activators) that bind to
the enhancer sequence
RNA polymerase
Transcription factors that
bind to RNA polymerase
Promoter region of DNA
Enhancer sequence
of DNA
Coding region of gene
Role of
Transcription
Factors
Eukaryotic RNA polymerase cannot, on its
own, initiate transcription.
It depends on transcription factors to recognize
and bind to the promoter.
Transcription factors also bind to the enhancer
sequence of DNA RNA polymerase
Transcription factors
(activators) that bind
to the enhancer
Transcription factors that
bind to RNA polymerase
Promoter region of DNA
Enhancer
sequence of DNA
Coding region of gene
Activating Transcription
Transcription is activated when a hairpin loop in the DNA brings the
transcription factors on the enhancer sequence (activators) in
contact with the transcription factors bound to the RNA polymerase at
the promoter.
Protein-protein interactions are crucial to eukaryotic tanscription.
The RNA polymerase can only produce a mRNA molecule once the
complete initiation complex is assembled.
Transcription factors
bound to RNA polymerase
Activators
Enhancer
Promoter
RNA polymerase
Initiation complex
Transcription proceeds
until a terminator
sequence is encountered.
Then transcription stops.

http://highered.mcgrawhill.com/olc/dl/120080/bio28.swf
Regulation
Prokaryotes
Eukaryotes
1.
Genes for a metabolic
pathway are linked together
in operons with a common
switch mechanism (operator).
1.
Pathways are separated, not
grouped by operons.
2.
Genes are switched on
separately.
3.
Genes have introns that are
removed in RNA processing.
4.
Transcription and translation
do not occur simultaneously.
5.
Genes have a large number of
control elements.
2. No introns-no RNA processing
3. The structural genes undergo
transcription and translation
simultaneously.
4. Regulation occurs by switching
all genes of a pathway on or
off.