Controling-Gene-Expression-operon-types of regulation..ppt

Gene on and off mechanism.

1. Controlling
Gene Expression
Timothy G. Standish, Ph. D.

2. All Genes Can’t be Expressed
At The Same Time
Some genes are needed for the function of all
cells all the time. These genes are called
constitutive genes and are expressed by all cells.
Other genes are only needed by certain cells or at
specific times. The expression of these inducible
genes is tightly controlled in most cells.
For example, beta cells in the pancreas make the
protein insulin by expressing the insulin gene. If
neurons expressed insulin, problems would result.

3. Operons Are Groups Of Genes
Expressed By Prokaryotes
The genes grouped in an operon are all
needed to complete a given task
Each operon is controlled by a single
control sequence in the DNA
Because the genes are grouped together,
they can be transcribed together then
translated together

4. The LacOperon
Genes in the lacoperon allow E. colibacteria to
metabolize lactose
Lactose is a sugar that E. coliis unlikely to
encounter, so it would be wasteful to produce the
proteins needed to metabolize it unless necessary
Metabolizing lactose for energy only makes sense
when two criteria are met:
–Other more readily metabolized sugar (glucose) is
unavailable
–Lactose is available

5. The LacOperon -Parts
The lacoperon is made up of a control region and
four genes
The four genes are:
–LacZ-b-galactosidase -An enzyme that hydrolizes
the bond between galactose and glucose
–LacY-Codes for a permease that lets lactose across
the cell membrane
–LacA-Transacetylase -An enzyme whose function in
lactose metabolism is uncertain
–Repressor -A protien that works with the control
region to control expression of the operon

6. The LacOperon -Control
The control region is made up of two parts:
Promoter
–These are specific DNA sequences to which RNA
Polymerase binds so that transcription can occur
–The lac operon promoter also has a binding site for
another protein called CAP
Operator
–The binding site of the repressor protein
–The operator is located down stream (in the 3’
direction) from the promoter so that if repressor is
bound RNA Polymerase can’t transcribe

7. The LacOperon:
When Glucose Is Present But Not Lactose
Repressor Promoter LacYLacALacZOperator
CAP
Binding
RNA
Pol.
Repressor
Repressor
Repressor
mRNA
Hey man, I’m
constitutive
Come on,
let me through
No way
Jose!
CAP

8. The LacOperon:
When Glucose And Lactose Are Present
Repressor Promoter LacYLacALacZOperator
CAP
Binding
Repressor
Repressor
mRNA
Hey man, I’m
constitutive
CAP
Repressor
Repressor
X
RNA
Pol.
RNA
Pol.
Great, I can
transcribe!
Some transcription
occurs, but at a slow rate
This lactose has
bent me
out of shape

9. The LacOperon:
When Lactose Is Present But Not Glucose
Repressor Promoter LacYLacALacZOperator
CAP
Binding
Repressor
Repressor
mRNA
Hey man, I’m
constitutive
CAP
cAMP
Repressor
Repressor
X
This lactose has
bent me
out of shape
CAP
cAMP
CAP
cAMP
Bind to me
Polymerase
RNA
Pol.
RNA
Pol.
Yipee…!

10. The LacOperon:
When Neither Lactose Nor Glucose Is Present
Repressor Promoter LacYLacALacZOperator
CAP
Binding
CAP
cAMP
CAP
cAMP
CAP
cAMP
Bind to me
Polymerase
RNA
Pol.
Repressor
Repressor
mRNA
Hey man, I’m
constitutive
Repressor
STOP
Right there
Polymerase
Alright, I’m off to
the races . . .
Come on, let
me through!

11. The TrpOperon
Genes in the trpoperon allow E. colibacteria
to make the amino acid tryptophan
Enzymes encoded by genes in the trp operon
are all involved in the biochemical pathway that
converts the precursor chorismate to
tryptophan.
The trpoperon is controlled in two ways:
–Using a repressor that works in exactly the opposite
way from the lacoperon repressor
–Using a special attenuator sequence

12. The Tryptophan
Biochemical Pathway
O
-
OOC
OH
HN
H
H
-2
O
3P
OH
H
H
CH
2O
5-Phosphoribosyl-
a-Pyrophosphate
PP
i
N-(5’-
Phosphoribosyl)
-anthranilate
COO
-
COO
-
H
CH
2
C
HO
H
O
Chorismate
-
OOC
OH
-2
O
3POCH
2
N
H
C
H
C C
H
OH
C
H
OH
Enol-1-o-
Carboxyphenylamino
-1-deoxyribulose phosphate
N
H
-
OOC CH
2
NH
3+
C
H
Tryptophan
H
2OSerine
Antrhanilate
COO
-
NH
2
Glutamate +
Pyruvate
Glutamine
CO
2+H
2O
-2
O
3POCH
2
C
H
C C
H
OH
C
H
OH
N
H
Indole-3-glycerol phosphate
Glyceraldehyde-
3-phosphate
N
H
Indole
Anthranilate synthetase
(trpEand D)
Anthranilate synthetase
N-(5’-Phosphoribosyl)-anthranilate
isomerase Indole-3’-glycerol
phosphate synthetase (trpC)
Tryptophan synthetase
(trpBand A)
N-(5’-Phosphoribosyl)-
Anthranilate isomerase Indole-
3’-glycerol phosphate synthetase
Tryptophan synthetase

13. The TrpOperon:
When Tryptophan Is Present
STOP
Right there
Polymerase
Trp
Trp
Repressor
Repressor
RepressorPromo. trpD trpBLead.Operator trpAtrpCtrpEAten.
RNA
Pol.
Foiled
Again!
Repressor
mRNA
Hey man, I’m
constitutive

14. Attenuation
The trp operon is controlled both by a
repressor and attenuation
Attenuation is a mechanism that works only
because of the way transcription and
translation are coupled in prokaryotes
Therefore, to understand attenuation, it is
first necessary to understand transcription
and translation in prokaryotes

15. 3’
5’
5’
3’
Transcription And Translation
In Prokaryotes
Ribosome
Ribosome
5’
mRNA
RNA
Pol.

16. Met-Lys-Ala-Ile-Phe-Val-
AAGUUCACGUAAAAAGGGUAUCGACA -AUG-AAA-GCA-AUU-UUC-GUA-
Leu-Lys-Gly-Trp-Trp-Arg-Thr-Ser-STOP
CUG-AAA-GGU-UGG-UGG-CGC-ACU-UCC-UGA-AACGGGCAGUGUAUU
CACCAUGCGUAAAGCAAUCAGAUACCCAGCCCGCCUAAUGAGCGGGCUUUU
Met-Gln-Thr-Gln-Lys-Pro
UUUU-GAACAAAAUUAGAGAAUAACA -AUG-CAA-ACA-CAA-AAA-CCG
trpE. . .
Terminator
The Trp Leader and
Attenuator
4
1 2
3

17. The mRNA Sequence Can
Fold In Two Ways
4
12
3
Terminator
haripin
4
12
3

18. 3’
5’
5’
3’
The Attenuator
When Starved For Tryptophan
4
1
2
3
RNA
Pol.
Ribosome
Help,
I need
Tryptophan

19. 3’
5’
5’
3’
The Attenuator
When Tryptophan Is Present
4
1
2
3
RNA
Pol.
Ribosome
RNA
Pol.

20. Control Of Expression In
Eukaryotes
Some of the general methods used to control
expression in prokaryotes are used in eukaryotes,
but nothing resembling operons is known
Eukaryotic genes are controlled individually and
each gene has specific control sequences
preceding the transcription start site
In addition to controling transcription, there are
additional ways in which expression can be
controlled in eukaryotes

21. Eukaryotes Have Large
Complex Geneomes
The human genome is about 3 x 10
9
base
pairs or ≈ 1 m of DNA
Because humans are diploid, each nucleus
contains 6 3 x 10
9
base pairs or ≈ 2 m of
DNA
That is a lot to pack into a little nucleus!

22. Eukaryotic DNA Must be
Packaged
Eukaryotic DNA exhibits many levels of
packaging
The fundamental unit is the nucleosome,
DNA wound around histone proteins
Nucleosomes arrange themselves together
to form higher and higher levels of
packaging.

23. Highly Packaged DNA Cannot
be Expressed
The most highly packaged form of DNA is
“heterochromatin”
Heterochromatin cannot be transcribed,
therefore expression of genes is prevented
Chromosome puffs on some insect
chomosomes illustrate where active gene
expression is going on

24. Only a Subset of Genes is
Expressed at any Given Time
It takes lots of energy to express genes
Thus it would be wasteful to express all
genes all the time
By differential expression of genes, cells
can respond to changes in the environment
Differential expression, allows cells to
specialize in multicelled organisms.
Differential expression also allows
organisms to develop over time.

25. DNA
Cytoplasm
Nucleus
G AAAAAA
Export
Degradation etc.
G AAAAAA
Control of Gene Expression
G AAAAAA
RNA
Processing
mRNA
RNA
Transcription
Translation
Packaging
Modification
Transportation
Degradation

26. Logical Expression Control Points
DNA packaging
Transcription
RNA processing
mRNA Export
mRNA masking/unmasking
and/or modification
mRNA degradation
Translation
Protein modification
Protein transport
Protein degradation
Increasing cost
The logical
place to
control
expression is
before the
gene is
transcribed

27. A “Simple” Eukaryotic Gene
Terminator
Sequence
Promoter/
Control Region
Transcription
Start Site
3’5’
RNA Transcript
Introns
Exon 2 Exon 3Int. 2Exon 1Int. 1
3’ Untranslated Region
5’ Untranslated Region
Exons

28. 5’
DNA
3’
Enhancers
Enhancer Transcribed Region
3’5’ TFTF
3’5’ TF TF
5’
RNA
RNA
Pol.
RNA
Pol.
Many bases
Promoter

29. Eukaryotic mRNA
Protein Coding Region
3’ Untranslated Region5’ Untranslated Region
Exon 2Exon 3Exon 1 AAAAAG 3’5’
3’ Poly A Tail5’ Cap
RNA processing achieves three things:
Removal of introns
Addition of a 5’ cap
Addition of a 3’ tail
This signals the mRNA is ready to move out
of the nucleus and may control its life span
in the cytoplasm