Transcript Lecture 20: Gene Regulation in Eukaryotic Cells Dr. A. Abouelmagd

• Enumerate the differences
between Lactose & tryptophan operons?
Lac operon
1)
2)
3)
4)
5)
6)
7)
8)
9)
10)
Inducible gene complex.
Catabolic system (converts lactose
into glucose).
Contains 3 structural Genes.
Produces 3 inducible enzymes.
Repressor protein is active.
High lactose turn transcription ON,
in low glucose level.
Lactose act as inducer.
Has 2 types of gene regulation:
- Negative & positive control.
Usually OFF, active under certain
condition.
cAMP is necessary for CAP to switch
transcription ON.
Tryptophan operon
1) Repressible gene complex.
2) Anabolic system (synthesizes
tryptophan).
3) Contains 5 structural Genes.
4) Produces 5 repressible enzymes.
5) Repressor protein is inactive.
6) High tryptophan turn transcription
OFF.
7) Tryptophan act as co-repressor.
8) Has one type of gene regulation:
- Negative control.
9)
Usually ON, inactive at high level of
tryptophan
10)
cAMP is not necessary
Gene Regulation
In Eukaryotic Cells
General features:
In Eukaryotic Cells, it is more complicated and
multifaceted.
1) The changes in environment; turn a set of genes ON or OFF
(viral infection & heat shock).
2) Tissue specific regulation; specialization and organization of the
cells in organs require activation of some genes & inactivation of
others.
3) Operons are not present in eukaryotic cells.
4) Specific regulatory sequence; for each gene.
5) House-keeping enzymes; are encoded by constitutive genes.
6) Temporal regulation mechanism, some genes are active only
during certain period of life (inducible).
Levels of Gene Expression
Control In Eukaryotic Cells
1)
2)
3)
4)
5)
DNA or Chromosomal organization.
Gene transcription level.
Post-transcription level.
Translation control.
Post-translation control.
a) Gene expression control at level
of DNA organization
1)
2)
2)
Formation multiple copies of genes.
e.g.: rRNA & tRNA
(the cell contains 150 – 450 transcription units).
Specific protein
Gene amplification (self –replication of some genes
to increase their product).
Gene inactivation by:
a) Change chromatin structure.
formation a compact mass of inactive heterochromatin.
e.g. x- chromosome form Barr body.
b) DNA –methylation.
- addition of methyl group to DNA-cytosine
methyl cytosine.
- Methy cytosine + specific protein
methyl cytosine protein complex
.
Methyl cytosine
Blocks
transcription
b) Gene expression control at
Transcription level

Control of the rate of transcription depends on:
1) Efficiency of promoter
2) Enhancer
3) Regulatory protein
(transcription requires multiple regulatory proteins called
general transcription machinery or regulatory protein complex).
Promoter
DNA
Enhancer
UPEs
TATA -box
Transcription
initiation site
30 base pairs
About 100 base pairs
About several thousands of base pairs
1) Promoter

Composition:
1)
TATA –box :
2)
UPEs (upstream promoter elements):




• Binding site of RNA –polymerase.
• Formed of T & A.
• About 30 base pairs in upstream from
transcription initiation site.
• Each one about 8 – 12 base pairs
• About 100 base pairs in upstream from initiation site.
Activity of promoter depends on the number & type of UPEs.
Weak promoter contains few UPEs.
Strong promoter contains several UPEs.
UPEs are required for accurate & efficient initiation of mRNA –
synthesis.
2) Enhancer
•
•
DNA –sequence
Increase rate of Gene transcription, when it interacts with
transcription machinery complex in presence of the activator
protein
3) Regulatory protein
•
•
•
May be activator or repressor protein.
Transcription requires multiple regulatory proteins, R. Protein complex.
Regulatory protein complex binds to TATA –box to facilitate the
binding of RNA polymerase, that results in formation of
transcription machinery complex.
4) Activator protein
•
•
Have 2 functional domains; one binds to Enhancer & the other
connects the transcription machinery complex.
Increase rate of transcription & mRNA synthesis.
TATA –box
Initiation site
Regulatory protein
complex (multiple proteins)
enhancer
DNA
1) R.P. complex is
required for
binding of RNA
polymerase
RNA-polymerase
2)
No transcription, although
RNA-polymerase &
Transcription M.C. are
bound to TATA-box.
( Transcription is very low
or not at all )
enhancer
DNA
3) Activator protein facilitates interaction
between Enhancer & T.M.C. to accelerate
rate of transcription.
DNA –loop allows
interaction between:
- Activator protein,
- Enhancer
- Transcription .M.C.
Transcription
machinery
complex
activator
DNA
Transcription ON
Gene Expression at
Post-translation level
1)
Proteolytic process:
Pepsinogen (inactive)
Proteolytic enzymes
Pepsin (active)
2) Selective degradation:
To maintain a suitable concentration of protein within the
cell and removal the unuseful parts.
3) Chemical modification:
removal or addition functional group to activate or
inactivate some enzymes, such as phosphate group.