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Regulation of Gene expression
Gene Regulation
Learning Goals:
To know and explain:
 Constitutive ( house keeping) vs. Controllable genes
OPERON structure and its role in gene regulation
Regulation of Eukaryotic Gene Expression at different levels:
DNA methylation
Histone modifications(Chromatin Remodeling)
Increasing the number of gene copies (gene amplification)
Changing the rate of initiation of transcription
Alternate splicing
mRNA stability
Changing the rate of initiation of translation
Protein stability
Hormonal regulation
•
11-Regulation by protein stability
Gene Regulation
What makes one cell different from another cell?
• Regulation not only involves which genes are active or inactive,
but also determines the level of activity and the amount of
protein that is available in a cell.
• Genes which are always active are called constitutive genes, or
housekeeping genes.
Control Mechanisms
Over 20,000 human genes – but not all are needed as
proteins at the same time
• It would be energy inefficient to synthesize all of
them all the time!
• Thus, gene regulation:
– The turning on or off of specific genes as required by an
organism
• But there are still housekeeping genes:
– Genes that are continually transcribed and translated
because they are vital to the organism’s life functions
• And to help, there are transcription factors:
– Proteins that bind to DNA that needs to be transcribed to
help RNA polymerase bind and transcribe more easily
Gene regulation
Gene expression/ regulation can occur at 4 levels:
1. Transcriptional
2. Post-transcriptional
3. Translational
4. Post-translational
A promoter is a region of DNA that attracts the RNA polymerase
complex to bind and begin transcription.
Gene regulation
Regulation in prokaryotes
Regulation is required when switching metabolic pathway
dependent upon what nutrients are available
3 levels of regulation:
1. During transcription
2. During translation
3. After protein synthesis
Gene regulation
OPERON:
In prokaryotes, an area that includes a promoter and many
genes clustered together under control of a single promoter
Genes involved in the same metabolic pathway are often
found in the same operon.
Genes can be expressed in a single mRNA strand called
polycistronic mRNA.
**Polycistronic – multiple gene products on the same piece of
DNA.
OPERON in gene regulation of prokaryotes
Definition: a few genes that are controlled collectively by one
promoter
Its structure: Each Operon is consisted of few structural genes(
cistrons) and some cis-acting element such as promoter (P) and
operator (O).
Its regulation: There are one or more regulatory gene outside of
the Operon that produce trans-acting factors such as repressor or
activators.
Classification:
1- Catabolic (inducible) such as Lac OPERON
2- Anabolic (repressible) such as ara OPERON
3- Other types
An operator is a DNA segment that turns the gene "on" or "off." It interacts with proteins
that increase the rate of transcription or block transcription from occurring.
General structure of an OPERON
The lac Operon
E.Coli can adjust gene expression according to the sugar is available.
Genes that encode the enzymes for are needed to break down
lactose are found in the lac operon. The lac operon consists of a
coding region and a regulatory region
Also contains:
•Promoter
•Operator- a DNA sequence to which a protein binds to inhibit
transcription initiation (repressor)
The CAP is a DNA sequence to which a specific protein also binds.
The binding of CAP increases the rate of transcription of a gene or
genes.
The activity of an Operon in the presence or
the absence of repressor
No repressor
With repressor
Figure 8.13
To summarize…
Lac Operon
• Inducible operon
• Repressor (LacI) is usually
ON the operator
• Repressor changes shape
and falls off when the
inducer (lactose) binds to
the repressor, allowing
transcription
Trp Operon
• Repressible operon
• Repressor is usually OFF the
operator
• Co-repressor (tryptophan)
changes the repressor’s
shape so it can attach to the
operator, blocking
transcription
Gene Regulation in Eukaryotes
Gene Regulation in Eukaryotes
Regulation is much more complex:
•
•
•
•
•
Pre-transcriptional
Transcriptional
Post-transcriptional
Translational
Post translational
1- Control at DNA level by DNA
methylation
– Heterochromatin is the most tightly packaged form of DNA.
transcriptionally silent, different from cell to cell
– Methylation is related to the Heterochromatin formation
•
Small percentages of newly synthesized DNAs (~3% in mammals)
are chemically modified by methylation.
•
Methylation occurs most often in symmetrical CG sequences.
•
Transcriptionally active genes possess significantly lower levels of
methylated DNA than inactive genes.
•
Methylation results in a human disease called fragile X syndrome;
FMR-1 gene is silenced by methylation.
2- Control at DNA level by Histone
modifications(Chromatin Remodeling)
•
Acetylation by HATs
and coactivators leads to
euchromatin formation
•
Methylation by HDACs
and corepressors leads to
heterochromatin
formation
3-Control at DNA level by gene
amplification
Repeated rounds of DNA replication yield multiple
copies of a particular chromosomal region.
4- Control at transcription
initiation
By using different sequences (promoter, enhancer or silencer
sequences) and factors, the rate of transcription of a gene is controlled
gene X
promoter
gene control region for gene X
5- Control at mRNA splicing
(alternate splicing)
(four exons)
1
2
1, 2 & 4
cell 2
1, 2 & 3
cell 1
4
3
Calcitonin
gene-related
peptide
32 amino acids
Reduces bone resorption
37 amino acids
Vasodilator
61
6- Control at mRNA stability
• Some hormones which enhance the production of
proteins also increase the half life of the protein’s
mRNA.
Estrogen : ovalbumin
t1/2 from 2- 5hr to >24hr
Prolactin : casein
t1/2 from 5 hr to 92hr
7- Control at initiation of translation
3’ UTR
5’ UTR
AUG
UAA
Specific sequences make specific secondary structures
Specific protein factors bind to these secondary structures
8-Regulation by protein stability
•Ubiquitin-dependent proteolysis. Cyclins control of cell cycle.
• Protein molecule is tagged for degradation by attachment of a 20 kDa
protein, ubiquitin
ATP
NH2
NH2
+
Doomed protein
molecule
CO NH
COOH
ubiquitin protein
ligase
CO NH
26S
proteasome
• The stability of a protein depends upon its N-terminal amino acid
(the N-end rule).
N-terminal : For example arginine , lysine : protein t1/2 = 3 min
N-terminal : For example methionine, alanine, : t1/2 >20 hrs.
Regulation by water soluble Hormones
Regulation by lipid soluble Hormones
Steroid hormones pass through the cell membrane
and bind cytoplasmic receptors, which together
bind directly to DNA and regulate gene expression.
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