Post on 14-Dec-2014
description
REGULATION OF GENE EXPRESSION
By:
Khaled El MasryAssistant Lecturer of Human Anatomy & Embryology
Mansoura Faculty of Medicine
What is the GENE????
A gene is the sequence of nucleotides in DNA encoding one polypeptide chain or one mRNA molecule.
Gene expression is carried out in 2 steps:
1 .transcription.2 .translation.
Factors inducing gene expression:
1 .Environmental factors as heat and light.2 .Signaling molecules as hormones and
growth factors.
HEAT:Exposure to high temperature
Transcription of heat shock genes
Synthesis of heat shock proteins
Stabilize the internal cellular environment
LIGHT:In plants exposure to light
Activate transcription of the gene for
Ribulose Carboxylase The enzyme that plays a critical
role in
PHOTOSYNTHESIS
HORMONES
”STEROID HORMONES”
Hormone receptor complex
Act as a transcription factor
“PEPTIDE HORMONES”
Activate a signaling system
Regulation of eukaryotic gene expression is dependent on 2 factors:
1 .Regulatory elements:Promoters & Enhancers
2 .Transcription factors:Basal & Special
Regulatory elements
PromotersThe region necessary to initiate transcription.
Consists of short nucleotide sequence that serve as the recognition point for binding of RNA polymerase.
Located immediately adjacent to the genes they regulate, upstream from the transcription startpoint.
There are significant differences in number , orientation and distance between promoters in different genes.
Promoters for RNA polymerase II include:
TATA box,CAAT box,GC box,
&Octamer box.
Promoters for RNA polymerase I & III have a different sequence and bind different transcription factors.
Importance Structure Site
Mutations in this sequence greatly reduce transcription
(Loosing the ability to bind to transcription factors)
8 bp sequences composed only of T=A pairs.
25-30 bp upstream
(from the initial point of transcription
TATA box
Mutations in this sequence greatly reduce transcription
CAAT or CCAAT sequence.
70-80 bp upstream
(from the initial point of transcription
CAAT box
Documented by mutational analysis
GGGCGG sequence, often present in multiple copies.
110 bp upstream(from the initial
point of transcription
GC box
Affects the efficiency of promoter in initiating transcription.
ATTTGCAT sequence.
120-130 bp upstream
(from the initial point of transcription
Octamer box
2 .Enhancers
DNA sequences interact with regulatory proteins
increase the efficiency of initiation of transcription
increase its rate .
Enhancers:
up to several hundred bp long).) Large >>>>
Tissue- specific >>>> ( stimulate transcription only in certain tissues).
Mechanism of action of Enhancers???
1 .The proteins that bind to enhancers affect the activity of proteins that bind to promoters???.
2 .Enhancers may allow RNA polymerase to bind to DNA and move along the chromosome till it reaches a promoter site.
3 .May respond to molecules outside the cell ( e.g : steroid hormones).
4 .May respond to molecules inside the cell ( e.g : during development thus the gene participates in cell differentiation).
How enhancers can control transcription although they
are located away from the transcription site???
Enhancers bind to transcription factors by at
Least 20 different proteins
Form a complex
change the configuration of the chromatin
folding, bending or looping of DNA.
DNA looping will bring the distal enhancers close to the promoter site to form activated transcription complexes, then the transcription is activated, increasing the overall rate of RNA synthesis.
Differentiation ( ) Enhancers and Promoters
1 .The enhancers affect the gene expression independent of their position or orientation.
2 .The enhancers operate from a distance away from their target genes.
Transcription factors
Def: . “they are proteins essential for initiation of
the transcription, but they are not part of RNA polymerase molecule that carry out
the transcription process.”
Function:
Each RNA polymerase requires a number of transcription factors which help in:
1 .Binding of the enzyme to DNA template.
2 .Initiation and maintenance of transcription.
3 .Control the rate of gene expression.
Structure & Mechanism of action
These proteins contain 2 functional domains (a.as that perform specific function).
1 .DNA binding domain: binds to DNA sequences present in regulatory regions (e.g : TATA binding protein).
2 .Transcriptional activating domain: activate transcription via protein-protein interaction
Types of transcription factors:
1 .Basal transcription factors:
The initiation of transcription by RNA polymerase II requires the assistance of several basal transcription factors.
Each of these proteins binds to a sequence within the promoter to facilitate the proper alignment of RNA polymerase on the template strand of DNA.
The basal TFs must interact with the promoters in the correct sequence to initiate transcription effectively.
TFIID is the 1st basal TF that interact with the promoter ; it contains TATA- Binding Protein.
Followed by TFII B, F, E, H & J.
2 .Special TFs:Involved in regulation of heat, light, and hormone inducible genes.They bind to:
a. enhancers.b. Basal TFs.c. RNA polymerase that bind to the gene promoter.
Thus, special TFs can regulate the transcriptional activity of the gene.
Chromatin Conformation and Gene Expression
The normal structure of the chromatin suppresses the gene activity, making the DNA relatively inaccessible to transcription factors, and thus active transcription complex can’t occur.Thus chromatin remodeling is needed
(it is a change in chromatin conformation in which proteins of nucleosomes are released from DNA , allowing DNA to be
accessible for TFs and RNA polymerase .)
Ways of Chromatin Remodeling Inactive chromatin remodeled into active
chromatin by 2 biochemical modifications:
1 .Acetylation of histone proteins by histone acetyl transferases which loosen the association between DNA and histone.
2 .Specialised protein complexes disrupt
the nucleosome structure near the gene’s promoter site.
(this protein complex slides histone along DNA
transfer the histone to other location on DNA molecule.
Active chromatin can be deactivated by 3 biochemical reactions:
1 .Histone deacetylation ( catalysed by histone deacetylase).
2 .Histone methylation ( catalysed by histone methyl transferases).
3 .Methylation of some DNA nucleotides by DNA methyl transferases.
(Chromatin subjected to these modifications tends to be transcriptionaly silent)
Post-transcriptional regulation of gene expression by RNA INTERFERENCE
siRNA & miRNA
These molecules 21-28 bp long.
Able to interact with specific mRNA molecules.
This interaction occurs by base pairing ( ) a single strand of iRNA and a complementary sequence in the mRNA molecule.
siRNA
Base pair perfectly with the target
sequence in mRNA
mRNA cleavage and degradation
miRNA
Base pair imperfectly with the target
sequence in mRNA
mRNA inhibition of translation