Lectures 7-8: Post-transcriptional control of gene expression (pt1) Flashcards
What are the 4 main controls during protein synthesis?
1 - transcription control
2 - RNA processing control
3 - translational control
4 - protein activity control
What are the problems with the different protein synthesis control processes?
Genetic diseases are caused by defects in these events
Viruses alter or use these events to their advantage
Do transcription levels effect the amount of mRNA produced?
No
When are the Cap and poly A tail added?
Added post-transcriptionally (not encoded in the genome)
Why is the poly A tail added?
For stability
What are the different primary transcript processes?
Capping, Splicing, Polyadenylation and editing
Why does primary transcript processing take place and how?
They are coupled to transcription via the RNA polymerase II CTD (C-terminal domain) which acts as a landing pad
What is the structure and synthesis of the m7G cap?
Takes place on all RNA polymerase II RNAs
RNA initially contains triphosphate at 5’ end
2 step event: GpppN structure then methylation
Methylation alters chemical behaviour of the base
What are the functions of the m7G cap?
Protects mRNA from degradation by 5’-3’ nucleases
Facilitates splicing
Facilitates export from the nucleus
Critical for translation of most mRNAs
Functions mediated through protein binding (CBP80/20 nucleus and eIF4 cytoplasm)
Switches function based on location
How are exons presented in the pre-mRNA sequence?
They are not a continuous coding stream
What happens to mRNA if introns are not spliced out?
A truncated protein forms that can be degraded or have a detrimental effect
What is the gene with the largest mRNA and introns?
Dystrophin - gene linked to duchenne muscular dystrophy
What are the different conserved sequences in introns?
5’ splice site
3’ splice site
Branch site
What is the function of the conserved sequences in introns?
They define the limits of exons and introns
Recruit splicing machinery required to remove introns and join exons
What are the 2 steps of splicing introns?
Step 1: cut at 5’ splice site
Creation of bond between 5’ end of intron and branch site
Step 2: cut at 3’ slice site to release intron lariat
Ligation of 2 exons
What are the reactions that take place when splicing exons?
2 trans-esterification reactions (chemical)
What is the spliceosome?
An enzymatic complex that catalyses the removal of introns
Requiring ATP
>200 proteins that assemble onto each intron
Proteins: RNA-binding proteins, ATPases, GTPases
Contain several snRNPs (U1, U2, U4, U5 & U6)
What are snRNPs?
Small nuclear ribonucleoprotein particles
Stable RNA - protein complexes in the nucleus
Do not code for protein
Base-pair wit conserved sequences in the intron
(splicing catalysed by snRNAs NOT snRNPs)
What structural properties do snRNPs have?
Main: Sm ring
Sm binding site - conserved sequence on all molecules
Allows the binding of subset proteins
How was the function of snRNP ring determined?
Anti-Sm and Lupus
Anti-Sm antibodies react against the Sm proteins
Anti-Sm antibodies are very rare unless you have systemic lupus erythematosis (SLE)
How is splicing catalysed?
By RNA
How is the proteome expanded in alternative splicing?
Different genes can be mixed and matched to produce different isoforms meaning different proteins are produced
What are the different types of alternative splicing?
Exon skipping
Intron retention
Mutually exclusive exons
Alternative 5’ splice site
Alternative 3’ splice site
How do mutations impact alternative splicing?
It can lead to less isoforms of genes
