L15 - Control of Gene Expression - RNA Flashcards
(38 cards)
What 3 things can achieve different isoforms?
Alternative splice sites
Start sites
Poly-adenylation sites
What is subcellular localisation used for?
Used to target translation to the part of the cell its needed
What 2 things can translation be regulated by?
Directly by sequences in untranslated regions
Globally by regulation of eIFs
Some mRNAs have a second open reading frame that can be?
Regulated independently
What % of Drosophilla and human genes are alternatively spliced?
40% Drosophila
75% human
What are the 4 types of alternative splicing?
Optional exon
Optional intron
Mutually exclusive exon
Internal splice site
Why are splice donor and acceptor sequences found so frequently?
They are only two bases
What is a cryptic splice site?
A site that is favoured over neighbouring splice sites
What else effects the choice of splice site in RNA?
Other sequences
Regulation of alternative splicing - sex determination in Drosophila genes
Sxl – sex lethal
Tra – transformer
Dsx – double sex
Sex determination in Drosophila males method
Transcripts for sxl and tra are spliced to produce inactive isoforms
Transcripts for dsx are active and spliced to produce a male specific repressor protein
- Represses transcription of genes required for female development
Sex determination in Drosophila females method
Two X chromosomes allow a small amount of active Sxl protein - alternative promoter
Sxl represses splicing by blocking binding by U2AF
This feeds back on the sxl transcript to make more of itself and also binds to tra transcripts
Results in a female specific isoform of Dsx
Site of polyadenylation on mRNA - B lymphocytes
They produce two antibody isoforms
Antibody gene has two positions for cleavage and polyadenylation of RNA transcript
Site of polyadenylation - when the cell produces the long transcript
First stop codon is spliced out
Results in the translation of a transmembrane domain
Site of polyadenylation - when the cell produces the short transcript
Splice acceptor is lost and first stop codon isn’t lost
Results in the antibody being secreted
Alternative start sites - optimal sequence
Kozak sequence - ACCAUGG
What happens if the start sequence is not perfect - leaking scanning
Small ribosome can scan past the first AUG
- Stops at a second or third AUG
All in the same reading frame, so isoforms differ only by the sequence of there N-terminus
What favours the first AUG?
High levels of eIF-4F in a cell
Regulated nuclear transport - regulating mRNA - HIV small genome integration
HIV has a small genome that is integrated into the host genome
After integration the entire genome is transcribed in one piece
- Alternative splicing allows for many different protein products to be made
Regulated nuclear transport- why is full length RNA needed to make new virons?
Un-spliced RNAs cannot leave the nucleus
Rev protein binds to HIV introns and interacts with nuclear pore to allow unspliced RNA to leave
Rev levels distinguish two phases of infection
What is Rev protein involved in?
Involved in binding to HIV introns to allow unspliced RNA to leave
Signals in the untranslated region of mRNA can target it to part of the cell – regulating mRNA
Intermolecular base pairing within the 3’ UTR forms stem loops
These are recognized by cellular proteins
Gives rise to localised translation
Translated control elements in mRNA - regulating mRNA - ferritin
Stores iron in the cell - reducing the available Fe
Translated control elements in mRNA - regulating mRNA - transferrin
Receptor imports iron into the cell - increasing the available Fe
