RNA lecture Flashcards
What happens to the mRNA after its transcription?
Three main events
- 5’ end capping
- 3’ end processing - addition of poly A tail
- splicing
- RNA editing
What key feature of RNAPII is needed for RNA processing
At the carboxy-terminal domain of the RNAPII there is a series of serine residues that become phosphorylated. The phosphorylation of these residues is essential for critical stages of transcription
At what size is a nascent RNA capped?
A pre-mRNA is capped when it is 22-40 nucleotides long
Describe the structure of the 5’ cap
The 5’ cap is a 7 methylguanosine molecule linked by an unusal 5’-5’ triphosphate linkage
Describe the capping reactions
1) RNA triphosphatase cleaves the gamma phosphate from the 5; end of the RNA
2) RNA guanyltransferase adds GMP from GTP to RNA diphosphate
3) Methylation of guanine
What is the function of the 5’ cap?
It counteracts attack by 5’ exonucleases
It increases the efficiency of splicing and polyadenylation
In the nucleus the cap is bound by the cap binding complex (CBC) which is recognised by the nuclear pore complex facilitating the export of newly made RNAs into the cytoplasm. RNAs that are capped are more efficiently transported to the cytoplasm
In the cytoplasm, CBC is replaced by the translation initiation factor eIF4E - essential for translation
Describe the two sequences required for polyadenylation
- AAUAA
- downstream G/U rich region
separated by 20 nucleotides
Describe the transacting factors that bind to the cis acting sequences needed for polyadeynlation (AAUAAA, GU rich region)
CPSF - cleavage and polyadenylation specific factor binds to AAUAAA and cuts the mRNA between the two signals
CtsF - cleavage stimulation factor
Describe three functions of the polyA tail
- To protect the mRNA from degradation by 3’ exonucleases
- To promote nuclear export
- To promote translation
Alternative polyadenylation sometimes results in alternative terminal exons, describe the two types
1) Skipped Terminal Exons - alternative exons are selected through splicing to be the terminal exon
2) Composite terminal exon - extension of an internal exon, into the next intron through inhibition of the 5’ splice site
Describe the consequence of having APAs in 3’ UTRs
mRNA containing different 3’ UTRs are regulated differently
mRNA with shorter 3’ UTRs are usually translated at higher levels
Describe the two types of RNA editing
Base modification
A–>I (ADAR)
C–>U (CDAR)
Insertion/Deletion
