RNA processing in eukaryotes Flashcards
What is changed in response to RNA processing regulation?
The amount of polypeptide produced, or which polypeptide is produced
What do changes to RNA processing alter about the transcript?
mRNA stability, translation efficiency, localization of the mRNA, changes to the coding region to produce a different polypeptide
What are the 3 main mRNA processing events that are added to every single transcript?
5’ methylguanosine cap, splicing, 3’ polyadenosine tail
What determines when mRNA processing events occur?
The phosphorylation state of the C-term of RNAP, which determines the mRNA processing proteins that get recruited
What are 2 reversible modifications to an mRNA transcript?
m6A methylation and mRNA editing
What does the m6A methylation modification do?
Gets read by RNA binding proteins and regulates stability, export, translation initiation, and miRNA biogenesis
What mRNA editing event occurs in primate mRNA?
A to I editing by ADAR proteins. Occurs at Alu sequences
What mRNA editing events occur in flowering plants?
C to U editing in the chloroplasts and mitochondria
When does the 5’ cap get added to a transcript?
As soon as the new transcript emerges from RNAP II
What protein adds the 5’ cap to a transcript?
The capping enzyme complex (CEC). Includes 3 proteins and is bound to RNAP II before transcription initiation
What are the 3 steps in the capping process?
- 5’ phosphate gets removed
- GTP gets added in its place, forming a 5’-‘5’ phosphate bond
- The guanosine gets methylated
What are the 4 functions of the 5’ cap?
- Protect the transcript from 5’ exonucleases
- Binding to proteins needed to initiate translation
- Binding to proteins needed for nuclear export
- Some involvement in splicing
When does the 3’ polyA tail get added?
As soon as the finished transcript gets cleaved
What are the 3 enzymes that coordinate the addition of the polyA tail?
CPSF (cleavage and polyadenylation specificity factor), PAP (polyA polymerase), PABII (poly A binding protein)
What is the polyadenylation sequence?
AAUAAA
What are the 4 steps in the polyadenylation process?
- CPSF and other cleavage factors bind to the polyadenylation sequence as soon as it gets transcribed
- The transcript gets cleaved
- CPSF recruits PAP to the cleaved transcript
- PAP extends the polyA tail with help from PABII
How long is the finished polyA tail?
200 adenine residues
What are 3 functions of the polyA tail?
- Protect from 3’ exonucleases
- Required for translation
- Required for nuclear export
How can polyadenylation be a point of regulation?
It can be used to control when a transcript gets translated
How does cytoplasmic polyadenylation control when maternal mRNAs in Xenopus get translated?
The tail gets removed in the cytoplasm so that translation can’t occur. Only when the cell receives a progesterone signal does the tail get added again the cytoplasm and translation can proceed.
What are the steps seen in cytoplasmic polyadenylation regulatory mechanism seen in Xenopus maternal mRNAs?
- Maternal mRNA gets transcribed and processed, then exported into the cytoplasm
- PARN (a deadenylase) removes most of the polyA tail until there’s only about 20 A’s left
- A signalling cascade in response to progesterone triggers a change in the activity of CPE binding protein (CPEB), so it binds to a CPE (cytoplasmic polyadenylation element)
- CPEB recruits CPSF and cytoplasmic PAP to add a full polyA tail
- The transcript now has a full tail and can be translated
