What is the CTD? What ind of sequence does it have?
On large subunit, it’s phosphorylated form is associated with highly transcribed genes
- composed of heptapeptide sequences
What phosphorylation is actively taking place in event of transcription?
- CTD: the serine at position 5 then serine at position 2
Describe the process of 5’ capping
- Capping enzyme recognizes serine 5, and sits near the 5’ end
- CDK7 phosphorylates on serine 5, and the phosphorylated P caps the 5’ end
Why is 5’ capping important?
- protects pre-mRNA
- Facilitates nuclear export
- aids in recognition by translation factors
What is an important step of RNA pol II after initiation?
RNA pol II pauses after inititiation
- allows a “changing of the guard” from Factors blocking elongation (NELF) to factors enhancing it (DSIF, SPT6 and PAF)
- This switch is mediated by CDK9
What are the roles of CDK9 and CDK7?
-CDK7 phosphorylates on serine 5 after DNA is melted by TF2H- leading to capping
CDK9 adds additional phosphates to serine 2, causing conformational changes (changing of the guard)
Explain the spliceosome cycle
Why is it necessary?
Eukaryotes have introns and exons- meaning that DNA -> premRNA (introns) -> mRNA -> proteins
RNA processing of premRNA takes place in order to remoe introns and join exons in correct order
Explain how the spliceosome cycle takes place?
- First: 5’cap is added, poly(A) tail on 3’ end, introns are removed
- Exon-intron junctions almost always have GU(5’) and AG(3’) of intron, and an A upstream on the 3’ splice site (branch point)
- snRNPs (U1, U2, U5, U4 and U6) carry out spliceosome cycle
- U1 binds to 5’ splice site, U2 binds to A residue
- U5, U4 and U6 bind U1 and U2 together, creating the spliceosome complex, with a loop in the intron
- U1 and U4 are expelled, U6 pairs with U2 at 5’ splice site
- 5’ splice site is cleaved, then 3’ end is cleaved
- Ends of exons bind together
Intron is released as ‘lariat’
