Week 1

Why is the 5’ end of mRNA capped?

• Stabilises the structure

• Enhances mRNA export and its translation in the cytoplasm

• Prevents its degredation

What is the role of the BBP?

• Branchpoint Binding Protein binds to the branch point sequence early in the splicing process

• Forms a pre splicing complex with:

  • U1 snSNP at the 5’ splice site

  • U2 AF proteins at the polyprimidine tract and 3’ splice site

• Initial arangement sometimes called the E(early) complex

What is BBP replaced by later on in the splicing process?

U2 snRNP

• preforms a more stable recognition of the branch point

What is the role of U1 snRNP?

• recognises and binds the 5’ splice site

What is the role of U2 snRNP?

• Replaces the BBP at the branch point and positions the branch-point-adenosine for catalysis 

• Requires ATP for its stable association

What forms the active catlyitic spliceosome?

• U4/U5/U6 snRNP

What sequence is important for 3’end formation of mRNA?

AAUAAA

• found 10-30 nucleotides upstream of cleavage site 

• Signals cleavage and the addition of ~250 A residues (polyA tail)

What is the CTD?

Carboxy Terminal Domain

• Consists of repeats of YSPTSPS, varying in repeat number by species

• Serves as a binding platform for proteins involved in splicing and mRNA export

What does co-transcriptional mRNA processing mean?

• mRNA is processed (Capped, Spliced ect…) whilst transcription is still taking place

Which bit of Pol II is the CTD associated with?

Large subunit

What is the rough correlation between CTD repeat number and organism complexity?

• Humans have 52

• Budding yeast have 26

• Rough correlation between repeat number and organims complexity (more = more complex)

What are the 7 AAs within the CTD repeats?

• Y - Tyrosine 

• S - Serine 

• P - Proline

• T - Theronine

• S - Serine 

• P - Proline 

• S - Serine

What does phosporylation of Ser5 on the CTD trigger?

Spliceosome association 

What happens when phosphorylation of the CTD is inhibited?

• Co-transcriptional processing does not occur

• But processing not to do with CTD still occurs 

What happens to the CTD repeats throughout transcription?

They are dynamically modified

What is triggerd by phosphorylation of Ser5 on the CTD?

• Requitment of capping enzyme

• 5’ end of the mRNA capped 

What is CPSF and where is it found?

Cleavage and Polyandenlyation Specificity Factor 

• Is bound to Pol II, waiting for AAUAAA signal

What happens to Ser2p levels when AAUAAA signal is transcribed?

• CTD has some Ser2p before polyadenylation but not alot

• Kinase Cdk12 helps increase phosphorylation of Ser2 after passing the stop signal

What is recruited when Ser2p levels increase?

CSF (cleavage stimulation factor)

• Joins with CPSF to create full 3’ end processing machinery