Unit 6: RNA Processing

Question 1

What RNA processing occurs in the nucleus?

Answer 1

• 5’ cap addition
• 3’ poly(A) tail addition
• splicing to remove introns
• sometimes RNA editing

Question 2

What does the 5’cap do? How is it added?

Answer 2

The 5’ cap prevents recognition of the 5’ end by the 5’ - 3’ exonuclease. It is added when the 5’ carbon of GTP links to the 5’ terminal base of pre-mRNA by 5’, 5; triphosphate linkage and the methylation of guanine on N-7

Question 3

What does the guanylyltransferase do?

Answer 3

Adds the cap after being recruited by phosphorylated CTD of Pol II

Question 4

How is the Poly(A) tail signal recongnized?

Answer 4

• Ploy A is recognized by an endonuclease associated with Pol II CTD
• The endonuclease translocates to RNA
• The endonuclease cleaves between poly(A) and GU-rich region

Question 5

How is the polyA tail synthesized?

Answer 5

Poly(A) polymerase (PAP):

• PAP is an enzyme associated with Pol II CTD
• after RNA cleavage, PAP adds A residues at 3’ end of the RNA by adding one A at a time without a template

Question 6

How is the PolyA tail add protection?

Answer 6

-Poly(A) tail is a binding site for PABPs that block exonucleases

Question 7

What does alternative splicing do?

Answer 7

Enables 1 gene to make several different proteins

Question 8

What does every intron have?

Answer 8

• A 5’ splice site
• A 3’ splice site
• An internal ‘A’ just upstream of the 3’splice site at the branch point

Question 9

What are the steps of the splicing reaction?

Answer 9

1. The branch-point 2’-OH attacks the 5’ splice site
2. The 5’ splice site is now activated to attack the 3’ splice site
3. The intron is released from the spliced mRNA

Question 10

What is the splicesome made of?

Answer 10

A complex of 5 small nuclear snRNPs: U1, U2, U4, U5, U6

Question 11

What are the steps of the splicesome?

Answer 11

1. (“folders”) -U1: binds to 5’ splice site with base pairing & U2: binds to branch point with base pairing
2. U4-U5-U6 trimeric displaces U1, then U4 dissociates
3. U6 and U2 catalyze attack of the branch point at the 5’ splice site
4. The 5’ splice site attacks the 3’ splice site
5. Intron excision, joining of the 5’ and 3’ exons

Question 12

What is TREX?

Answer 12

TRanscription EXport: a large protein complex that exports mRNAs. Recruited to actively transcribed genes through binding to Pol II and to the Cap binding proteins

Question 13

What are EJCs?

Answer 13

Splicesome deposits a complex of proteins called exon junction complex on mRNAs that recruit TREX complex. EJCs are removed in cytoplasm by first round of translation

Question 14

What is RNA degradation?

Answer 14

Complete hydrolysis to NTPs, catalyzed by ribonucleases

Question 15

What is the exosome?

Answer 15

A complex of up to 10 proteins that degrades RNAs from 3’ to 5’

Question 16

Why are most proteins with a premature STOP codon degraded?

Answer 16

Normally, EJCs are removed by ribosomes during first translation. If premature STOP, EJCs downstream of STOP codon signal for degradation