RNA processing in eukaryotes

Question 1

What is changed in response to RNA processing regulation?

Answer 1

The amount of polypeptide produced, or which polypeptide is produced

Question 2

What do changes to RNA processing alter about the transcript?

Answer 2

mRNA stability, translation efficiency, localization of the mRNA, changes to the coding region to produce a different polypeptide

Question 3

What are the 3 main mRNA processing events that are added to every single transcript?

Answer 3

5’ methylguanosine cap, splicing, 3’ polyadenosine tail

Question 4

What determines when mRNA processing events occur?

Answer 4

The phosphorylation state of the C-term of RNAP, which determines the mRNA processing proteins that get recruited

Question 5

What are 2 reversible modifications to an mRNA transcript?

Answer 5

m6A methylation and mRNA editing

Question 6

What does the m6A methylation modification do?

Answer 6

Gets read by RNA binding proteins and regulates stability, export, translation initiation, and miRNA biogenesis

Question 7

What mRNA editing event occurs in primate mRNA?

Answer 7

A to I editing by ADAR proteins. Occurs at Alu sequences

Question 8

What mRNA editing events occur in flowering plants?

Answer 8

C to U editing in the chloroplasts and mitochondria

Question 9

When does the 5’ cap get added to a transcript?

Answer 9

As soon as the new transcript emerges from RNAP II

Question 10

What protein adds the 5’ cap to a transcript?

Answer 10

The capping enzyme complex (CEC). Includes 3 proteins and is bound to RNAP II before transcription initiation

Question 11

What are the 3 steps in the capping process?

Answer 11

1. 5’ phosphate gets removed
2. GTP gets added in its place, forming a 5’-‘5’ phosphate bond
3. The guanosine gets methylated

Question 12

What are the 4 functions of the 5’ cap?

Answer 12

1. Protect the transcript from 5’ exonucleases
2. Binding to proteins needed to initiate translation
3. Binding to proteins needed for nuclear export
4. Some involvement in splicing

Question 13

When does the 3’ polyA tail get added?

Answer 13

As soon as the finished transcript gets cleaved

Question 14

What are the 3 enzymes that coordinate the addition of the polyA tail?

Answer 14

CPSF (cleavage and polyadenylation specificity factor), PAP (polyA polymerase), PABII (poly A binding protein)

Question 15

What is the polyadenylation sequence?

Answer 15

AAUAAA

Question 16

What are the 4 steps in the polyadenylation process?

Answer 16

1. CPSF and other cleavage factors bind to the polyadenylation sequence as soon as it gets transcribed
2. The transcript gets cleaved
3. CPSF recruits PAP to the cleaved transcript
4. PAP extends the polyA tail with help from PABII

Question 17

How long is the finished polyA tail?

Answer 17

200 adenine residues

Question 18

What are 3 functions of the polyA tail?

Answer 18

1. Protect from 3’ exonucleases
2. Required for translation
3. Required for nuclear export

Question 19

How can polyadenylation be a point of regulation?

Answer 19

It can be used to control when a transcript gets translated

Question 20

How does cytoplasmic polyadenylation control when maternal mRNAs in Xenopus get translated?

Answer 20

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.

Question 21

What are the steps seen in cytoplasmic polyadenylation regulatory mechanism seen in Xenopus maternal mRNAs?

Answer 21

1. Maternal mRNA gets transcribed and processed, then exported into the cytoplasm
2. PARN (a deadenylase) removes most of the polyA tail until there’s only about 20 A’s left
3. 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)
4. CPEB recruits CPSF and cytoplasmic PAP to add a full polyA tail
5. The transcript now has a full tail and can be translated