Chapter 20 - mRNA stability

Question 1

mRNAs have characteristic structures that are related to

Answer 1

their stability in the cell

Question 2

Stability of mRNA is related to

Answer 2

ribonuclease activity
exoribonucleases and endoribonucleases

Question 3

mRNA-specific stability information is encoded in

Answer 3

cis-sequences

Question 4

stability is measured in

Answer 4

half-life

Question 5

mRNA stability can range

Answer 5

100-fold or more

Question 6

Eukaryotic mRNA exists as

Answer 6

mRNPs

Question 7

RBP

Answer 7

Ribonucleoprotein particles than contain RNA and RNA-binding proteins (RBPs)

Question 8

Eukaryotic mRNAs are associated with a distinct combination of

Answer 8

RBPs that serve to classify the mRNA

Question 9

Eukaryotic mRNAs are protected from degradation by

Answer 9

3’ tail and 5’ cap

Question 10

Most mRNA degradation is initiated by removing the

Answer 10

3’ tail

Question 11

Tail is gradually shortened as soon as it enters the cytoplasm by

Answer 11

poly(A) nucleases

Question 12

polyA nucleases

Answer 12

PAN2/3 slow initial degradation
CCR4-NOT quickly removes remaining tail

Question 13

Nuclear CCR4-NOT complexes promote

Answer 13

transcription elongation

Question 14

5’ to 3’ decay pathway

Answer 14

Digestion of the poly(A) tail down to 10-12 nt
Lsm1-7 decapping enhancer binds to short poly(A) tail
Lsm1-7 activates the decapping reaction on the 5’ end
Removal of the cap produces a 5’ monophosphorylated RNA
5’ to 3’ Xrn1 exonuclease rapidly degrades the mRNA

Question 15

The 5’ cap is usually resistant to decapping while it is being translated because

Answer 15

it is bound to a cytoplasmic cap-binding protein

Question 16

Translation and decapping machineries compete for

Answer 16

the cap

Question 17

initiation of translation involves an

Answer 17

interaction between the PABPs and the eIF4F initiation complex at the 5’ end
Removal of the tail and release of PABPs destabilizes the eIF4F-cap interaction
Cap is more exposed to decapping enzymes

Question 18

3’ to 5’ decay pathway

Answer 18

Digestion of the poly(A) tail down to 10-12 nt triggers exosome action
The exosome is a multiprotein complex that contains a 3’to5’ exonuclease
The exosome degrades the mRNA from the 3’ end

Question 19

Deadenylation independent decapping and degradation

Answer 19

Decapping and Xrn1 digestion proceeds in the presence of a full-length tail
Requires a mechanism to prevent eIF4F binding to the cap

Question 20

RPS28B mRNA

Answer 20

S28 is the protein product of this mRNA
Small ribosomal subunit protein

Question 21

Stem-loop in 3’-UTR of RPS28B mRNA recruits a decapping enhancer

Answer 21

Only recruited when bound by S28 protein
Destabilizes eIF4F

Question 22

Cell-cycle regulated histone mRNAs in mammalian cells

Answer 22

High levels required during early S phase and should be rapidly degraded in late S
Histone expression should be reduced after DNA and chromatin synthesis is complete
These mRNAs have a 3’ structure like that of the stem-loop observed in bacterial mRNAs
No poly-A tail
Have a short poly(U) tail added on the 3’ end of the mRNA in late S
“Platform” for Lsm1-7
Standard 3’5’ poly-A dependent decay pathways are then activated

Question 23

PolyA removal-independent degredation

Answer 23

1. Deadenylation independent decapping and degradation
2. Cell-cycle regulated histone mRNAs in mammalian cells