Translation Regulation (Kieft L2)

Question 0

Translation control can be ___________ or __________ to specific mRNA’s

Answer 0

global (turn all translation on or off)

specific (a single mRNA)

Question 1

Where does most translation regulation occur?

Answer 1

At the initiation step

Question 2

What factors can drive translation regulation?

Answer 2

1. mRNA structure and sequence
2. Varying activities of elongation or initiation factors
3. Modification of the ribosome
4. Binding of proteins to mRNA
5. Action of small molecules such as antibiotics.

Question 3

What does the wobble base of the anticodon do?

Answer 3

allows for some flexibility in which tRNA will be matched to a codon.
determines the number of codons that a tRNA can recognize

Question 4

Regarding pairing flexibility in tRNA codon recognition;
C can pair with \_\_\_\_\_\_\_\_
A can pair with \_\_\_\_\_\_\_\_\_
U can pair with \_\_\_\_\_\_\_\_
G can pair with \_\_\_\_\_\_\_\_\_

Answer 4

G only
U only
A and sometimes G
C and sometimes U

Question 5

What is an inosine?

Why is it special?

Answer 5

modified nucleotide

can pair with A, U or C

Question 6

How does abundance of tRNA’s regulate translation?

Answer 6

different amounts of tRNA’s present and the degree to which certain codons are used (when redundancy is possible) allows cells to regulate (slow down or speed up) mRNA translation.

Question 7

What are the different types of mutations?

Answer 7

Missense
Silent
Frameshift
Nonsense
Sense

Question 8

What is a missense mutation?

Answer 8

change in base resulting in incorrect aa. coding

Question 9

What is a frame shift mutation?

Answer 9

insertion or deletion of a base (or two) that shifts the reading frame. now the codons are not being read for the correct aa’s

Question 10

What is a silent mutation?

Answer 10

change in base pair such that codon still codes for same amino acid.

may slow down translation due to tRNA choice associated with the alternate codon

Question 11

What is a nonsense mutation?

Answer 11

introduction of a stop codon due to an insertion, deletion or base change

Question 12

What is a sense mutation?

Answer 12

Loss of a stop codon due to insertion, deletion or base change.

Question 13

List two diseases caused my mRNA mutations in hemoglobin and what mutation each arises from?

Answer 13

1. Hemoglogin Wayne - 3’ terminal frameshift mutation

2. Hemoglobin Constant Spring - UAA stop codon to CAA (Gln)

Question 14

What is the Kozak sequence and why is it important?

Answer 14

Specific sequence preceding the AUG start codon that is favored by the ribosomes

GCCRCC_AUG_G

R = purine A or G

Question 15

Give some ways in which elements of mRNA can affect its translation.

Answer 15

1. Some 5’UTR’s are harder to scan (RNA ‘folding’)
2. Kozak sequence
3. Different AUG start codons have different “strengths” due their neighboring bases.

Question 16

What is “recoding” during translation?

Answer 16

Frameshift induced by “slippery sequences” and/or specific RNA sequences embedded in the mRNA

Can be utilized by viruses
Can happen at different levels withing a cell

Question 17

Describe how Iron (Fe) levels can be regulated by translation.

Answer 17

Under high Fe conditions, IRE-BP’s are bound to Fe and cannot bind to IRE mRNA.

Under low Fe conditions, IRE-BP’s are not bound to Fe so can bind IRE mRNA

Question 18

What does the transferrin receptor do? (TFR)

Answer 18

Transfers iron into cell when levels are low.

Question 19

What is the IRE vs. IRES?

Answer 19

IRE = Iron Response Element (mRNA seq.; [Fe] pathway)
IRES = Internal Ribosomal Entry Site (cap-ind. translation initiation)

Question 20

How does transferrin levels affect Fe concentration and how is this regulated by translation?

Answer 20

Low Fe: IRE-BP can bind to IRE of transferrin receptor mRNA
this protects mRNA from degredation
synthesis of transferrin receptor proceeds
transferrin receptor transfers Fe into cell

High: IRE-BP bound to Fe. IRE-BP cannot bind to IRE. Transferrin not synthesized.

Question 21

What does Ferritin do?

Answer 21

sequesters Fe when levels are (too) high

Question 22

What does transferrin do?

Answer 22

transports Fe into the cell

Question 23

Where is the IRE found in transferrin receptor mRNA?

Answer 23

3’ UTR just downstream of the stop codon

Question 24

Where is the IRE found in the ferritin mRNA?

Answer 24

in the 5’UTR, near the start codon

Question 25

Suppose low levels of Iron in the cell. How does this affect translation of transferrin and ferritin?

Answer 25

IRE-BP not bound to iron. IRE-BP can bind to IRE (iron response element) in mRNA.

Since IRE is near stop codon (3’UTR) for transferrin, its mRNA is protected from degredation and lots of transferrin is made.

Since IRE is near start codon (5’UTR) for ferritin, translation for ferritin is blocked.

Question 26

What is an IRES?

Answer 26

Internal Ribosomal Entry Site
rRNA in the rib. allows cap-independent entry
Can be exploited by viruses after they shut down the normal 5’-cap-dependent entry of mRNA into the ribosome (by cleaving eIF4E)

Question 27

How does interferon work in broad terms?

Answer 27

virus-infected cell releases interferon which diffuses to other cells as a signal to mount immune response. original cell probably dies, but others are able to survive.

Question 28

How does mTOR work in regards to 4E-bp (binding protein)

Answer 28

4E-bp represses translation when it binds to the eIF4E complex

mTOR phosphorylates 4E-bp –> 4E does not bind to eIF4F –> eIF4F can do its job and initiate translation.

no mTOR —> 4E-bp not phosphorylated —> binds to eif4F —> eIF4F cannot do its job of translation initiation.

Question 29

Give some examples of viruses that exploit cap-independent translation initiation.

Answer 29

poliovirus
HIV-1
Hep C
Hep A
foot & mouth virus
human rhinovirus

Question 30

What are two specific anti-viral pathways induced by interferon?

Answer 30

1. The “warned” cells procude PKR which phosphorylates eIF-2 rendering it inactive, thus shutting down translation of the viral protein
2. Induces synthesis of 2-5A which activates an endonuclease that degrades viral mRNA.

Question 31

Give an example of mRNA editing?

Answer 31

A protein made by apoB mRNA is edited by cytosine deamination and introduction of a stop codon in certain tissue types.

Upshot: In liver cells the protein is longer and lasts longer. In the gut the protein is shorter and degrades faster.

Question 32

What does Rapamycin do?

Answer 32

Targets mTOR.
mTOR —> upregulates translation
Rapamycin turns off mTOR —> translation is downregulated
In particular, turns off the immune response. Prevents activation of T & B-cells. Used as anti-rejection drug in organ transplantation.