Translation control can be ___________ or __________ to specific mRNA’s
global (turn all translation on or off)
specific (a single mRNA)
Where does most translation regulation occur?
At the initiation step
What factors can drive translation regulation?
- mRNA structure and sequence
- Varying activities of elongation or initiation factors
- Modification of the ribosome
- Binding of proteins to mRNA
- Action of small molecules such as antibiotics.
What does the wobble base of the anticodon do?
allows for some flexibility in which tRNA will be matched to a codon.
determines the number of codons that a tRNA can recognize
Regarding pairing flexibility in tRNA codon recognition; C can pair with \_\_\_\_\_\_\_\_ A can pair with \_\_\_\_\_\_\_\_\_ U can pair with \_\_\_\_\_\_\_\_ G can pair with \_\_\_\_\_\_\_\_\_
G only
U only
A and sometimes G
C and sometimes U
What is an inosine?
Why is it special?
modified nucleotide
can pair with A, U or C
How does abundance of tRNA’s regulate translation?
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.
What are the different types of mutations?
Missense Silent Frameshift Nonsense Sense
What is a missense mutation?
change in base resulting in incorrect aa. coding
What is a frame shift mutation?
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
What is a silent mutation?
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
What is a nonsense mutation?
introduction of a stop codon due to an insertion, deletion or base change
What is a sense mutation?
Loss of a stop codon due to insertion, deletion or base change.
List two diseases caused my mRNA mutations in hemoglobin and what mutation each arises from?
- Hemoglogin Wayne - 3’ terminal frameshift mutation
2. Hemoglobin Constant Spring - UAA stop codon to CAA (Gln)
What is the Kozak sequence and why is it important?
Specific sequence preceding the AUG start codon that is favored by the ribosomes
GCCRCC_AUG_G
R = purine A or G
Give some ways in which elements of mRNA can affect its translation.
- Some 5’UTR’s are harder to scan (RNA ‘folding’)
- Kozak sequence
- Different AUG start codons have different “strengths” due their neighboring bases.
What is “recoding” during translation?
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
Describe how Iron (Fe) levels can be regulated by translation.
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
What does the transferrin receptor do? (TFR)
Transfers iron into cell when levels are low.
What is the IRE vs. IRES?
IRE = Iron Response Element (mRNA seq.; [Fe] pathway) IRES = Internal Ribosomal Entry Site (cap-ind. translation initiation)
How does transferrin levels affect Fe concentration and how is this regulated by translation?
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.
What does Ferritin do?
sequesters Fe when levels are (too) high
What does transferrin do?
transports Fe into the cell
Where is the IRE found in transferrin receptor mRNA?
3’ UTR just downstream of the stop codon
Where is the IRE found in the ferritin mRNA?
in the 5’UTR, near the start codon
Suppose low levels of Iron in the cell. How does this affect translation of transferrin and ferritin?
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.
What is an IRES?
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)
How does interferon work in broad terms?
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.
How does mTOR work in regards to 4E-bp (binding protein)
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.
Give some examples of viruses that exploit cap-independent translation initiation.
poliovirus HIV-1 Hep C Hep A foot & mouth virus human rhinovirus
What are two specific anti-viral pathways induced by interferon?
- The “warned” cells procude PKR which phosphorylates eIF-2 rendering it inactive, thus shutting down translation of the viral protein
- Induces synthesis of 2-5A which activates an endonuclease that degrades viral mRNA.
Give an example of mRNA editing?
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.
What does Rapamycin do?
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.
