CMB2001/L08 Post-transcriptional Control II

Question 1

Describe polyadenylation. (2)

Answer 1

Endonuclease cleavage in nascent RNA
Addition of As by polyA polymerase

Question 2

What is ‘USE’?

Answer 2

U-rich upstream element (5’ end)

Question 3

Where is the G/U rich tract located?

Answer 3

Just downstream from the polyA site

Question 4

Where is the conserved AAUAA sequence located?

Answer 4

10-35 nucleotides upstream of the polyA site

Question 5

Which sequences are bound by proteins required for polyadenylation? (3)

Answer 5

Cleavage and polyadenylation specificity factor (CPSF) binds AAUAA
Cleavage stimulatory factor (CstF) binds G/U
PolyA polymerase

Question 6

Give 3 functions of the polyA tail.

Answer 6

Enhances export of RNA
Stabilises 3’ end of mRNA
Enhances translation of mRNA

Question 7

Which mRNAs have a 3’ polyA tail?

Answer 7

All except histones

Question 8

How long is the polyA tail?

Answer 8

Approx 350 nts

Question 9

Define RNA editing.

Answer 9

Nucleotide alterations which rsult in different or additional nucleotides in mature RNA

Question 10

In which 3 major classes of RNA does RNA editing occur?

Answer 10

mRNA
tRNA
Ribosomal RNA (rRNA0

Question 11

Give the 2 classes of editing.

Answer 11

Insertion/deletion
Modification (A to I, C to U, U to C)

Question 12

Why is RNA editing in medicine and development significant?

Answer 12

Disease - atherosclerosis
Brain function - human higher brain function and depression
Development - Drosophila
Parasites
Mitochondria - potential target for drugs

Question 13

Name 2 kinds of base modification.

Answer 13

Marked nucleotide
Altered identity

Question 14

Give 2 effects of mRNA editing on start codons.

Answer 14

Creation of start codons by U insertion
Creation of start codons by C to U changes

Question 15

Give 2 effects of mRNA editing on stop codons.

Answer 15

Creation of stop codons by U insertion
Removal of stop codons by base conversions
Creation of stop codons by C to U changes

Question 16

Give 2 other effects of mRNA editing on nucleotide sequence.

Answer 16

Creation of new ORFs by nucleotide insertion
Changes in encoded amino acids and splice site choice by base conversion

Question 17

Describe RNA editing by deamination.

Answer 17

Adenosine converted to inosine by adenosine deaminases
C to U by cytidine deaminases

Question 18

How is inosine read during translation?

Answer 18

As guanosine

Question 19

What enzyme carries out cytidine deamination?

Answer 19

APOBEC-1

Question 20

What is the enzyme APOBEC-1 linked to?

Answer 20

Cholesterol control
Cancer development
Inhibition of viral replication

Question 21

What level of editing occurs in the intestine and in the liver?

Answer 21

Intestine - editing to produce ApoB-48
Liver - no editing produces ApoB-100

Question 22

Which long form major component of low density lipoproteins is linked to artherosclerosis?

Answer 22

ApoB-100 including LDL-binding receptor

Question 23

Which enzyme carries out A to I editing in the Q/r site of glutamate receptors?

Answer 23

ADAR2

Question 24

How does A to I editing in the Q/R site of glutamate receptors affect Ca2+ permeability?

Answer 24

Decrease in Ca2+ permeability of channels containing ‘R’ version

Question 25

What is the result of mutations in the mouse ADAR2 gene result in?

Answer 25

Seizures
Post-natal death
Neurodegeneration in hippocampus

Question 26

Give 3 reasons for mRNA localisation.

Answer 26

Localised protein synthesis
Generate cell polarity
Prevents expression in the wrong place
Promotes efficiency of subsequent protein targeting
Local control of translation

Question 27

Describe diffusion based localisation.

Answer 27

mRNAs freely diffuse in cytoplasm and are locally entrapped by anchor proteins

Question 28

Describe active transport based localisation. (2)

Answer 28

mRNA recognised by specific trans-acting factors in the nucleus
Cytoplasmic factors ensure transport along a polarised cytoskeleton

Question 29

How is mRNA localised asymmetrically in the cytoplasm?

Answer 29

Localised to dendrites or axons enabling synaptic protein synthesis
In embryonic development, specific mRNAs localised to ant/posterior poles of cell to establish polarity and facilitate axis formation

Question 30

Which mechanisms are used to localise mRNAs?

Answer 30

Active or passive transport