Title: RNA Editing
1RNA Editing
- Definition any process, other than splicing,
that results in a change in the sequence of a RNA
transcript such that it differs from the sequence
of the DNA template - Discovered in trypanosome mitochondria
- Also common in plant mitochondria
- Also occurs in a few chloroplast genes of higher
plants, and at least a few nuclear genes in
mammals
L. Simpson
K. Stuart
2Discovery of RNA Editing in Trypanosome
Mitochondria
- Unusual Mitos. called Kinetoplasts
- DNA
- Maxicircles (22 kb in T. brucei), contains most
of the genes - Minicircles (1-3 kb), heterogenous
- Sequencing of genomic Mt DNA (Maxicircles)
revealed apparent pseudogenes - Full of Stop codons
- Deletions of important amino acids
3Kinetoplast DNA from a trypanosome visualized by
EM
Fig. 16.13
4- Where were the real functional genes?
- Investigators generated cDNA clones to some of
the kinetoplast mRNAs and sequenced them - Sequences were partially complementary to
pseudogenes on maxicircle DNA - cytochrome oxidase
- subunit II
- the COXII DNA sequence above is missing 4 Us
found in the mRNA - Called this Editing because it produced
functional mRNAs and proteins from pseudogenes
5Some genes are very heavily edited!
COXIII Cytochrome oxidase III From Trypanosoma
brucei
Lower case Us were inserted by editing. The
deleted Ts (found in the DNA) are indicated in
upper case.
Fig. 16.15
6Editing Mechanism
- Post-transcriptional
- Guide RNAs (gRNAs) direct editing
- gRNAs are small and complementary to portions of
the edited mRNA - Base-pairing of gRNA with unedited RNA gives
mismatched regions, which are recognized by the
editing machinery - Machinery includes an Endonuclease, a Terminal
UridylylTransferase (TUTase), and a RNA ligase
- Editing is directional, from 3 to 5
7Guide RNAs Direct Editing in Trypanosomes.
Editing is from 3 to 5 along an unedited RNA.
16.17,18
8Editing Mechanism with the enzymes.
TUTase, or terminal uridylyl transferase, adds
U(s) to the 3 end created by cleavage of the
pre-mRNA
from Fig. 16.20
9Other Systems with RNA Editing
- Land plant (C ? U) and Physarum (slime mold)
mitochondria (nt insertions) - Chloroplasts of angiosperms (C ? U)
- Some nuclear genes in mammals
- Apolipoprotein B, C ? U
- Glutamate receptor B, A ? I (inosine)
- Hepatitus delta virus (A ? I)
- Paramyxovirus (G insertions)
10Editing of Oenothera mitochondrial RNAs
Determined by comparing sequences of cDNA copies
of mt RNAs with the corresponding genomic gene.
11Editing of Angiosperm Mt RNAs
- Most RNAs are edited
- Most events are C ? U, but also U ? C
- Preferential editing of coding regions, but
introns and untranslated regions are also
edited. - Editing produces translatable RNAs, and restores
conserved amino acids (i.e, functional proteins).
12Possible mechanism for plant Mt editing
Deamination of cytosine (to uracil) by a cytidine
deaminase
NH2
O
N
N
H20
O
O
N
N
Cytosine
Uracil
13Plant mt RNA Editing Mechanism (cont.)
- Cytidine deaminases are known, and in fact one is
involved in ApoB editing in mammals. Plant enzyme
not identified yet. - How are editing sites recognized?
- No guide RNAs have yet been found in angiosperm
mitochondria.
14Editing of Apolipoprotein B in Mammals
- Large nuclear gene
- Editing is C6666 ? U6666 in exon 26 of the 14 Kb
mRNA - This creates a Stop codon, producing a truncated
form of the protein - - both forms circulate in blood but have
different functions - - the long form is endocytosed via the LDL
receptor the short form is not
15Molecular Consequences of Editing ApoB pre-mRNA
(Splicing precedes editing)
Produced by Unedited mRNA
Produced by Edited mRNA
16Editing of Apolipoprotein B The Editosome
- A cytidine deaminase activity is involved
- apobec (apoB mRNA editing enzyme catalytic
subunit) - Another protein, ACF (apobec complementation
factor) is also required - Both recognize sequences flanking the C to be
edited
17RNA editing in brain tissue Adenosine to inosine
ADA
Adenosine Inosine
18Inosine has long been known from purine metabolism
Inosine also acts as a signaling molecule.
ADA deficiency is a metabolic disease.
19A to I Editing in RNA
- 1st case Glutamate Receptor B
- I read as G during translation, R instead of Q
- Affects Ca2 permeability, intracellular
trafficking of receptor
20Important Examples of A to I Editing in Mammals
21Mechanism of A to I Editing
- dsRNA-dependent adenosine deaminase (ADAR)
- converts A ? I in 2 Glut Receptor B exons
(changes the amino acids I read as G during
translation) - recognizes secondary structure around site to be
edited - requires intron and exon sequences - acts on
unspliced receptor pre-mRNA - has dsRNA binding domains as well as a catalytic
center similar to the cytosine deaminase
22ADARs adenosine deaminases that work on RNA
lt- key for editing of GlutR
Other possible functions - RNA modification
(other types) - RNAi - Chromatin remodeling
dsRBD- dsRNA binding Z Zn, DNA binding R-rich
Arg rich