RNA editing modifies the RNA sequence after transcription, primarily through site-specific deamination and guide RNA-directed uridine insertion or deletion. These processes alter protein coding information, influencing key biological functions such as cholesterol transport and brain development. The fully processed mRNA then undergoes transport from the nucleus to the cytoplasm, facilitated by specific proteins and energy requirements.

1. RNA EDITING
RNA editing, like RNA splicing, can change the sequence of the RNA after it has
been transcribed. Instead of stretches of the mRNA being reassorted, during
editing, individual bases are either inserted, deleted or changed. That is the
coding information in the RNA is altered.
There are two mechanisms that mediate editing:
1. Site specific deamination of adenines or cytosines
2. Guide RNA directed uridine insertion or deletion
1. Site specific deamination of adenines
➢ The mammalian apolipoprotein B gene has several exons among which
one is CAA codon that is targeted for editing.
➢ It is the C within the code that gets deaminated. The deamination is
performed by the enzyme cytidine deaminase, that converts C to U. the
deamination occurs in a tissue-specific manner: messages are edited in
intestinal cells but not in liver cells.

2. ➢ The CAA codon, which is translated as glutamine in the unedited message
in the liver, is thus converted to UAA- a stop codon in the intestine. The
result is full length protein is formed in the liver but a truncated
polypeptide is made in the intestine.
➢ The longer form of apolipoprotein, in the liver is involved in the transport
of endogenously synthesized cholesterol and triglycerides. The shorter
form, in the intestine transports dietary lipids to various tissues.
Fig: Site specific deamination.
Site specific deamination of cytosines
➢ This reaction is performed by the enzyme ADAR (adenosine deaminase
acting on RNA) to generate inosine from adenine.
➢ Inosine can base pair with cytosine, and thus this change can readily alter
the sequence of the protein encoded by the mRNA.
➢ An ion channel expressed in mammalian brains is the target of this type
of editing. A single edit in its mRNA elicits a single amino acid change in

3. the protein, which in turn alters the Ca2+
permeability of the channel. In
the absence of this editing, brain development is seriously impaired.
2. Guide RNA directed uridine insertion
or deletion
➢ In this case, multiple Us are inserted into specific regions of mRNA after
transcription. The addition of Us to the message changes codons and
reading frames, completely altering the meaning of the message.
➢ In a specific region of the mRNA of the trypanosome coxII gene, four Us
are inserted between adjacent sites at three sites.
➢ Us are inserted in to the message by so called guide RNAs (gRNAs). Each
gRNAs are divided in to three regions:
1. The anchor at the 5’ end. It directs the gRNA to the region of the
mRNA it will edit.
2. The region that determines exactly where the Us will be inserted
within the edited sequence.
3. Poly- U stretch at the 3’ end.
Fig: Parts of gRNA

4. ➢ A stretch of gRNA complementary to the region in the message to be
edited contains additional As. The As are at positions in the gRNA opposite
where Us will be inserted in to the mRNA.
➢ The gRNA and mRNA form an RNA-RNA looped out single stranded regions
opposite where Us will be inserted. An endonuclease recognizes and cuts
the mRNA opposite these loops.
➢ Editing involves the transfer of Us in to the gap in the message. This
process is catalysed by the enzyme 3’ terminal uridylyl transferase
(TUTase).
➢ After the addition of Us, the two halves of the mRNA are joined by an
mRNA ligase, and the editing region of the gRNA continues its action along
the mRNA in to its 3’-to-5’ direction.
Fig: Guide RNA directed uridine insertion or deletion

5. mRNA Transport
Once it has been fully processed- capped, spliced and polyadenylated- an
mRNA is transported out of the nucleus and in to the cytoplasm where it
is translated to give its protein product. The fully processed mRNA
represents only a small proportion of the RNA found in the nucleus and
many of the other RNAs would be detrimental (damaged or misprocessed
RNAs) if exported.
The moment an RNA molecule starts to be transcribed; it
becomes associated with proteins of various sorts. Some
proteins are replaced but others are not.
Moreover, more proteins join. Thus, the mature mRNA carries a
collection of proteins that identifies it as being mRNA destined
for transportation.
Other mRNAs not only lack these signature proteins but also
often carries hnRNPs (repressors of splicing) and these mark
such RNA for nuclear retention and destruction.
In addition to SR proteins (Serine-argine rich splicing regulators),
the mature mRNA carries another group of proteins that binds
to exon-exon junctions (which are only found in spliced species).
This emphasizes the fact that it is the set of proteins, not any
individual protein, that marks RNA for either export or retention
in the nucleus.
Export takes place through a special structure in the nuclear
membrane called the nuclear pore complex.

6. Some of the proteins associated with the RNA carry nuclear
export signals that are recognized by export receptors that
guide the RNA out through the pore.
Once in the cytoplasm, the proteins are discarded and are then
recognized for import back in to the nucleus, where they
associate with other mRNA and repeat the cycle.
Export requires energy, and this is supplied through hydrolysis
of GTP by a GTPase protein called Ran.
Fig: mRNA transport from the nucleus to the cytoplasm.
Reference:
Watson, J, D. Molecular Biology of The Gene. Pearson Publication. Fourth
edition 2019.