11.3
RNA Editing
RNA editing is a post-transcriptional modification where a precursor mRNA (pre-mRNA) nucleotide sequence is changed by base insertion, deletion, or mo…
RNA editing is a process where a nucleotide sequence in precursor, or pre-mRNA, is changed after transcription. It allows an organism to produce different forms of a protein without altering the DNA sequence.
mRNA editing in vertebrates is the result of site-specific base deamination, a reaction where an amino group is removed from a nitrogenous base, adenine or cytosine.
When adenosine is deaminated, it is converted to inosine. Inosine closely resembles guanosine and can trick the translation machinery into reading inosine as guanosine.
This reaction is the most common type of RNA editing in animals and is catalyzed by the enzyme, adenosine deaminase acting on RNA or ADAR.
The ADAR recognizes a pre-mRNA hairpin loop formed at an exon-intron junction and edits a specific adenine present on the exon.
In vertebrates, ADAR edits the pre-mRNA of the glutamate receptor. A specific C-A-G codon is modified to C-I-G, which is then read as C-G-G by the ribosome. This substitution replaces a glutamine with an arginine in the final protein.
The second type of mRNA editing occurs when cytidine is deaminated to uridine. The editing of the apolipoprotein B pre-mRNA is a well-studied example.
There are two types of apolipoprotein B – the larger, liver-specific ApoB-100 and the smaller, intestine-specific ApoB-48.
The same pre-mRNA encodes both proteins; however, an intestine-specific complex of enzymes acts on a specific cytosine near the middle of the pre-mRNA turning a CAA codon into UAA, a stop codon.
This results in the truncated ApoB-48 protein being produced in the intestine, whereas the unaltered apolipoprotein B pre-mRNA produces the full-length ApoB-100 protein in the liver.
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Q1: What is RNA editing and how does it differ from DNA mutations?
RNA editing is a post-transcriptional process where nucleotide sequences in pre-mRNA are changed after transcription, allowing organisms to produce different protein forms without altering DNA. Unlike DNA mutations, which permanently change genetic code, RNA editing modifies only the mRNA transcript, enabling flexible protein diversity from a single gene without affecting the underlying genome.
Q2: How does adenosine deamination work in RNA editing?
Adenosine deamination converts adenosine to inosine by removing an amino group from the nitrogenous base. Inosine resembles guanosine closely enough to trick the translation machinery into reading it as guanosine. The enzyme ADAR recognizes a pre-mRNA hairpin structure at an exon-intron junction and catalyzes this deamination at specific adenine sites, changing codons and ultimately altering amino acids in the final protein.
Q3: What role does ADAR play in vertebrate RNA editing?
ADAR, or adenosine deaminase acting on RNA, is the primary enzyme catalyzing adenosine-to-inosine conversion in vertebrates. Three ADAR types exist: ADAR1 and ADAR2 are found in various tissues, while ADAR3 is brain-specific in some species. ADAR recognizes double-stranded RNA hairpin structures and edits specific adenines, such as in glutamate receptor pre-mRNA, where a C-A-G codon becomes C-I-G, read as C-G-G by ribosomes.
Q4: How does cytidine deamination create protein diversity in apolipoprotein B?
Cytidine deamination converts cytidine to uridine, exemplified in apolipoprotein B editing. An intestine-specific enzyme complex converts a CAA codon to UAA, a stop codon, producing truncated ApoB-48 in intestines. The unedited pre-mRNA produces full-length ApoB-100 in the liver. This single base modification generates two distinct proteins from the same gene, demonstrating how RNA editing enables protein recoding.
Q5: What is insertional and deletional RNA editing in trypanosomes?
Insertional and deletional RNA editing involves adding or removing nucleotides from pre-mRNA, particularly in trypanosome mitochondria. The editosome enzyme complex, guided by guide RNA molecules with complementary anchor sequences, adds or deletes uridine residues according to template instructions. Over 50% of some trypanosomal mitochondrial RNA is formed through uridine additions, enabling dramatic recoding of genetic information.
Q6: What structural feature does ADAR require to recognize editing sites?
ADAR requires a double-stranded RNA structure formed between the target region and a downstream complementary intron region of the pre-mRNA. This hairpin loop, typically formed at an exon-intron junction, acts as a recognition signal that directs ADAR to specific adenine residues for deamination. The secondary structure is essential for site-specific editing accuracy and substrate recognition.
Q7: What diseases are associated with defects in RNA editing?
Defects in RNA editing processes can cause several central nervous system disorders, including amyotrophic lateral sclerosis, epilepsy, depression, and schizophrenia. Though RNA editing is relatively rare in vertebrates, its dysregulation has significant neurological consequences. These associations highlight the critical role of precise RNA editing in maintaining proper neural function and protein expression.