RNA editing is a molecular process through which some cells can make discrete changes to specific nucleotide sequences within a RNA molecule after it has been generated by RNA polymerase.
RNA editing is relatively rare, and common forms of RNA processing include the insertion, deletion, and base substitution of nucleotides within the edited RNA molecule.
It has been shown in previous studies that the only types of RNA editing seen in the plants’ mitochondria and plastids are conversion of C-to-U and U-to-C (very rare). RNA-editing sites are found mainly in the coding regions of mRNA, introns, and other untranslated regions.
In fact, RNA editing can restore the functionality of tRNA molecules.
The editing sites are found primarily upstream of mitochondrial or plastid RNAs.
While the specific positions for C to U RNA editing events have been fairly well studied in both the mitochondria and plastid,
The identity and organization of all proteins comprising the editosome have yet to be established.
Members of the expansive PPR protein family (pentatricopeptide repeat is a 35 amino acid sequence motif) have been shown to function as trans-acting factors for RNA sequence recognition.
Specific members of the MORF (Multiple Organellar RNA editing Factor) family are also required for proper editing at several sites.
As some of these MORF proteins have been shown to interact with members of the PPR family, it is possible MORF proteins are components of the editosome complex.
An enzyme responsible for the trans- or deamination of the RNA transcript remains elusive, though it has been proposed that the PPR proteins may serve this function as well.
RNA editing is essential for the normal functioning of the plant’s translation and respiration activity.
Editing can restore the essential base-pairing sequences of tRNAs, restoring functionality.
It has also been linked to the production of RNA-edited proteins that are incorporated into the polypeptide complexes of the respiration pathway.
Therefore, it is highly probable that polypeptides synthesized from unedited RNAs would not function properly and hinder the activity of both mitochondria and plastids.
C-to-U RNA editing can create start and stop codons, but it cannot destroy existing start and stop codons. A cryptic start codon is created when the codon ACG is edited to AUG
Because rRNAs and tRNAs are non-coding chemical modification to their nucleotides affect only the structural feature and possibly catalytic activities of the molecules, With mRNAs the situation is very different: chemical modification has the potential of change the coding properties of the transcript, resulting in an equivalent alteration in the amino acid sequence of the protein that is specified.
A notable example of RNA editing occurs with the human mRNA for apolipoprotein B.
The gene for this protein codes for a 456-amino-acid polypeptide, called apolipoprotein B100, which is synthesize: d in liver cells and secreted into the bloodstream where it transports lipids around the body.
A related protein, apolipoprotein B48, is made by intestinal cells.
This protein is only 2153 amino acids in length and is synthesized from an edited version of the mRNA for the full-length protein. In intestinal cells this mRNA is modified by deamination of a cytosine, converting this into an uracil.
This changes a CAA codon, specifying glutamine, into a UAA codon, which causes translation to stop, resulting in the truncated protein.
The deamination is carried out by an RNA-binding enzyme which, in conjunction with a set of auxiliary protein factors, binds to a sequence immediately downstream of the modification position within the mRNA. Although not common, RNA editing occurs in a number of different organisms and includes a variety of different nucleotide changes.
Some editing events have a significant impact on the organism: in humans, editing is partly responsible for the generation of antibody diversity and has also been implicated in control of the HIV-1 infection cycle.
One particularly interesting type of editing is the deamination of adenosine to inosine, which is carried out by enzymes called adenosine deaminases acting on RNA (AGARs). Some of the target mRNAs for these enzymes are selectively edited at a limited number of positions,
These positions are apparently specified by double-stranded segments of the pre-mRNA, formed by base-pairing between the modification site and sequences from adjacent introns.
This type of editing occurs for example, during processing of the mRNAs for mammalian glutamate receptors.
Selective editing contrasts with the second type of modification carried out by ADARs, in which the target molecules become extensively deaminated, over 50% of the adenosines in the RNA becoming converted to inosines.
Hyper Editing has so far been observed mainly, but not exclusively, with viral RNAs and is thought to occur by chance, these RNAs adopting base-paired structures that fortuitously act as substrates for ADAR. It may, however, have physiological importance in the etiology of disease caused by the edited viruses.
This possibility is raised by the discovery that viral RNAs associated with persistent measles infections
MCQ’s
What is RNA editing?
A) A process of splicing introns
B) A post-transcriptional modification that alters RNA sequences
C) A method of DNA replication
D) A form of transcription initiation
Answer: B
Which enzyme catalyzes transcription of RNA before RNA editing occurs?
A) DNA polymerase
B) RNA polymerase
C) Reverse transcriptase
D) Ligase
Answer: B
RNA editing involves modifications at which level?
A) DNA
B) RNA
C) Protein
D) Chromosome
Answer: B
Which of the following is NOT a common form of RNA editing?
A) Base substitution
B) Insertion
C) Deletion
D) Phosphorylation
Answer: D
Which of the following is a rare form of RNA editing in plant mitochondria?
A) C-to-U conversion
B) U-to-C conversion
C) A-to-G conversion
D) G-to-A conversion
Answer: B
RNA Editing Mechanisms
What is the most common type of RNA editing in plants?
A) A-to-G editing
B) C-to-U editing
C) G-to-C editing
D) U-to-A editing
Answer: B
Which of the following proteins is involved in RNA sequence recognition in plants?
A) Histones
B) PPR proteins
C) DNA polymerase
D) Ribosomal proteins
Answer: B
The enzyme responsible for adenosine-to-inosine RNA editing is called:
A) ADAR (Adenosine Deaminase Acting on RNA)
B) RNA ligase
C) Reverse transcriptase
D) DNA helicase
Answer: A
RNA editing is essential for which of the following plant processes?
A) Photosynthesis
B) Respiration
C) Translation
D) All of the above
Answer: D
What role do MORF proteins play in RNA editing?
A) DNA replication
B) RNA sequence recognition
C) Assisting PPR proteins in RNA editing
D) Protein degradation
Answer: C
RNA Editing in Organisms
In which organelles is RNA editing most commonly found in plants?
A) Nucleus and cytoplasm
B) Mitochondria and plastids
C) Ribosomes and endoplasmic reticulum
D) Golgi apparatus and lysosomes
Answer: B
What is the function of RNA editing in mitochondria?
A) Helps DNA replication
B) Regulates cell division
C) Ensures proper translation of proteins
D) None of the above
Answer: C
Which of the following organisms is well-known for extensive RNA editing?
A) Humans
B) Trypanosomes
C) Bacteria
D) Yeast
Answer: B
Which viral infection is associated with hyper-editing?
A) Measles
B) HIV
C) Influenza
D) COVID-19
Answer: A
In mammals, RNA editing is crucial for the function of which receptor?
A) Dopamine receptor
B) Glutamate receptor
C) Insulin receptor
D) Estrogen receptor
Answer: B
Examples of RNA Editing
Which human protein undergoes RNA editing to produce two different forms?
A) Hemoglobin
B) Apolipoprotein B
C) Insulin
D) Keratin
Answer: B
What is the consequence of C-to-U RNA editing in apolipoprotein B?
A) A stop codon is introduced, producing a shorter protein
B) The protein is degraded
C) Translation does not occur
D) No effect
Answer: A
What type of modification does ADAR-mediated editing perform?
A) C-to-U
B) A-to-I
C) U-to-G
D) T-to-C
Answer: B
Where does ADAR-mediated RNA editing commonly occur?
A) Intron sequences
B) Exon sequences
C) Non-coding RNA regions
D) All of the above
Answer: D
What is inosine recognized as during translation?
A) Cytosine
B) Guanine
C) Uracil
D) Thymine
Answer: B
Miscellaneous
Which molecule is not directly involved in RNA editing?
A) tRNA
B) rRNA
C) mRNA
D) DNA polymerase
Answer: D
What is the function of RNA-binding proteins in editing?
A) Structural support
B) Recognizing target sequences for modification
C) Breaking down RNA
D) None of the above
Answer: B
How does RNA editing contribute to genetic diversity?
A) By modifying the amino acid sequence of proteins
B) By altering DNA sequences
C) By duplicating genes
D) None of the above
Answer: A
Can RNA editing change start and stop codons?
A) Yes, always
B) Only in plants
C) Yes, but it cannot remove existing start and stop codons
D) No
Answer: C
In humans, which disease has been linked to abnormal RNA editing?
A) Cancer
B) Alzheimer’s disease
C) Amyotrophic lateral sclerosis (ALS)
D) All of the above
Answer: D
RNA Editing in Different Organisms
In which organelle of trypanosomes does extensive RNA editing occur?
A) Nucleus
B) Mitochondria
C) Cytoplasm
D) Endoplasmic reticulum
Answer: B
Which of the following is a key characteristic of RNA editing in trypanosomes?
A) Only base substitutions occur
B) U-insertion and deletion occur extensively
C) It happens only in plastids
D) It does not alter protein-coding sequences
Answer: B
What type of RNA editing is commonly seen in squid nervous system genes?
A) C-to-U
B) U-to-C
C) A-to-I
D) G-to-C
Answer: C
Which organism exhibits guide RNA (gRNA)-mediated RNA editing?
A) Humans
B) Plants
C) Trypanosomes
D) Bacteria
Answer: C
Which of the following best describes guide RNAs (gRNAs)?
A) They help edit mitochondrial RNAs in trypanosomes
B) They degrade RNA molecules
C) They prevent transcription
D) They function only in bacteria
Answer: A
Proteins and Enzymes Involved in RNA Editing
What does ADAR stand for?
A) Adenosine Degradation and Repair
B) Adenosine Deaminase Acting on RNA
C) Adenosine Deletion and RNA Modification
D) Adenosine Detection and RNA Regulation
Answer: B
How does ADAR-mediated editing affect protein translation?
A) It introduces premature stop codons
B) It changes amino acid sequences
C) It has no effect on protein function
D) It removes all introns
Answer: B
What is the primary function of pentatricopeptide repeat (PPR) proteins?
A) DNA replication
B) RNA sequence recognition
C) Protein degradation
D) Lipid metabolism
Answer: B
Which of the following is NOT a known function of RNA editing?
A) Generating protein diversity
B) Repairing damaged DNA
C) Regulating gene expression
D) Enabling adaptive responses in organisms
Answer: B
Which cofactor is essential for the function of many RNA-editing enzymes?
A) Magnesium ions (Mg²⁺)
B) Calcium ions (Ca²⁺)
C) Zinc ions (Zn²⁺)
D) Sodium ions (Na⁺)
Answer: A
Types and Consequences of RNA Editing
How does inosine (I) affect translation when present in mRNA?
A) It is recognized as adenine
B) It is recognized as guanine
C) It is translated as thymine
D) It is ignored during translation
Answer: B
How does RNA editing influence alternative splicing?
A) It removes exons from mRNA
B) It modifies splice sites, leading to different protein isoforms
C) It blocks transcription
D) It prevents splicing altogether
Answer: B
Which of the following is an example of site-specific RNA editing in humans?
A) Apolipoprotein B mRNA editing
B) Hemoglobin mRNA splicing
C) Actin mRNA degradation
D) Histone RNA processing
Answer: A
How does RNA editing impact mitochondrial gene expression in plants?
A) It changes protein-coding sequences to restore function
B) It inhibits all mitochondrial gene expression
C) It only affects tRNAs
D) It removes mitochondrial DNA mutations
Answer: A
Why is RNA editing considered an evolutionary advantage?
A) It allows organisms to create multiple proteins from a single gene
B) It prevents gene mutations from occurring
C) It eliminates the need for transcription factors
D) It replaces DNA recombination
Answer: A
RNA Editing and Diseases
Which neurological disorder is linked to defects in RNA editing?
A) Parkinson’s disease
B) Alzheimer’s disease
C) Amyotrophic lateral sclerosis (ALS)
D) All of the above
Answer: D
How is RNA editing involved in viral infections?
A) It helps the virus evade the immune system
B) It repairs viral RNA mutations
C) It prevents viral replication
D) It stops viral entry into cells
Answer: A
How does hyper-editing of viral RNA affect measles virus persistence?
A) It stabilizes viral RNA
B) It creates mutations that allow long-term infection
C) It prevents the virus from replicating
D) It removes viral proteins
Answer: B
Why is RNA editing important in immune system function?
A) It helps generate antibody diversity
B) It suppresses immune responses
C) It removes viral DNA from host cells
D) It blocks antigen presentation
Answer: A
Which of the following is a consequence of defective ADAR-mediated editing?
A) Increased inflammation
B) Neurodevelopmental disorders
C) Autoimmune diseases
D) All of the above
Answer: D
Experimental and Clinical Applications of RNA Editing
What is a potential therapeutic application of RNA editing?
A) Correcting genetic mutations
B) Enhancing mRNA stability for vaccines
C) Modifying protein function without altering DNA
D) All of the above
Answer: D
How does CRISPR technology relate to RNA editing?
A) CRISPR can be adapted to edit RNA sequences instead of DNA
B) CRISPR prevents RNA from being edited
C) CRISPR only modifies DNA and does not affect RNA
D) CRISPR degrades all edited RNA
Answer: A
Which type of RNA editing could be used to treat genetic disorders without altering DNA?
A) A-to-I editing
B) U-to-G editing
C) G-to-T editing
D) None of the above
Answer: A
What is a major challenge in developing RNA editing therapies?
A) Off-target effects
B) Low efficiency of editing enzymes
C) Delivery of editing tools to specific cells
D) All of the above
Answer: D
What is the future potential of RNA editing in medicine?
