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RNA Editing 

• 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.