RNA editing: A process by which nucleotides coded by a gene are introduced at specific position in a RNA molecule
after transcription is called RNA editing. Chemical modification of nucleotides of rRNA of tRNA’s effects only their structures & only catabolic activities but chemical modification of nucleotides of mRNA has the potential to change to coding properties of the transcript. Resulting in an alternation in the amino acid sequence of the protein i.e., specified. This is known as RNA editing.
The phenomenon of RNA editing was 1st identified in tripenosoma, the protozoa that causes sleeping sickness. Since that time RNA editing has been shown to occurring various organisims & in a variety of
ways. An example of RNA editing is found to occur in human RNA for a polypoprotein B. The gene code a polypeptide of 4563 amino acid which is synthesis in the tuber cells. In the intestinal cell some gene codes another lipoprotein containing only 2153 amino acids & this found to be very similar in their end terminal amino acid. This is because of the change in the codon CAA into a terminal codon UAA due to a process of deamination of cytosine converting in to urasil. Such a RNA editing causes the translation of the trunokated polypeptide. The deamination is carried by as RNA binding enzyme along with some auxiliary factor.
In human, RNA editing is responsible for the generation of the antibody diversity & also have been implicated on the control HIV1 infaction cycle by changing conclou. The enzyme adenosine diaminase acting on RNA(ADAR) selectively edited at a limited no. Of position convert adenosine into inosine. This occurs during the processing of the rRNA for mammalian glutamine receptor.
Extensive deamination by ADAR is observed in viral RNA for ex. The measles virus which undergo hyperteting.
Many other complex type of RNA editing are found-
1) PAN editing: It involves the extensive insertion of nucleotide into RNA’s in order to produced functional molecule. It occurs in the mitochondria of tripenosomes & the editing process involves a guide RNA. A short RNA that can base pair to the pre-RNA & can insert one or more U-residues into an RNA
or deletion of one or more U-residues.
Processing of tRNA: Pre tRNA’s are large precursors of tRNA synthesis from the tRNA genes in the nucleoplasm by RNA poly. III. During the processing, mature tRNA is produced by splicing & fielding of the RNA chain. Simultaneously modification of some bases & replacement of terminal UU residues of CCA.
1) Pre tRNA are larger in length than mature tRNA. The 5’ terminal end has a sequence of variable length. This extra nucleotides are removed by RNaseP.Ecoli RNaseP is 14KD protein with377 nucleotide RNA called M1-RNA. At high concentration of mg2+ recognised & cleaves the 5’ terminal nucleotide of pretRNA.
2) About 10% of the bases of pre tRNA are enzymatically modified during processing. 4types of base modification occur namely 2 methyle groups are added to Guanine to form N,N’-dimethyleguanine, 2-H are added to Uridine to form dehydrouridine, deamination of guanine to form inosine & replacement of Oxygen by Sulphur in Uridine to form Thiouridine.
3) Replacement of terminal 3’ 2-Uridine molecules by CCA sequence which is found in all tRNA’s. The trimming & splicing of pre-tRNA molecules occur through a cut & paste mechanism. That is catalysed by same proteins & enzymes. The splicing process is not similar to that of mRNA splicing. First the pre mRNA is cleaved at 2 places on each site of the intro. The cleavage generates a 2’,3’-cyclic phosphomonoester at the 3’ end of the 5’ axon. Further phosphorylation by ATP activated ligase causes the 3’-> 5’ linkage between the 2 axons. The 2’ phosphate of 5’ axon is then removed by 2’ phophotransferase enzyme. In such processing no lariat formation or snRNA are required.
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