RNA editing
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RNA editing






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RNA editing RNA editing Presentation Transcript

  • ♫ Viruses & lower eukaryotes exhibit several unexpected mechanisms for the control of gene expression. ♫ The most spectacular one is RNA editing, which was first found in the mitochondria of trypanosomes. INTRODUCTION
  • ♫The corresponding primary transcripts are extensively modified by insertion or by deletion of uridine nucleotides to yield the final mRNA that is translated in to protein. ♫These newly inserted uridine nucleotides can make up over half of the total number of nucleotides of the mature mRNA. ♫The initial RNA transcript is specifically altered with the help of “guide RNA”.
  • RNA editing is a process in which information changes at the level of mRNA. RNA editing changes the sequence of an RNA after or during its transcription. RNA editing describes those molecular process in which the information content in an mRNA molecule is altered through a chemical change in the base makeup. RNA EDITING
  • It is revealed by situations in which the coding sequence in an RNA differs from the sequence of DNA from which it was transcribed. Changes have been observed in tRNA ,mRNA ,rRNA & micro RNA molecules of eucaryotes .
  • RNA editing occurs by 2 distinct mechanisms O Substitution editing. O Insertion / Deletion editing.
  • SUBSTITUTION EDITING Substitutions of individual bases occur in mammalian systems. It is a chemical alteration of individual nucleotides. These alterations are catalyzed by enzymes that recognize a specific target sequence of nucleotides .
  • Cytidine deaminases that convert a C in the RNA to U (uracil ). Adenosine deaminases that convert an A to I (inosine )which the ribosome translates as a G. Thus a CAG codon (for Gln) can be converted to a CGG codon (for Arg ).
  • Example :The Human apo B gene Humans have a single locus encoding the apo B gene. It contains 29 exons . The exons contain a total of 4564 codons. Codon 2153 is CAA ,which is a codon for the aminoacid glutamine (Gln ).
  • The gene is expressed in cells of both liver & the intestine. In both locations, transcription produces a pre messenger RNA that must be spliced to produce the mRNA to be translated in to protein. In the liver: Here the process occurs normally producing apolipoprotein B-100 -a protein containing 4563 amino acids- that is essential for the transport of cholesterol &other lipids in the blood .
  • In the intestine: In the cells of intestine, an additional step of pre-mRNA processing occurs. i.e, the chemical modification of the C nucleotide in codon 2153 (CAA) in to a U. This RNA editing converts a glutamine codon (CAA) to a stop codon (UAA) & thus leads to a truncated protein.
  • The modifications is catalyzed by the enzyme cytidine deaminase that Recoganizes the sequences of RNA at that one place in the molecule. & catalyses the deamination of C thus forming U. Translation of the mRNA stops at codon2153 forming apolipoprotein B-48 -a protein containing 2152 amino acids- that aids in the absorption of dietary lipids from the contents of the intestine.
  • INSERTION /DELETION EDITING (Role of Guide RNA [gRNA] ) Example: The gene for one of the subunits of cytochrome oxidase. Particularly seen in the mitochondrial genes of Trypanosoma & leishmania. Several genes having mutations, which were supposed to render these genes inactive (due to origin of premature stop signals or due to loss of start signals ) were still active.
  • The sequences of mRNA molecules in several cases where such which could not have been derived from the DNA sequence of the corresponding genes .These genes are called “Cryptogenes”. Editing requires a special class of RNA molecules called guide RNA(gRNA ). Both in Trypanosoma & Leishmania, mtRNA sequences were found that encode small RNA molecules less than 40 nucleotides in length.
  • This RNA apparently carried information for uridine insertions & deletion called Guide RNA. A guide RNA contains a sequence that is complementary to the correctly edited mRNA. Complementarity is more extensive on the 3’side of the edited region& rather short on 5’side .
  • Editing usually proceeds from 3’end of the transcript toward the 5’end. Extensive editing reactions occur in trypanosomes in which as many as half of the bases in an mRNA are derived from editing. The editing reaction uses a template consisting of a guide RNA that is complementary to the mRNA sequence .
  • The reaction is catalyzed by an enzyme complex. Endonuclease Terminal uridyltransferase(TUTase) RNA ligase First pairing between the substrate RNA &guide RNA occurs. The substrate RNA is cleaved at a site that is identified by the absence of pairing with the guide RNA, a uridine is inserted or deleted to base pair with the guide RNA.
  • Then the substrate RNA is ligated. UTP provides the source for the uridyl residue. It is added by the TUTase activity. When the reaction is completed, the guide RNA separates from the mRNA which become available for translation. The inserted nucleotides cause a frameshift & result in a translated protein that differs from its gene.
  • • In RNA editing the nucleotide sequence of a pre mRNA is altered in the nucleus. • In vertebrates, this process is fairly rare and results in deamination of a single base in the mRNA sequence, resulting in a change in the aminoacid specified by the corresponding codon & production of a functionally different protein.