Capping & Polyadenylation

CAPPING AND POLYADENYLATION

Capping

  • Most eukaryotic and viral m RNA have a cap at their 5’ end. The cell enzymes that carry out the capping activities are guanylyl transferases (add guanosine 5’ triphosphate) and methyl transferases (add methyl groups).
  • Most viruses carrying out transcription in the nucleus use the cell enzymes. Some of the viruses replicating in cytoplasm eg: pox,reo and corona viruses encode their own capping and methylating enzymes.
  • Negative sense RNA viruses with segmented genomes “ snatch” caps from cell m RNAs eg: influenza. In this case the viral RNA polymerase binds to the cellular capped m RNA and cleaves the RNA 10-20 nucleotides from the 5’ end.
  • The capped oligonucleotide acts as primers to initiate transcription of viral m RNA. Not all m RNAs are capped. For example picorna viruses donot cap their m RNAs.

Polyadenylation

  • Series of adenosine residues are to 3’ end of most m RNAs of eukaryotes and viruses. This probably increase stability of m RNA and plays a role in the initiation of translation.
  • Some viruses (eg: reoviruses) do not polyadenylate their m RNA.
Last modified: Sunday, 10 April 2011, 11:41 AM