Nuclear pre-mRNA introns or spliceosomal introns

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  1. Phylogenetic distribution

  2. Eukarya
  3. Isolated cases in Eukaryotic viruses (pre-mRNA genes of Chlorella viruses)

  4. Location
  5. Nucleus of eukaryotic organisms, interrupting the pre-mRNAs of protein-coding genes.
  6. Recently found in genes encoding the snRNAs components of some fungi, and in the small- and large-subunit ribosomal DNA genes of a limited number of ascomycetes.

  7. Size
  8. From 25 nt over 1002 nt. Most are around 150-300 nt long.

  9. Sequence: Very limited conservation
  10. Not highly constraint in sequence except at critical sites.
  11. All have nearly invariant sequences at splice sites.
  12. 5' splice site: |GUAUGU or GCAUGU (underline = invariant)
  13. Branch site : UACUAAC
  14. 3' splice site: YAG|
  15. Some have internal guide sequences (polyT tracts).
  16. Splicing mechanism: Spliceosome mediated
  17. Require an elaborate external assembly of small nuclear RNAs and associated proteins which is called the spliceosome.
  18. Mechanism similar to that of Group II introns, suggesting that Group II introns could be the ancestors of spliceosomal introns:
    - Initiate splicing with an internal A (branch site)
    - Use a phosphoester transfer mechanism for splicing, i.e. two trans-esterification reactions.

  19. Intron product of splicing is a lariat.

  20. Requirement for ATP hydrolysis.

  21. Non-Coding sequences

Last modified: Tue Feb 10 11:22:13 CET 2004