Group II introns



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  1. Phylogenetic distribution: Restricted
  2. Not found in higher eukaryotes.
  3. Organellar genomes of plants, fungi and protists: Most in chloroplasts and higher plants mitochondria. A minority in fungal mitochondria.
  4. Eubacterial genomes.


  5. Location
  6. Eukaryotes: Most in protein-coding genes (mRNA). Few in chloroplast tRNA and large rRNA genes.
  7. Bacteria: mRNAs.


  8. Conserved sequences at splice junctions.
  9. 5' splice site: |GUGCG    (underline = invariant)
  10. 3' splice site: YNAU|


  11. Conserved secondary and tertiary structures (different from group I).

  12. Splicing mechanism: Self-splicing, may be protein-assisted.
  13. Initiate splicing with an internal A.
  14. Uses a phosphoester transfer mechanism, i.e. two trans-esterification reactions, similar to the one occurring for nuclear pre-mRNA introns.
  15. Self-splicing occurs in vitro (for a few of the group II introns), but protein machinery is required in vivo.


  16. Intron product of splicing is a lariat
  17. Splicing of group II introns occurs via a lariat intermediate and two trans-esterification reactions. This similarity has led to the widespread belief that group II introns were the ancestors of spliceosomal introns.


  18. Does not require an external source of energy (ATP hydrolysis).

  19. Some are mobile
  20. Eukaryotic organellar introns: Most are ORF-less and non-mobile. However, a subset of Group II introns have a remarkable property: they encode reverse transcriptase (RT) ORFs and are active mobile elements. Such mobile introns can insert into defined sites of intronless alleles at high efficiencies (retrohoming), or can invade unrelated novel (ectopic) sites at low frequencies (retrotransposition) according to a mobility mechanism similar to non-LTR retroelements. The intronic ORF encodes function for splicing and mobility (RNA maturase, reverse transcriptase, endonuclease).
  21. Bacterial introns : Almost always encode RT ORFs and are retroelements.




Last modified: Tue Feb 10 11:08:55 CET 2004