Group I introns

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  1. Phylogenetic distribution: Wide but irregular
  2. Organellar genomes: Most in fungal (e.g., yeasts) mitochondrial genome. They also occur in plant organelles (mitochondria and chloroplasts). In metazoan: only in mitochondrial genes (mRNA) of a few anthozoans (e.g., sea anemone).
  3. Nuclear genome of some protists, fungi, and lichens (large and small rRNA genes).
  4. Eubacterial & bacteriophage genomes.
  5. Not found in vertebrate genes.

  6. Location
  7. Mitochondrial introns, in genes encoding components of the electron transport system (mt mRNA genes of Cox1 and Cob genes) and large rRNA genes (mt rRNA).
  8. Chloroplastic introns, mainly in large rRNA and tRNA genes.
  9. Nuclear introns, in both large and small rRNA genes.
  10. Phage introns, in genes encoding proteins involved in DNA metabolism (mRNAs).
  11. Bacterial introns, in tRNA genes.

  12. Size: Variable
  13. From 68 over 3000 nt. Most are over 400 nt.

  14. Sequence: Limited conservation
  15. All have four blocks of conserved sequences in the catalytic core.
  16. All have additional complementary but non-conserved sequences.
  17. Most have internal guide sequences.
  18. No splice site consensus.

  19. Conserved secondary and tertiary structures (different from group II)
  20. All have well defined secondary structure.
  21. All folds into a specific 3D structure.
  22. Not all helices are present in all group I introns.

  23. Splicing mechanism: Self-splicing introns
  24. Most are able to splice themselves in the absence of proteins, i.e. the RNA itself is catalytic ("ribozymes").
  25. Initiate splicing with an external G nucleotide (cofactor).
  26. Uses a phosphoester transfer mechanism, i.e. two successive transesterification steps catalysed by RNA in vitro.
  27. Not all group I introns are truly catalytic. Splicing of some group I introns in vivo is modulated by a number of proteins encoded either by various genes independent from the introns or by the introns themselves. Maturases (proteins) are required (play a role in folding and 3D structure) although they may not be required for in vitro activity.

  28. Intron product of splicing is a linear intron

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

  30. Mobile
  31. Group I introns are able to propagate themselves in the genome. The DNA of some Group I introns includes an open reading frame (ORF) that encodes a transposase-like protein that can make a copy of the intron and insert it elsewhere at predetermined positions into intronless sites of genes. The intronic ORF encodes functions for splicing (RNA maturase) and mobility (?homing endonuclease'), also host-encoded machinery is recruited for splicing.

Last modified: Tue Feb 10 11:10:07 CET 2004