Nuclear pre-mRNA introns or spliceosomal introns
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- Phylogenetic distribution
- Isolated cases in Eukaryotic viruses (pre-mRNA genes of Chlorella viruses)
- Nucleus of eukaryotic organisms, interrupting the pre-mRNAs of protein-coding genes.
- 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.
- From 25 nt over 1002 nt. Most are around 150-300 nt long.
- Sequence: Very limited conservation
- Not highly constraint in sequence except at critical sites.
- All have nearly invariant sequences at splice sites.
- 5' splice site: |GUAUGU or GCAUGU (underline = invariant)
- Branch site : UACUAAC
- 3' splice site: YAG|
- Some have internal guide sequences (polyT tracts).
- Splicing mechanism: Spliceosome mediated
- Require an elaborate external assembly of small nuclear RNAs and associated proteins which is called the spliceosome.
- 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.
- Intron product of splicing is a lariat.
- Requirement for ATP hydrolysis.
- Non-Coding sequences
Last modified: Tue Feb 10 11:22:13 CET 2004