Nuclear pre-mRNA introns or spliceosomal introns

BACK to 'Hemiascomycetous Yeast Spliceosomal Introns'

1. Phylogenetic distribution



Eukarya

Isolated cases in Eukaryotic viruses (pre-mRNA genes of Chlorella viruses)




2. Location
   

   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.




3. Size
   

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




4. 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).

5. 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.




6. Intron product of splicing is a lariat.
   
   
7. Requirement for ATP hydrolysis.
   
   
8. Non-Coding sequences