Terminal intron dinucleotide sequences do not distinguish between U2- and U12-dependent introns

Area of Science:

• Molecular Biology
• Genetics
• RNA Splicing

Background:

• Eukaryotic nuclear introns are classified into U2-dependent (GU/AG termini) and U12-dependent (AU/AC termini) types based on their splice site sequences.
• The specific spliceosomal system responsible for intron removal was thought to be determined by these terminal dinucleotides.

Purpose of the Study:

• To investigate the flexibility of the U12-dependent splicing system regarding intron termini.
• To determine if introns with non-canonical termini exist naturally and are evolutionarily conserved.
• To re-evaluate the criteria for classifying introns into U2- or U12-dependent mechanistic classes.

Main Methods:

• Analysis of naturally occurring introns across various eukaryotic species.
• Functional assays to test the splicing capabilities of U12-dependent systems with different intron termini.
• Comparative genomics to assess evolutionary conservation of intron classes.

Main Results:

• The U12-dependent splicing system can effectively process introns with the canonical GU/AG termini, and these introns are found naturally.
• U2-dependent introns with AU/AC termini also exist naturally and show evolutionary conservation.
• Intron classification into U2- or U12-dependent mechanistic groups is achievable by examining the complete set of conserved splice site sequences, not solely terminal dinucleotides.

Conclusions:

• The terminal dinucleotide sequence is not the sole determinant for U2- or U12-dependent intron splicing.
• A broader range of intron types exists within both U2- and U12-dependent splicing pathways.
• Conserved splice site sequences provide a more accurate basis for mechanistic classification of eukaryotic introns.