TERMINAL INTRON DINUCLEOTIDE SEQUENCES DO NOT DISTINGUISH BETWEEN U2-DEPENDENT AND U12-DEPENDENT INTRONS

Two types of eukaryotic nuclear introns are known: the common U2-depen dent class with /GU and AG/ terminal intron dinucleotides, and the rar e U12-dependent class with /AU and AC/ termini. Here we show that the U12-dependent splicing system can splice introns with /GU and AG/ term ini and that such introns occur naturally. Further, U2-dependent intro ns with /AU and AC/ termini also occur naturally and are evolutionaril y conserved. Thus, the sequence of the terminal dinucleotides does not determine which spliceosomal system removes an intron. Rather, the fo ur classes of introns described here can be sorted into two mechanisti c classes (U2- or U12-dependent) by inspection of the complete set of conserved splice site sequences.