Spliceosomal RNAs are a family of small nuclear RNAs (snRNAs) that are essential for pre-mRNA splicing.All vertebrate spliceosomal snRNAs are extensively pseudouridylated after transcription.Pseudouridines in spliceosomal snRNAs are generally clustered in regions that are functionally important during splicing.Many of these modified nucleotides are conserved across species lines.Recent studies have demonstrated that spliceosomal snRNA pseudouridylation is catalyzed by two different mechanisms:an RNA-dependent mechanism and an RNA-independent mechanism.The functions of the pseudouridines in spliceosomal snRNAs (U2 snRNA in particular) have also been extensively studied.Experimental data indicate that virtually all pseudouridines in U2 snRNA are functionally important.Besides the currently known pseudouridines (constitutive modifications),recent work has also indicated that pseudouridylation can be induced at novel positions under stress conditions,thus strongly suggesting that pseudouridylation is also a regulatory modification.