Trans-activation of group II intron splicing by nuclear U5 snRNA.

Similarities between RNA splicing during autocatalytic excision of group II introns and pre-mRNA processing led to the hypothesis that group II introns might be the evolutionary predecessors of spliceosomal small nuclear RNAs. The ID3 subdomain stem-loop structure of group II introns, the proposed analogue of the spliceosomal U5 snRNA, is thought to be essential for 5' splice site recognition and anchoring of the free 5' exon. Using the group II intron bI1 we have analysed the role of ID3 in splicing. In the absence of ID3 the 5' splice site was recognized accurately and efficiently, but exon anchoring was greatly reduced. This step was restored in the presence of RNA fragments consisting of either the terminal stem-loop structure of ID3 or spliceosomal U5 snRNA. This suggests that the predominant role of both RNAs is to anchor the 5' exon during exon ligation. Furthermore, as U5 complements for the loss of ID3, a similar network of structural RNAs may form the catalytic core of both group II introns and spliceosomes.