TRANSACTIVATION OF GROUP-II INTRON SPLICING BY NUCLEAR U5 SNRNA

Similarities between RNA splicing during autocatalytic excision of gro up II introns and pre-mRNA processing led to the hypothesis that group II introns might be the evolutionary predecessors of spliceosomal sma ll nuclear RNAs(1-4). The ID3 subdomain stem-loop structure of group I I introns, the proposed analogue of the spliceosomal U5 snRNA(5), is t hought to be essential for 5' splice site recognition and anchoring of the free 5' exon(6). Using the group II intron bI1 we have analysed t he role of ID3 in splicing. In the absence of ID3 the 5' splice site w as recognized accurately and efficiently, but exon anchoring was great ly reduced. This step was restored in the presence of RNA fragments co nsisting of either the terminal stem-loop structure of ID3 or spliceos omal U5 snRNA. This suggests that the predominant role of both RNAs is to anchor the 5' exon during exon ligation. Furthermore, as U5 comple ments for the loss of ID3, a similar network of structural RNAs may fo rm the catalytic core of both group II introns and spliceosomes.