Spliceosome, intron splicing role

Traditionally, a splicing construct contains an intron flanked by two exons. However, interactions across the exons play an important role in spliceosome assembly, a phenomenon termed exon definition.6 The presence of an upstream 3 -splice site generally stimulates the function of a weak downstream 5 -splice site. Similarly, a downstream 5 -splice site induces the usage of a weak upstream 3 -splice site. In this transactivation model, the 5 -cap structure of the first exon and the polyadenylation site in the terminal exons act as 3 - and 5 -splice sites, respectively, in promoting the spliceosome assembly. This view is reinforced by the observation that less efficiently spliced introns may be stimulated by moving the 5 -cap closer to the 5 -splice site or by inserting a functional 5 -splice- or polyadenylation site in the 3 -exon. However, exon definition seems only to play a minor role for efficiently spliced introns in vitro. 

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