Fig. 1.

The enforcement of exon linking order and the definition of splice sites are both controlled by SR proteins. (A) Exons in a multiexon pre-mRNA are always linked in order. Mechanisms are present that prevent the skipping of exons, such as joining exon 1 to exon 3. (B) Exons and splice sites are defined by interactions across the exon, between the U1snRNP and U2AF (a). SR proteins bound to ESEs also stimulate the assembly of these factors (b). SR proteins also inhibit the splicing of introns that contain an ESE (c). This inhibitory interaction prevents the splicing of nonadjacent exons and forces exon inclusion.