MULTIPLE INTERDEPENDENT SEQUENCE ELEMENTS CONTROL SPLICING OF A FIBROBLAST GROWTH-FACTOR RECEPTOR-2 ALTERNATIVE EXON

The fibroblast growth factor receptor 2 gene contains a pair of mutual ly exclusive alternative exons, one of which (K-SAM) is spliced specif ically in epithelial cells, Wet have described previously (F. Del Gatt o and R. Breathnach, Mol. Cell. Biol. 15:4825-4834, 1995) some element s controlling K-SAM exon splicing, namely weal; exon splice sites, an exon-repressing sequence, and an intron-activating sequence, We identi fy here two additional sequences in the intron downstream from the K-S AM exon which activate splicing of the exon. The first sequence (intro n-activating sequence 2 [IAS2]) lies 168 to 186 nucleotides downstream from the ex-on's 5' splice site. The second sequence (intron-activati ng sequence 3 [IAS3]) lies 933 to 1,052 nucleotides downstream from th e exon's 5' splice site. IAS3 is a complex region composed of several Darts, one of which (nucleotides 963 to 983) can potentially form an R NA secondary structure with IAS2. This structure is composed of two st ems separated by an asymmetric bulge, Mutations which disrupt either s tem decrease activation, while compensatory mutations which reestablis h the stem restore activation, either completely or partially, dependi ng on the mutation, We present a model for K-SAM exon splicing involvi ng the intervention of multiple, interdependent pre-mRNA sequence elem ents.