FIG. 2.

Exon 3 splicing requires the purine-rich sequence. (A) Diagram of RNA substrates used for in vitro splicing, showing the locations of exons 2 and 3. All substrates contain the exon 2 splice donor and exon 3 splice acceptor. The approximate location of the purine-rich sequence is highlighted. (B) After incubation for 2 h in HeLa cell splicing extracts, RNA products were electrophoresed through 4% polyacrylamide gels and visualized by autoradiography. The locations of spliced and unspliced products are shown. The fastest-migrating products in lanes 3 to 5 are intron products resulting from splicing. Sizes are shown at the left (in nucleotides).