Ordered splicing pathways for inclusion of exon 16.

(A) The 2 possible ordered splicing pathways by which exon 16 could be included and the structure of the partially spliced RNAs unique to each pathway. (B) The ability of the partially spliced RNAs to be chased into mature products in either HeLa nuclear extract (H) or oocytes (O). Lanes 1-3 show downstream-first RNA 13i16/17 before splicing (lane 1), or after splicing in HeLa nuclear extract (lane 2) or in oocytes (lane 3). Lanes 4-6 show upstream-first RNA 13/16i17 before splicing (lane 4), or after splicing in HeLa nuclear extract (lane 5) or oocytes (lane 6). Lower band corresponds to splicing at a cryptic 5′ splice site in exon 16.