FIG. 6.

Comparison of SC35 winner sequences and SC35 ESE motif in two different exonic contexts. (A) The 20-nt D2 winner sequence (see Fig. 3), a variant of D2 with two nucleotide changes to introduce a maximum-score consensus (D2C), and a 19-nt SC35 SELEX winner sequence (6-24) described in a previous study (29) were inserted into the IgM M1-M2 minigene in place of the natural ESE in exon M2, and the corresponding transcripts were spliced in nuclear extract (lanes 2 to 4). The control pre-mRNA lacking an ESE (ED) is shown in lane 1. (B) The same D2, D2C, and 6-24 sequences were tested in the context of an IgM C3-C4 minigene (26). The Ca pre-mRNA includes the first 38 nt of the C4 exon (lane 1). In the remaining pre-mRNAs, this segment of the C4 exon is followed by the next 38 nt of the C4 exon, which comprise a natural SC35-dependent ESE (CaCb; lane 2) or it is followed by the D2, D2C, or 6-24 sequence (lanes 3 to 5). The sequences of the relevant portions of the 3′ exons are shown below each panel. The two nucleotide changes in D2C, compared to D2, are underlined. The mobilities of the pre-mRNAs, mRNAs, and 5′ exon intermediates are indicated next to each autoradiogram.