FIGURE 10-19 Model of splicing activation by Tra protein and the SR proteins Rbp1 and Tra2. In female Drosophila embryos, splicing of exons 3 and 4 in dsx pre-mRNA is activated by the binding of Tra/Tra2/Rbp1 complexes to six exonic splicing enhancers in exon 4. Because Rbp1 and Tra2 cannot bind to the pre-mRNA in the absence of Tra, exon 4 is skipped in male embryos. See the text for discussion. A_n = polyadenylation. See T. Maniatis and B. Tasic, 2002, Nature 418:236.