FIGURE 10-18 A cascade of regulated splicing controls sex determination in Drosophila embryos. For clarity, only the exons (boxes) and introns (black lines) where regulated splicing occurs are shown. Splicing is indicated by red dashed lines above (female) and blue dashed lines below (male) the pre-mRNAs. Vertical red lines in exons indicate in-frame stop codons, which prevent synthesis of functional protein. Only female embryos produce functional Sxl protein, which represses splicing between exons 2 and 3 in sxl pre-mRNA (a) and between exons 1 and 2 in tra pre-mRNA (b). (c) In contrast, the cooperative binding of Tra protein and two SR proteins, Rbp1 and Tra2, activates splicing between exons 3 and 4 and cleavage/polyadenylation(A_n) at the 3′ end of exon 4 in dsx pre-mRNA in female embryos. In male embryos, which lack functional Tra, the SR proteins do not bind to exon 4, and consequently exon 3 is spliced to exon 5. The distinct Dsx proteins produced in female and male embryos as the result of this cascade of regulated splicing repress transcription of genes required for sexual differentiation of the opposite sex. See M. J. Moore et al., 1993, in R. Gesteland and J. Atkins, eds., The RNA World, Cold Spring Harbor Press, pp. 303–357.