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| Modern Genetic Analysis Chapter 16. The Genetic Basis of Development Drosophila Sex Determination: Every Cell for Itself Figure 16-4. (Facing page) The initiation and maintenance of the Sxl switch. (a) The Sxl switch. High levels of the NUMNUM transcription-factor dimers activate transcription from the early promoter (PE) of Sxl. (Activated promoters are represented by green rectangles, inactivate promoters by red rectangles.) By midembryogenesis, PE is turned off, and Sxl is transcribed in all cells of the animal from the constitutively active late promoter (PL). Through binding to the primary PL transcript and regulating its splicing pattern (preventing the male exon from being included in the final mRNA), preexisting active SXL protein ensures the further production of active SXL protein, which continues the splicing pattern leading to its own formation, thus creating an autoregulatory loop. On the other hand, when the X:A ratio is 0.5, PE is not activated. Thus, no SXL protein is present in the early embryo, and the RNA splicing pattern of the PL transcript generates an mRNA that includes the male exon. This exon contains a stop codon (UGA), terminating the Sxl polypeptide prematurely. The short SXL protein made in males is completely nonfunctional. AUG denotes the location of the translation initiation codon for the SXL polypeptide. (b) A plausible mechanism for the molecular basis of the X:A ratio. During early embryogenesis, the numerator and denominator genes are expressed. NUM subunits (red circles) encoded by X chromosome genes and DEM polypeptide subunits (green circles) encoded by autosomal genes form dimers at random. Only NUMNUM dimers form active transcription factor. If the X:A ratio is 1.0, high levels of these NUMNUM dimers form, bind to the PE enhancer, and activate transcription from PE. If, by chance, the X:A ratio is 0.5, most NUM subunits are part of NUMDEM heterodimers and do not function as active transcription factors.

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