The N-terminal domain of Sxl protein disrupts Sxl autoregulation in females and promotes female-specific splicing of tra in males

Sex determination in Drosophila depends upon the posttranscriptional regula tory activities of the Sex-lethal (Sxl) gene. Sri maintains the female dete rmined state and activates female differentiation pathways by directing the female-specific splicing of Sri and tra pre-mRNAs, While there is compelli ng evidence that Sri proteins regulate splicing by directly binding to targ et RNAs, previous studies indicate that the two Sri RNA-binding domains are not in themselves sufficient for biological activity and that an intact N- terminal domain is also critical for splicing function. To further investig ate the functions of the Sri N terminus, we ectopically expressed a chimeri c protein consisting of the N-terminal 99 amino acids fused to beta-galacto sidase. The N beta-gal fusion protein behaves like a dominant negative, int erfering with the Sri autoregulatory feedback loop and killing females. Thi s dominant negative activity can be attributed to the recruitment of the fu sion protein into the large Sxl:Snf splicing complexes that are found in vi vo and the consequent disruption of these complexes. In addition to the dom inant negative activity, the N beta-gal fusion protein has a novel gain-of- function activity in males: it promotes the female-specific processing of t ra pre-mRNAs. This novel activity is discussed in light of the blockage mod el for the tra splicing regulation.