Molecular identification of virilizer, a gene required for the expression of the sex-determining gene sex-lethal in Drosophila melanogaster

Sex-lethal (Sxl) is a central switch gene in somatic sexual development of Drosophila melanogaster. Female-specific expression of Sxl relies on autore gulatory splicing of Sxl pre-mRNA by SXL protein. This process requires the function of virilizer (vir). Besides its role in Sxl splicing, vb is essen tial for male and female viability and is also required for the production of eggs capable of embryonic development. We have identified vir molecularl y and found that it produces a single transcript of 6 kb that is ubiquitous ly expressed in male and female embryos throughout development. This transc ript encodes a nuclear protein of 210 kD that cannot be assigned to a known protein family. VIR contains a putative transmembrane domain, a coiled-coi l region and PEST sequences. We have characterized five different alleles o f vir. Those alleles that affect both sexes are associated with large trunc ations of the protein, while alleles that affect only the female-specific f unctions are missense mutations that lie relatively close to each other, po ssibly defining a region important for the regulation of Sxl.