The Drosophila fl(2)d gene, required for female-specific splicing of Sxl and tra pre-mRNAs, encodes a novel nuclear protein with a HQ-rich domain

The Drosophila gene female-lethal(2)d [fl(2)d] interacts genetically with t he master regulatory gene for sex determination, Sex-lethal. Both genes are required for the activation of female-specific patterns of alternative spl icing on transformer and Sex-lethal pre-mRNAs. We have used P-element-media ted mutagenesis to identify the fl(2)d gene. The fl(2)d transcription unit generates two alternatively spliced mRNAs that can encode two protein isofo rms differing at their amino terminus. The larger isoform contains a domain rich in histidine and glutamine but has no significant homology to protein s in databases. Several lines of evidence indicate that this protein is res ponsible for fl(2)d function. First, the P-element insertion that inactivat es fl(2)d interrupts this ORF. Second, amino acid changes within this ORF h ave been identified in fl(2)d mutants, and the nature of the changes correl ates with the severity of the mutations. Third, all of the phenotypes assoc iated with fl(2)d mutations can be rescued by expression of this cDNA in tr ansgenic flies. Fl(2)d protein can be detected in extracts from Drosophila cell lines, embryos, larvae, and adult animals, without apparent difference s between sexes, as well as in adult ovaries. Consistent with a possible fu nction in posttranscriptional regulation, Fl(2)d protein has nuclear locali zation and is enriched in nuclear extracts.