THE DROSOPHILA SPLICING REGULATOR SEX-LETHAL DIRECTLY INHIBITS TRANSLATION OF MALE-SPECIFIC-LETHAL-2 MESSENGER-RNA

Male-specific expression of the protein male-specific-lethal 2 (MSL-2) controls dosage compensation in Drosophila. msl-2 gene expression is inhibited in females by Sex-lethal (SXL), an RNA binding protein known to regulate pre-mRNA splicing. An intron present at the 5' untranslat ed region (UTR) of msl-2 mRNA contains putative SXL binding sites and is retained in female flies, Here we show that SXL plays a dual role i n the inhibition of msl-2 expression, Cotransfection of Drosophila Sch neider cells with an SXL expression vector and a reporter containing t he 5' UTR of msl-2 mRNA resulted in retention of the 5' UTR intron and efficient accumulation of the unspliced mRNA in the cytoplasm, where its translation was blocked by SXL, but not by the intron per se, Both splicing and translation inhibition by SXL were recapitulated in vitr o and found to be dependent upon SXL binding to high-affinity sites wi thin the intron, showing that SXL directly regulates these events, Our data reveal a coordinated mechanism for the regulation of msl-2 expre ssion by the same regulatory factor: SXL enforces intron retention in the nucleus and subsequent translation inhibition in the cytoplasm.