Direct activation of Sex-lethal transcription by the Drosophila Runt protein

Runt functions as a transcriptional regulator in multiple developmental pat hways in Drosophila melanogaster. Recent evidence indicates that Runt repre sses the transcription of several downstream target genes in the segmentati on pathway. Here we demonstrate that runt also functions to activate transc ription. The initial expression of the female-specific sex-determining gene Sex-lethal in the blastoderm embryo requires runt activity Consistent with a role as a direct activator, Runt shows sequence-specific binding to mult iple sites in the Sex-lethal early promoter. Using an in vivo transient ass ay, we demonstrate that Runt's DNA-binding activity is essential for Sex-le thal activation in vivo, These experiments further reveal that increasing t he dosage of runt alone is sufficient for triggering the transcriptional ac tivation of Sex-lethal in males. In addition, a Runt fusion protein, contai ning a heterologous transcriptional activation domain activates Sex-lethal expression, indicating that this regulation is direct and not via repressio n of other repressors. Moreover, we demonstrate that a small segment of the Sex-lethal early promoter that contains Runt-binding sites mediates Runt-d ependent transcriptional activation in vivo.