Repression by Suppressor of Hairless and activation by Notch are required to define a single row of single-minded expressing cells in the Drosophila embryo

Notch signal transduction appears to involve the ligand-induced intracellul ar processing of Notch, and the formation of a processed Notch-Suppressor o f Hairless complex that:binds DNA and activates the transcription of Notch target genes. This suggests that loss of either Notch or Su(H) activities s hould lead to similar cell fate changes. However, previous data indicate th at,in the Drosophila blastoderm embryo, mesectoderm specification requires Notch but not Su(H) activity. The determination of the mesectodermal fate i s specified by Single-minded (Sim), a transcription factor expressed in a s ingle row of cells abutting the mesoderm. The molecular mechanisms by which the dorsoventral gradient of nuclear Dorsal establishes the single-cell wi de territory of sim expression are not fully understood. We have found that Notch activity is required for sim expression in cellularizing embryos. In contrast, at this stage, Su(H) has a dual function. Su(H) activity was req uired to up-regulate sim expression in the mesectoderm, and to prevent the ectopic expression of sim dorsally in the neuroectoderm. We have shown that repression of sim transcription by Su(H) is direct and independent of Notc h activity. Conversely, activation of sim transcription by Notch requires t he Su(H)-binding sites. Thus, Notch signalling appears to relieve the repre ssion exerted by Su(H) and to up-regulate sim transcription in the mesectod erm. We propose a model in which repression by Su(H) and derepression by No tch are essential to allow for the definition of a single row of mesectoder mal cells in the blastoderm embryo.