Functional relationships between Notch, Su(H) and the bHLH genes of the E(spl) complex: the E(spl) genes mediate only a subset of Notch activities during imaginal development.

The basic helix-loop-helix proteins of the Enhancer of split complex constitute a link between activation of the transmembrane receptor Notch and the resulting effects on transcription of downstream genes. The Suppressor of Hairless protein is the intermediary between Notch activation and expression of all Enhancer of split genes even though individual genes have distinct patterns of expression in imaginal discs. A comparison between the phenotypes produced by Notch, Suppressor of Hairless and Enhancer of split mutations in the wing and thorax indicate that Suppressor of Hairless and Notch requirements are indistinguishable, but that Enhancer of split activity is only essential for a subset of developmental processes involving Notch function. Likewise, the ectopic expression of Enhancer of split proteins does not reproduce all the consequences typical of ectopic Notch activation. We suggest that the Notch pathway bifurcates after the activation of Suppressor of Hairless and that Enhancer of split activity is not required when the consequence of Notch function is the transcriptional activation of downstream genes. Transcriptional activation mediated by Suppressor of Hairless and transcriptional repression mediated by Enhancer of split could provide greater diversity in the response of individual genes to Notch activity.