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 c onstitute a link between activation of the transmembrane receptor Notc h and the resulting effects on transcription of downstream genes, The Suppressor of Hairless protein is the intermediary between Notch activ ation and expression of all Enhancer of split genes even though indivi dual genes have distinct patterns of expression in imaginal discs, A c omparison between the phenotypes produced by Notch, Suppressor of Hair less and Enhancer of split mutations in the wing and thorax indicate t hat Suppressor of Hairless and Notch requirements are indistinguishabl e, but that Enhancer of split activity is only essential for a subset of developmental processes involving Notch function. Likewise, the ect opic expression of sequences typical of ectopic Notch activation. We s uggest that the Notch pathway bifurcates after the activation of Suppr essor of Hairless and that Enhancer of split activity is not required when the consequence of Notch function is the transcriptional activati on of downstream genes. Transcriptional activation mediated by Suppres sor of Hairless and transcriptional repression mediated by Enhancer of split could provide greater diversity in the response of individual g enes to Notch activity.