Physical and functional definition of the Drosophila Notch locus by P element transformation.

Notch is a developmentally regulated locus which controls the differentiation of various Drosophila tissues, among them the embryonic nervous system. Molecular analysis has suggested that Notch is defined by an approximately 40-kb transcription unit which is spliced into a 10.2-kb mRNA composed of nine exonic regions and coding for a 2703-amino acid long transmembrane protein that shows homology to the mammalian epidermal growth factor. Here, we define the 5' end of the transcription unit and determine the sequences deleted in a Notch mutation involving the 5' nontranscribed region. Using a Notch cosmid vector we demonstrate by P element-mediated transformation that all sequences necessary for Notch function are confined in an approximately 40-kb long genomic region. cDNA sequences are used to construct a 15-kb "minigene" which lacks most, but not all, introns and its functionality is also tested by P element transformation. We show that, unlike the cosmid vector which is capable of rescuing completely all Notch mutations, only certain phenotypes can be rescued by the "minigene." The functional implications of our findings are discussed.