Temperature-sensitive mutations of the notch locus in Drosophila melanogaster.

Temperature-conditional mutations of the Notch locus were characterized in an attempt to understand the organization of a "complex locus" and the control of its function in development. Among 21 newly induced Notch alleles, about one-half are temperature-conditional for some effects, and three are temperature-sensitive for viability. One temperature-sensitive lethal, l(1)Nts1, is functionally non-complementing for all known effects of Notch locus mutations and maps at a single site within the locus. Among the existing alleles involved in complex patterns of interallelic complementation, Ax59d5 is found to be temperature-sensitive, while fag, spl, and l(1)N are temperature-independent. Whereas temperature-sensitive alleles map predominantly to the right-most fifth of the locus, fag, spl, and l(1)N are known to map to the left of this region. Temperature-shift experiments demonstrate that fag, spl, and l(1)N cause defects at specific, non-overlapping times in development. -- We conclude (1) that the Notch locus is a single cistron (responsible for a single functional molecule, presumably a polypeptide); (2) that the right-most fifth of the locus is, at least in part, the region involved in coding for the Notch product; (3) that the complexity of interallelic complementation is a developmental effect of mutations that cause defects at selected times and spaces, and that complementation occurs because the mutant defects are temporally and spatially non-overlapping; and (4) that mutants express selected defects due to critical temporal and spatial differences in the chemical conditions controlling the synthesis or function of the Notch product. The complexity of the locus appears to reside in controlling the expression (synthesis or function) of the Notch product in development.