We describe here the primary structure of HrNotch, an ascidian homologue of the Drosophila neurogenic gene Notch. HrNotch transcripts encode a protein of 2352 amino acids and share the principal features of the Notch gene family: extracellular epidermal growth factor (EGF)-like repeats, three Notch/Lin-12 repeats and six intracellular ankyrin repeats. Yet ascidian Notch contains only 33 EGF repeats in the putative extramembrane domain and specifically lacks the three EGF-like repeats. In situ hybridization shows that maternal HrNotch mRNA is distributed uniformly in the cytoplasm of the unfertilized egg. During cleavage, maternal HrNotch transcripts are ubiquitous in the ectoderm cells of the animal hemisphere, which contain less yolk granules. During gastrulation, maternal transcripts persist in most ectoderm lineage cells. Zygotic expression of HrNotch seems to start at the neural plate stage in both a-line cells (descendants of anterior-animal blastomeres) of the dorsal neuroectoderm and b-line cells (descendants of the posterior-animal blastomeres) that comprise the neural fold. Following this stage, transcripts are most evident in the descendants of these cells, that is, the brain lineage cells, precursors of a larval adhesive organ, and dorsal part of the nerve cord (roof plate). Brain lineage cells include the precursors of sensory pigment cells that are known to comprise an equivalence group in ascidian embryos. During tail elongation, transcripts disappear. Predominant expression of HrNotch in epidermal and neural cells is a common feature of chordate Notch genes. Furthermore, the timing of HrNotch expression in sensory pigment cell precursors suggests involvement in the determinative events in the sensory pigment cell equivalence group.
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