REGULATION OF THE DORSAL MORPHOGEN BY THE TOLL AND TORSO SIGNALING PATHWAYS - A RECEPTOR TYROSINE KINASE SELECTIVELY MASKS TRANSCRIPTIONAL REPRESSION

The dorsal (dl) nuclear gradient initiates the differentiation of the mesoderm, neuroectoderm, and dorsal ectoderm by activating and repress ing gene expression in the early drosophila embryo. This gradient is o rganized by a Toll signaling pathway that shares many common features with the mammalian IL-1 cytokine pathway. Here we present evidence tha t a second signaling pathway, controlled by the torso (tor) receptor t yrosine kinase, also modulates dl activity. Evidence is presented that the tor pathway selectively masks the ability of dl to repress gene e xpression but has only a slight effect on activation. Intracellular ki nases that are thought to function downstream of tor, such as D-raf an d the rolled map kinase, mediate this selective block in repression, N ormally, the Toll and tor pathways are both active only at the embryon ic poles, and consequently, target genes (zen and dpp) that are repres sed in middle body regions are expressed at these sites. Constitutive activation of the tor pathway causes severe embryonic defects, includi ng disruptions in gastrulation and mesoderm differentiation, as a resu lt of misregulation of dl target genes. These results suggest that RTK signaling pathways can control gene expression by antirepression, and that multiple pathways can fine-tune the activities of a single trans cription factor.