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Zusammenfassung:
Beta-catenin, a component of the wnt-signal-transduction pathway, is essential for the formation of the dorsal axis in Xenopus laevis embryos. On the dorsal side of the embryo, beta-catenin is translocated into the nuclei via a process linked to cortical rotation. When cortical rotation is blocked by UV-irradiation, nuclear beta-catenin is found in the vegetal pole of the embryo. Here we show that overexpression of beta-catenin in animal cap explants, in the absence of mesoderm induction, is sufficient to activate the expression of genes with dorsalizing activity such as siamois (sia) and nodal-related 3 (nr3) but not goosecoid (gsc). In embryos ventralized by UV-treatment, the expression of the dorsal-specific genes sia, nr3 and gsc is induced at the vegetal pole after the Mid-Blastula-Transition (MBT). While nr3 and sia expression continues in these embryos until gastrula stages, gsc transcription cannot be maintained. We propose that the spatial separation of the expression domains of genes with dorsalizing activities and the prospective mesodermal region results in the loss of dorsal structures in the embryo. The role of cortical rotation is to generate an overlap of the region with dorsal axis-forming activity, indicated by nuclear translocation of beta-catenin, and the prospective mesoderm in the marginal zone to assure the correct positioning of the Spemann organizer.
