A β-catenin/XTcf-3 complex binds to thesiamois promoter to regulate dorsal axis specification in Xenopus

  1. Mark Brannon1,3,
  2. Miranda Gomperts2,3,
  3. Lauro Sumoy1,
  4. Randall T. Moon2, and
  5. David Kimelman1,4
  1. 1Department of Biochemistry and 2Howard Hughes Medical Institute and Department of Pharmacology, University of Washington School of Medicine, Seattle, Washington 98195-7350 USA

Abstract

The Wnt pathway regulates the early dorsal–ventral axis in Xenopus through a complex of β-catenin and HMG box transcription factors of the Lef/Tcf family. We show that the promoter of the dorsalizing homeo box gene siamois is a direct target for the β-catenin/XTcf-3 complex, establishing a link between the Wnt pathway and the activation of genes involved in specifying the dorsal axis. By injectingsiamois reporter constructs into the animal pole ofXenopus embryos, we show that a 0.8-kb fragment of thesiamois promoter is strongly activated by β-catenin. The proximal 0.5 kb, which is also activated by β-catenin, contains three Lef/Tcf-binding sites. Mutations in these sites eliminate the β-catenin-mediated activation of siamois and show thatsiamois is regulated by the β-catenin/XTcf-3 complex, in combination with additional transcriptional activators. When expressed at the equator of the embryo, the siamoispromoter is activated to much higher levels on the dorsal side than the ventral side. Ectopic ventral expression of β-catenin raises the ventral expression of the siamois promoter to the dorsal levels. Conversely, ectopic dorsal expression of dominant-negative XTcf-3 abolishes the dorsal activation of the siamois promoter. Furthermore, elimination of the Lef/Tcf sites elevates the ventral expression of siamois, revealing a repressive role for XTcf-3 in the absence of β-catenin. Finally, we find that the endogenous siamois activator, although present throughout the dorsal side of the embryo, is most potent in the dorsal vegetal region. We propose that the dorsal activation of siamois by the β-catenin/XTcf-3 complex combined with the ventral repression of siamois by XTcf-3 results in the restriction of endogenous siamois expression to the dorsal side ofXenopus embryos.

Keywords

Footnotes

  • 3 These authors contributed equally to this work.

  • 4 Corresponding author.

  • E-MAIL kimelman{at}u.washington.edu; FAX (206) 685-1792.

    • Received June 23, 1997.
    • Accepted July 28, 1997.
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