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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 injecting siamois  reporter constructs into the animal pole of Xenopus  embryos, we show that a 0.8-kb fragment of the siamois  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 that siamois  is regulated by the β-catenin/XTcf-3 complex, in combination with additional transcriptional activators. When expressed at the equator of the embryo, the  siamois promoter 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 of Xenopus  embryos.

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