Wnt signaling and transcriptional control of Siamois in Xenopus embryos.

The Wnt-inducible homeobox gene Siamois is expressed in Xenopus embryos before gastrulation and is necessary for formation of the Spemann organizer. Here we show that 5'-flanking sequences of the Siamois coding region can specifically activate a heterologous reporter gene in dorsovegetal cells, thus mimicking Siamois's endogenous expression. A 245-bp DNA fragment is sufficient for activation by both Wnts and endogenous inducers. A dominant negative form of Xenopus T cell-specific factor 3 (XTCF-3) inhibited promoter activity, indicating that T cell-specific factor (TCF)/lymphocyte enhancer binding factor 1 (LEF-1) signaling is necessary for regulation of Siamois. Mutagenesis of two individual TCF sites in the -245 promoter revealed that the proximal, but not distal, site is necessary for dorsovegetal activation. These observations suggest that Siamois is directly regulated by TCFs during dorsoventral axis determination. Further deletion analysis identified a positive regulatory region that is required for dorsal activation, but not for Wnt inducibility, of the promoter. We also present evidence for autoregulation of Siamois transcription. Furthermore, the Siamois promoter was activated by Wnt signaling in 293T tissue culture cells, demonstrating that regulation of the promoter is functionally conserved.