Practice question

Siamois and Twin are immediate early zygotic genes expressed in Nieuwkoop center downstream of Wnt pathway. Beta-catenin stabilized dorsally enters nucleus displacing Groucho corepressor from Tcf3, forming beta-catenin-Tcf3/Lef activation complex binding specific TCF sites in siamois and twin promoters. Their transcription suppressed by dominant-negative Tcf3 or beta-catenin depletion, superinduced by LiCl or Wnt mRNA. Noggin, Activin or BMP act later or downstream, so primary activator complex responsible for their initiation is beta-catenin-Tcf3 complex, initiating organizer cascade. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.