Region-specific activation of the Xenopus Brachyury promoter involves active repression in ectoderm and endoderm: a study using transgenic frog embryos

Tissue specification in the early embryo requires the integration of spatia l information at the promoters of developmentally important genes, Although several response elements for signalling pathways have been identified in Xenopus promoters, it is not yet understood what defines the sharp herders that restrict expression to a specific tissue. Here we use transgenic frog embryos to study the spatial and temporal regulation of the Xbra promoter, Deletion analysis and point mutations in putative transcription factor-bind ing sites identified two repressor modules, which exert their main effects at different stages during gastrulation. One module is defined by a biparti te binding site for a Smad-interacting protein (SIP1) of the delta EF1 repr essor family and acts to confine expression to the marginal zone early in g astrulation, The other module is defined by two homeodomain-binding sites a nd is responsible for repression in dorsal mesoderm and ectoderm at mid-gas trula stages. In addition, an upstream region of the promoter is necessary to repress expression in neural tissues later in development, Together, our results show that repression plays an important role in the restriction of Xbra expression to the mesoderm, and we suggest that similar mechanisms ma y be involved in the spatial regulation of other genes in early embryonic d evelopment.