INHIBITION OF XBRA TRANSCRIPTION ACTIVATION CAUSES DEFECTS IN MESODERMAL PATTERNING AND REVEALS AUTOREGULATION OF XBRA IN DORSAL MESODERM

The Brachyury (T) gene is required for formation of posterior mesoderm and for axial development in both mouse and zebrafish embryos, In thi s paper, we first show that the Xenopus homologue of Brachyury, Xbra, and the zebrafish homologue, no tail (ntl), both function as transcrip tion activators, The activation domains of both proteins map to their carboxy terminal regions, and we note that the activation domain is ab sent in two zebrafish Brachyury mutations, suggesting that it is requi red for gene function, A dominant-interfering Xbra construct was gener ated by replacing the activation domain of Xbra with the repressor dom ain of the Drosophila engrailed protein, Microinjection of RNA encodin g this fusion protein allowed us to generate Xenopus and zebrafish emb ryos which show striking similarities to genetic mutants in mouse and fish, These results indicate that the function of Brachyury during ver tebrate gastrulation is to activate transcription of mesoderm-specific genes, Additional experiments show that Xbra transcription activation is required for regulation of Xbra itself in dorsal, but not ventral, mesoderm. The approach described in this paper, in which the DNA-bind ing domain of a transcription activator is fused to the engrailed repr essor domain, should assist in the analysis of other Xenopus and zebra fish transcription factors.