Goosecoid and Mix.1 repress Brachyury expression and are required for headformation in Xenopus

The Xenopus homologue of Brachyury, Xbra, is expressed in the presumptive m esoderm of the early gastrula, Induction of Xbra in animal pole tissue by a ctivin occurs only in a narrow window of activin concentrations; if the lev el of inducer is too high, or too low, the gene is not expressed. Previousl y, we have suggested that the suppression of Xbra by high concentrations of activin is due to the action of genes such as goosecoid and Mix.1. Here, w e examine the roles played by goosecoid and Mix.1 during normal development , first in the control of Xbra expression and then in the formation of the mesendoderm. Consistent with the model outlined above, inhibition of the fu nction of either gene product leads to transient ectopic expression of Xbra . Such embryos later develop dorsoanterior defects and, in the case of inte rference with Mix.1, additional defects in heart and gut formation. Gooseco id, a transcriptional repressor, appears to act directly on transcription o f Xbra, In contrast, Mix.1, which functions as a transcriptional activator, may act on Xbra indirectly, in part through activation of goosecoid.