ANALYSIS OF COMPETENCE AND OF BRACHYURY AUTOINDUCTION BY USE OF HORMONE-INDUCIBLE XBRA

Analysis of gene function in Xenopus development frequently involves o ver-expression experiments, in which RNA encoding the protein of inter est is microinjected into the early embryo, By taking advantage of the fate map of Xenopus, it is possible to direct expression of the prote in to particular regions of the embryo, but it has not been possible t o exert control over the timing of expression; the protein is translat ed immediately after injection. To overcome this problem in our analys is of the role of Brachyury in Xenopus development, we have, like Kolm and Sive (1995; Dev. Biol, 171, 267-272), explored the use of hormone -inducible constructs. Animal pole regions derived from embryos expres sing a fusion protein (Xbra-GR) in which the Xbra open reading frame i s fused to the ligand-binding domain of the human glucocorticoid recep tor develop as atypical epidermis, presumably because Xbra is sequeste red by the heat-shock apparatus of the cell. Addition of dexamethasone , which binds to the glucocorticoid receptor and releases Xbra, causes formation of mesoderm. We have used this approach to investigate the competence of animal pole explants to respond to Xbra-GR, and have fou nd that competence persists until late gastrula stages, even though by this time animal caps have lost the ability to respond to mesoderm-in ducing factors such as activin and FGF. In a second series of experime nts, we demonstrate that Xbra is capable of inducing its own expressio n, but that this auto-induction requires intercellular signals and FGF signalling, Finally, we suggest that the use of inducible constructs may assist in the search for target genes of Brachyury.