THE XENOPUS BRACHYURY PROMOTER IS ACTIVATED BY FGF AND LOW CONCENTRATIONS OF ACTIVIN AND SUPPRESSED BY HIGH-CONCENTRATIONS OF ACTIVIN AND BY PAIRED-TYPE HOMEODOMAIN PROTEINS

The mesoderm of Xenopus laevis arises through an inductive interaction in which signals from the vegetal hemisphere of the embryo act on ove rlying equatorial cells. One candidate for an endogenous mesoderm-indu cing factor is activin, a member of the TGF beta superfamily. Activin is of particular interest because it induces different mesodermal cell types in a concentration-dependent manner, suggesting that it acts as a morphogen. These concentration-dependent effects are exemplified by the response of Xbra, expression of which is induced in ectodermal ti ssue by low concentrations of activin but not by high concentrations. Xbra therefore offers an excellent paradigm for studying the way in wh ich a morphogen gradient is interpreted in vertebrate embryos. In this paper we examine the trancriptional regulation of Xbra2, a pseudoalle le of Xbra that shows an identical response to activin. Our results in dicate that 381 bp 5' of the Xbra2 transcription start site are suffic ient to confer responsiveness both to FGF and, in a concentration-depe ndent manner, to activin. We present evidence that the suppression of Xbra expression at high concentrations of activin is mediated by paire d-type homeobox genes such as gooseeoid, Mix.1, and Xotx2.