The Xenopus eomesodermin promoter and its concentration-dependent response to activin

Eomesodermin is an essential early gene in Xenopus mesoderm formation and shows a morphogen-like response to activin. Here we define the regions of the Eomesodermin promoter required for mesodermal expression and for concentration-dependent response to activin. We find an activin response element (ARE) located between -5.6 and -5.0 kb which contains two critical FAST2 binding sites. The ARE alone is necessary and sufficient for concentration-dependent response to activin. A 5.6 kb promoter recapitulates Eomes expression in normal mesoderm cells. A repressor element extinguishes Eomes expression in the endoderm. We relate our results to mesoderm patterning in early Xenopus development and to a mechanism of morphogen gradient response.