EOMESODERMIN, A KEY EARLY GENE IN XENOPUS MESODERM DIFFERENTIATION

Eomesodermin (Eomes) is a novel Xenopus T-domain gene. In normal devel opment, it is expressed in mesodermal cells in a ventral-to-dorsal gra dient of increasing concentration. It reaches its peak expression 1-2 hr before any other known panmesodermal gene. It is strongly inducible by normal vegetal cells and by mesoderm-inducing factors. Ectopic exp ression of Eomes in animal caps induces the transcription of nearly al l mesodermal genes in a concentration-dependent way. Overexpression of Eomes dorsalizes ventral mesoderm, inducing gsc and changing cell fat e to muscle and notochord. Blocking the function of Eomes causes gastr ulation arrest and defective mesoderm-dependent gene activation. We pr opose that Eomes fulfills an essential function in initiating mesoderm differentiation and in determining mesodermal cell fate.