A ROLE FOR THE VEGETALLY EXPRESSED XENOPUS GENE MIX.1 IN ENDODERM FORMATION AND IN THE RESTRICTION OF MESODERM TO THE MARGINAL ZONE

We have studied the role of the activin immediate-early response gene Mix.1 in mesoderm and endoderm formation. In early gastrulae, Mix.1 is expressed throughout the vegetal hemisphere, including marginal-zone cells expressing the trunk mesodermal marker Xbra. During gastrulation , the expression domains of Xbra and Mix.1 become progressively exclus ive as a result of the establishment of a negative regulatory loop bet ween these two genes. This mutual repression is important for the spec ification of the embryonic body plan as ectopic expression of Mix.1 in the Xbra domain suppresses mesoderm differentiation. The same effect was obtained by overexpressing VP16Mix.1, a fusion protein comprising the strong activator domain of viral VP16 and the homeodomain of Mix.1 , suggesting that Mix.1 acts as a transcriptional activator. Mix.1 als o has a role in endoderm formation. It cooperates with the dorsal vege tal homeobox gene Siamois to activate the endodermal markers edd, Xlhb ox8 and cerberus in animal caps. Conversely, vegetal overexpression of enRMix.1, an antimorphic Mix.1 mutant, leads to a loss of endoderm di fferentiation. Finally, by targeting enRMix.1 expression to the anteri or endoderm, we could test the role of this tissue during embryogenesi s and show that it is required for head formation.