MESODERM AND ENDODERM DIFFERENTIATION IN ANIMAL CAP EXPLANTS - IDENTIFICATION OF THE HNF4-BINDING SITE AS AN ACTIVIN-A RESPONSIVE ELEMENT IN THE XENOPUS HNF1-ALPHA PROMOTER

The gene encoding the tissue-specific transcription factor HNF1 alpha (LFB1) is transcriptionally activated shortly after mid-blastula trans ition in Xenopus embryos, We have now shown that the HNF1 alpha protei n is localized in the nuclei of the liver, gall bladder, gut and prone phros of the developing larvae, In animal cap explants treated with ac tivin A together with retinoic acid, we induced HNF1 alpha in pronephr ic tubules and epithelial gut cells, i.e. in mesodermal as well as in endodermal tissues, HNF1 alpha can also be induced by activin A, but n ot by retinoic acid alone, To define the promoter element responding t o the activin A signal, we injected various HNF1 alpha promoter lucife rase constructs into fertilized eggs and cultured the isolated animal caps in the presence of activin A, From the activity profiles of the p romoter mutants used, we identified the HNF4-binding site as an activi n-A-responsive element, As HNF4 is a maternal protein in Xenopus and l ocalized in an animal-to-vegetal gradient in the cleaving embryo, we s peculate that the activin A signal emanating from the vegetal pole coo perates with the maternal transcription factor HNF4 to define the embr yonic regions expressing HNF1 alpha.