XVENT-1 MEDIATES BMP-4-INDUCED SUPPRESSION OF THE DORSAL-LIP-SPECIFICEARLY RESPONSE GENE XFD-1' IN XENOPUS EMBRYOS

Ectopic expression of the ventralizing morphogen BMB-4 (bone morphogen etic protein-4) in the dorsal lip (Spemann organizer) of Xenopus embry os blocks transcription of dorsal-lip-specific early response genes. W e investigated the molecular mechanism underlying the BMP-4-induced in hibition of the fork head gene XFD-1'. The promoter of this gene conta ins a BMP-triggered inhibitory element (BIE) which prevents activation of this gene at the ventral/vegetal side of the embryo in vivo. In th e present study, we show that BMP-4-induced inhibition is not direct b ut indirect, and is mediated by Xvent homeobox proteins. Microinjectio ns of Xvent-1 RNA and XFD-1' promoter deletion mutants demonstrate tha t Xvent-1 mimics the effect of BMP-4 signalling not only by suppressio n of the XFD-1' gene, but also by utilizing the BEE. Suppression could be reverted using a dominant-negative Xvent-1 mutant. The repressor d omain was localized to the N-terminal region of the protein, Gel-shift and footprint analyses prove that Xvent-1 binds ilo the BIE. Moreover PCR-based target-site selection for the Xvent-1 homeodomain confirms distinct motifs within the BIE as preferential binding sites. Thus, bi ological and molecular data suggest that Xvent-1 acts as direct repres sor for XFD-1' transcription and mediates BMP-4-induced inhibition.