REGULATION OF THE ZEBRAFISH GOOSECOID PROMOTER BY MESODERM INDUCING FACTORS AND XWNT1

Goosecoid is a homeobox gene that is expressed as an immediate early r esponse to mesoderm induction by activin. We have investigated the ind uction of the zebrafish goosecoid promoter by the mesoderm inducing fa ctors activin and basic fibroblast growth factor (bFGF) in dissociated zebrafish blastula cells, as well as by different wnts in intact embr yos. Activin induces promoter activity, while bFGF shows a cooperative effect with activin. We have identified two enhancer elements that ar e functional in the induction of the goosecoid promoter. A distal elem ent confers activin responsiveness to a heterologous promoter in the a bsence of de novo protein synthesis, whereas a proximal element respon ds only to a combination of activin and bFGF. Deletion experiments sho w that both elements are important for full induction by activin. Nucl ear proteins that bind to these elements are expressed in blastula emb ryos, and competition experiments show that an octamer site in the act ivin responsive distal element is specifically bound, suggesting a rol e for an octamer binding factor in the regulation of goosecoid express ion by activin. Experiments in intact embryos reveal that the proximal element contains sequences that respond to Xwnt1, but not to Xwnt5c. Furthermore, we show that the distal element is active in a confined d orsal domain in embryos and responds to overexpression of activin in v ivo, as well as to dorsalization by lithium. The distal element is to our knowledge the first enhancer element identified that mediates the induction of a mesodermal gene by activin.