Activin-induced factors maintain goosecoid transcription through a paired homeodomain binding site.

Previous studies in both Xenopus and zebrafish have shown that goosecoid is one of the first genes to be transcribed at the onset of gastrulation. Goosecoid transcription still initiates when embryos are treated with protein synthesis inhibitors, indicating that it is mediated by preexisting factors and suggesting that goosecoid transcription is immediately downstream of the maternal mesoderm-inducing signal. However, goosecoid transcription continues long after this maternal signal has ceased to be active, indicating that there are mechanisms to maintain activin-induced transcription. Our study has focused on understanding the factors required to maintain this transcription. We have defined an element within the zebrafish goosecoid promoter that is sufficient for activin inducibility in both Xenopus and zebrafish embryos. This element, the goosecoid activin element, interacts with two developmentally regulated proteins from Xenopus embryos. A maternal protein interacts through cleavage stages until the midblastula transition, and a second protein binds from the onset of gastrulation. The second protein is zygotically expressed, and its binding is required for activin inducibility in our assay system. We suggest that the zygotic protein we have identified is a good candidate to be involved in the maintenance of goosecoid transcription. Furthermore, this zygotic protein is likely to contain a paired class homeodomain since a consensus binding site for such proteins is present within the goosecoid activin element and is essential for its function.