Interaction of goosecoid and brachyury in Xenopus mesoderm patterning.

Detailed in situ analyses reveal overlapping expression of gsc and Xbra in the early Spemann's organizer. Coexpression is lost during gastrulation suggesting an interaction between these genes. Ectopic expression of gsc ventrally suppresses endogenous Xbra expression and transcription from Xbra promoter reporter gene constructs. Suppression is mediated, at least partially, by a gsc-binding site within the first 349 bp of the promoter. Xbra reporter gene transcription is also suppressed in the region of endogenous gsc expression, whereas high-level ectopic Xbra expression has no effect on endogenous gsc expression. We suggest that early patterning of the vertebrate mesoderm, like early patterning of the Drosophila embryo, occurs by first establishing broad domains of gene expression which are subsequently refined by intergenic interactions to further delimit tissue boundaries.