Activin induces the expression of different genes in a concentration-dependent manner. In this paper, we show that the initial response of cells to activin, whether assayed in dispersed cells or in a bead-implantation regime in intact animal caps, is to activate expression of both Xbra and goosecoid. However, differential expression of the two genes, with down-regulation of Xbra, occurs very rapidly and certainly within 3 h of the initial phase of expression. This rapid refinement of gene expression can occur in dispersed cells and thus does not require cell-cell interactions. Refinement of gene expression does, however, require protein synthesis but not goosecoid function. Together, our results place the burden of threshold formation not on the initial induction of different genes but on regulatory interactions between the genes once they have been activated.
Copyright 2000 Academic Press.