Fig. 8.

A cis- and trans-regulatory model for the temporal and spatial expression of nodal. (A) Cis-regulatory inputs into the nodal gene and disposition of its output. The three inputs demonstrated in the present study are indicated: initial input (1), positive feedback input (2), and vegetal repressor input (3). Other implied or proposed inputs (see text) are: a ubiquitous activator or an activating input driven by a vegetal signal, mediated by a 5P SMAD site; an extracellular aboral inhibition of Nodal signaling (open circle); an apical repressor input, and the input into INT of whatever activator accounts for ectopic expression in the vegetal region in the absence of the 3′ region of INT module. (B) Locations in the embryo of the trans factors that convey these inputs at blastula stage.