THE INTERPRETATION OF POSITION IN A MORPHOGEN GRADIENT AS REVEALED BYOCCUPANCY OF ACTIVIN RECEPTORS

Xenopus blastula cells activate different mesodermal genes as a concen tration-dependent response to activin, which behaves like a morphogen. To understand how cells recognize morphogen concentration, we have bo und naturally labeled activin to cells and related this to choice of g ene activation. We find that the increasing occupancy of a single rece ptor type can cause cells to switch gene expression. Cells sense ligan d concentration by the absolute number of occupied receptors per cell (100 and 300 molecules of bound activin induce Xbra and Xgsc, respecti vely, i.e., 2% and 6% of the total receptors) and not by a ratio of oc cupied to unoccupied receptors. The long duration of occupancy explain s a previously described ratchet effect. Our results suggest a new con cept of morphogen gradient formation and interpretation that is partic ularly well suited to the needs of early development.