Figure 8.

Model for Sens action. (A) Schematic of proneural and Sens expression in a spatial and temporal fashion. The darkness of the colors is meant to reflect levels of proteins. Arrows indicate temporal events. We have been unable to establish whether, in the SOP, the elevated Sens expression precedes elevated proneural expression, whether they occur simultaneously, or whether elevated proneural expression precedes elevated Sens expression. (B) Schematic of the mechanism by which the Sens binary switch operates. N signaling and low levels of Sens keep the levels of proneural proteins low in the proneural field. Upon N signaling, the N^icd together with Su(H) increase the expression of E(spl) proteins. Proneural proteins also increase the expression of E(spl). E(spl) in turn prevents proneural protein up-regulation by inhibiting expression and function of Sens and proneurals. Low levels of Sens also repress the expression of proneural genes. At a later stage, Sens levels in the presumptive SOP reach a point that is sufficient to initiate synergism (red box) between proneurals and Sens. Increased levels of proneurals in the presumptive SOP prevents this cell from receiving the N signal, and Su(H) together with Hairless and corepressors repress the expression of E(spl). In addition, high levels of Sens prevent the E(spl) proteins from repressing the proneurals.