FIG 8.

Model of mutual dependency of SBF binding. Diagrams of the HO promoter with rectangles indicating URS1 and URS2. Swi5 site locations in URS1 are indicated, and numbers indicate SBF sites in URS2. Swi5 and SBF transcription factors are represented by dark- and light-gray ovals, respectively. Signal propagation along the HO promoter is indicated by arrows. Generation of HO RNA is specified by the solid arrow, while the absence of production of HO expression is represented by dashed arrows. Top (wild type), predicted signal propagation in the native promoter. Swi5-dependent coactivator recruitment to URS1 allows for SBF loading onto the left half of URS2 via an unknown mechanism. Left-half SBF loading allows for downstream right-half URS2 SBF binding and ultimate activation of HO. Middle, loss of URS2-L SBF sites blocks URS2-L SBF loading and prevents downstream SBF signal propagation. Bottom, loss of URS2-R sites allows for URS2-L SBF loading but prevents the URS2-R SBF binding that is required for HO activation.