Imaginal disc silencers from Ultrabithorax: evidence for Polycomb response elements

Silencers from the Drosophila homeotic gene Ultrabithorax (Ubx) require hunchback (hb) and Polycomb (Pc) to suppress the activity of embryonic enhancers outside the Ubx domain. Embryonic silencing is initiated by hb protein which binds to the silencers to repress Ubx, thereby defining the Ubx domain. Here, we study silencing during subsequent development by examining expression patterns in imaginal discs conferred by individual Ubx fragments and pair-wise combinations thereof. We find that fragments which mediate silencing in anterior regions of imaginal discs contain embryonic silencers and hb target sites. One exception to this is a fragment called BXD which is not under hb control itself, but whose silencing activity depends on combination with fragments containing hb protein binding sites. Since silencing by BXD also requires Pc function, this suggests that BXD contains target sites for Pc or for Pc-like proteins. We propose that stable silencing of Ubx is achieved through cooperation between hb and Pc target sites.