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 embryoni c enhancers outside the Ubx domain. Embryonic silencing is initiated b y hb protein which binds to the silencers to repress Ubx, thereby defi ning the Ubx domain. Here, we study silencing during subsequent develo pment by examining expression patterns in imaginal discs conferred by individual Ubx fragments and pair-wise combinations thereof. We find t hat fragments which mediate silencing in anterior regions of imaginal discs contain embryonic silencers and hb target sites. One exception t o this is a fragment called BXD which is not under hb control itself, but whose silencing activity depends on combination with fragments con taining hb protein binding sites. Since silencing by BXD also requires Pc function, this suggests that BXD contains target sites for Pc or f or Pc-like proteins. We propose that stable silencing of Ubx is achiev ed through cooperation between hb and Pc target sites.