Fig. 8. Model of how an excess of Ubx may trigger permanent Ubx repression. (A) In the wild type, Ubx protein expression downregulates, but does not suppress, Ubx transcription in the haltere disc. Ubx transcription through the PRE prevents the function of Pc-group complexes either by reducing binding of some proteins to the PRE or by preventing the assembly of such complexes. (B) When Ubx protein levels are highly raised, Ubx transcription is repressed and this leads to the binding and/or activation of the Pc-group complexes at the PRE. (C) Pc-group activity prevents the expression of the Gal4 gene in the Ubx-Gal4^SS.2 insertion and, eventually, of the Ubx gene. The UbdA domain of the decaying Ubx protein may help in this role.