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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-Gal4SS.2 insertion and, eventually, of the
Ubx gene. The UbdA domain of the decaying Ubx protein may help in
this role.