Fig. 7. (A) Heterozygous CtBP mutants completely abolished YY1 repression. The GBUZ hbGALYY1 recombinant line was crossed into the either ctbp or groucho mutant backgrounds. Heterozgous CtBP mutants completely abolished YY1 repression, whereas Groucho mutants showed little effect. (B) YY1 physically interacts with CtBP. GST pull-down experiments were performed with recombinant YY1 prepared by in vitro translation, and GST–CtBP prepared in bacteria. The arrow indicates full-length YY1. (C) Heterozygous CtBP mutation abolishes Pc repression. The hbGAL-Pc BGUZ recombinant chromosome was crossed into a heterozygous ctbp mutant background, resulting in loss of Pc repression. (D) Potential mechanism linking YY1, CtBP and the PcG complex. In this model, YY1 binds a homodimer of CtBP that interacts with the PcG complex to recruit the complex to PRE sequences, thereby repressing transcription. Arrows show potential dual functions of CtBP in recruiting PcG proteins and repressing transcription.