The DNA-binding Polycomb group protein Pleiohomeotic mediates silencing ofa Drosophila homeotic gene

Polycomb group (PcG) proteins repress homeotic genes in cells where these g enes must remain inactive during development. This repression requires cia- acting silencers, also called PcG response elements. Currently, these silen cers are ill-defined sequences and it is not known how PcG proteins associa te with DNA. Here, we show that the Drosophila PcG protein Pleiohomeotic bi nds to specific sites in a silencer of the homeotic gene Ultrabithorax. In an Ultrabithorax reporter gene, point mutations in these Pleiohomeotic bind ing sites abolish PcG repression in vivo. Hence, DNA-bound Pleiohomeotic pr otein may function in the recruitment of other non-DNA-binding PcG proteins to homeotic gene silencers.