The iab-7 polycomb response element maps to a nucleosome-free region of chromatin and requires both GAGA and pleiohomeotic for silencing activity

In the work reported here we have undertaken a functional dissection of a P olycomb response element (PRE) from the iab-7 cis-regulatory domain of the Drosophila melanogaster bithorax complex (BX-C). Previous studies mapped th e iab-7 PRE to an 860-bp fragment located just distal to the Fab-7 boundary . Located within this fragment is an similar to 230-bp chromatin-specific n uclease-hypersensitive region called HS3. We have shown that HS3 is capable of functioning as a Polycomb-dependent silencer in vivo, inducing pairing- dependent silencing of a mini-white reporter. The HS3 sequence contains con sensus binding sites for the GAGA factor, a protein implicated in the forma tion of nucleosome-free regions of chromatin, and Pleiohomeotic (Pho), a Po lycomb group protein that is related to the mammalian transcription factor YY1. We show that GAGA and Pho interact with these sequences in vitro and t hat the consensus binding sites for the two proteins are critical for the s ilencing activity of the iab-7 PRE in vivo.