De novo establishment of Polycomb-mediated repression. Jumana S AlHaj Abed, Judith Benes, Richard Jones. Dept. Of Biology, Southern Methodist University, Dallas, TX.

   Polycomb group proteins (PcG) are conserved epigenetic regulators that control target genes by taking over repression from gene-specific transcription factors. Once PcG-mediated repression is established, it is maintained through many cell divisions. Most studies have focused on the activities of PcG proteins during the maintenance phase. Much less is known about the mechanisms and molecular events by which PcG proteins initially recognize the repressed state of a gene and lead to the establishment of PcG-mediated silencing. The challenge to understanding PcG silencing mechanisms in vivo is the difficulty of acquiring a homogeneous population of cells in which all cells are exhibiting PcG-mediated repression of a particular gene. This study focuses on understanding the molecular events that lead to the initiation of PcG-mediated repression of giant (gt). Maternal-effect mutations are used to generate embryos in which gt is uniformly repressed. Chromatin immunoprecipitiation (ChIP) assays are being used to examine the distribution of PcG proteins and other transcription factors at gt. By perfroming ChIP assays on a time course of embryos from nuclear cleavage stages, and throught cellular blastoderm, it is possible to track the events at gt as PcG proteins take control of repression from maternal Hunchback (Hb). In addition, transgenic reporter lines have been generated in order to map the location of Polycomb Response Elements (PREs) within the upstream regulatory region of gt.