Roles of Individual Protein in de novo Polycomb Protein Recruitment

Polycomb Group (PcG) proteins are evolutionarily conserved epigenetic transcriptional regulators that maintain the transcriptional repression of silenced genes by maintaining heritable chromatin states. PcG proteins first discovered as repressors of Hox genes in Drosophila, later were shown to regulate a wide range of genes. In mammals, PcG proteins are involved in maintaining pluripotent state of stem cells and controlling cell differentiation. Misexpression of PcG protein leads to cancers like lymphoma and melanoma. PcG proteins maintain rather than initiate transcriptional repression, once PcG – mediated repression is established it can be maintained through an unlimited number of cycles. Most studies of PcG proteins are in vitro or focus on maintenance phase of repression. Little is known about the molecular mechanisms by which PcG proteins are initially recruited to target genes. The challenge of obtaining a homogenous population of cells in a certain developmental stage in which a target gene is uniformly repressed by PcG proteins, creates a major difficulty in studying recruitment of PcG proteins in vivo. To solve this problem, our lab previously generated a genetic system in which giant (gt), a PcG target gene, is ubiquitously repressed. In embryos produced by bcd osk tsl homozygous mother, maternal Hunchback (Hb) is ubiquitously expressed due to lack of osk. There is no zygotic Hb due to lack of bcd and tsl. gt remained repressed after maternal Hb is completely degraded at nuclear cycle 14, at the same time PcG proteins take over repression. Time course chromatin immunoprecipitation (ChIP) assay has previously been done on bcd osk tsl system to determine proteins distribution when PcG proteins take over repression. To study contributions of various proteins involved in recruitment, we knocked down each protein by RNAi in bcd osk tsl background followed by ChIP assays on embryos of different stages. The goal of my research is to define the roles of individual PcG proteins (as complexes) as well as participated transcription factors in facilitating de novo establishment of PcG silencing