Mapping the distribution of polycomb-group proteins at the Giant gene in early Drosophila embryos

Polycomb group proteins (PcG) are transcriptional repressors of various genes required during development. They are highly conserved from Drosophila to mammals. They are known to maintain pluripotent state of stem cells and their misexpression is known to cause several cancers. Polycomb group proteins do not initiate the repression of target genes but maintain the repression of target genes following initial repression by gene specific transcription factors.;The initial setup of gene expression in Drosophila embryos is caused by maternal and zygotic transcription factors. For instance, hunchback (hb), a transcription factor, initially represses Ultrabithorax (Ubx), a Hox gene. Zygotic hb decays ∼2 hrs after initiating Ubx repression. However, the silenced state of Ubx is maintained throughout life by PcG proteins. Most of the studies done to date have focused on studying the role of the PcG during the maintenance phase of repression using tissue or cultured cells in which a target gene is uniformly repressed. However, almost nothing is known about how PcG proteins initially recognize and take over the repressed state of target genes and the molecular steps that lead to maintenance of repression. To be able to understand the molecular mechanisms that take place when the PcG initially takes over repression of target genes, a homogenous population of cells in which the target gene is uniformly repressed is required.;Giant (gt) is a important segmentation gene that is expressed in early Drosophila embryos. It is a known target gene of PcG proteins and is initially repressed by a maternally expressed, hb at the syncytial blastoderm. Normally, zygotically expressed hb takes over gt repression by the cellular blastoderm stage. In mutant backgrounds that lack zygotic hb expression (embryos derived from bcd tsl females), PcG proteins assume the responsibility of maintenance of repression.;In this study, distribution of PcG proteins have been mapped across the giant gene to lay a foundation for further studies into the mechanisms by which PcG proteins recognize the repressed state of a target gene and the sites where transcription factors such as repressors and activators bind with respect to the PcG.