The Polycomb Group Mutant esc Leads to Augmented Levels of Paused Pol II in the Drosophila Embryo

Many developmental control genes contain paused RNA polymerase II (Pol II) and are thereby poised for rapid and synchronous activation in the early Drosophila embryo. Evidence is presented that Polycomb group (PcG) repressors can influence paused Pol II. ChIP-Seq and GRO-Seq assays were used to determine the genome-wide distributions of Pol II, H3K27me3, and H3K4me3 in extra sex combs (esc) mutant embryos. ESC is a key component of the Polycomb repressive complex 2 (PRC2), which mediates H3K27me3 modification. Enhanced Pol II occupancy is observed for thousands of genes in esc mutant embryos, including genes not directly regulated by PRC2. Thus, it would appear that silent genes lacking promoter-associated paused Pol II in wild-type embryos are converted into poised genes with paused Pol II in esc mutants. We suggest that this conversion of silent genes into poised genes might render differentiated cell types susceptible to switches in identity in PcG mutants.