The Polycomb group mutant esc leads to augmented the levels of paused Pol II in the Drosophila embryo.

The role of Polycomb group (PcG) was studied on RNA Polymerase II (Pol II) pausing phenomenon. Wild type and Polycomb mutant (esc) embryos were used for ChIP-Seq experiments using Pol II and histone methylation antibodies. Enhanced Pol II occupancy was observed for thousands of genes in esc mutant embryos, including genes not known to be bound by PRC2 or having H3K27me3 associated. Most of these genes exhibit a concomitant reduction in H3K27me3 and increase in H3K4me3 at promoter-proximal regions. 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.