Histone methylation in polycomb gene silencing

Polycomb (PcG) group proteins are chromatin-modifying factors that collaborate to repress gene expression during development. PcG proteins are crucial for silencing during animal embryogenesis, maintenance of stem cell populations and are also implicated in cancer epigenetics.;PcG proteins operate in at least three molecular complexes: PHORC, PRC2 and PRC1. Current models suggest that PRC1 is recruited at target loci by PRC2 and is most directly responsible for gene silencing. Our study uses the Drosophila model system to investigate the precise role of PRC2 in PRC1 targeting.;One key function of PRC2 is to catalyze methylation of histone H3 on lysine-27. A common model suggests that this creates a binding site for the recruitment of PRC1. The loss of PRC2 leads to disruption in PcG mediated silencing. We wished to assess if PRC1 is recruited at PcG targets by a direct interaction with PRC2 or by binding to the methyl mark. We have used enzyme-dead versions of PRC2 that can stably accumulate at target loci and shown that PRC1 is also found at these targets suggesting that the initial recruitment of PRC1 is independent of PRC2 catalytic function. We have also engineered a heterologous enzyme, called vSET, to create methylated histones in the absence of PRC2. This heterologous enzyme can be used to test for PRC1 accumulation without PRC2.