Distinct roles of Polycomb group gene products in transcriptionally repressed and active domains of Hoxb8.

To address the molecular mechanisms underlying Polycomb group (PcG)-mediated repression of Hox gene expression, we have focused on the binding patterns of PcG gene products to the flanking regions of the Hoxb8 gene in expressing and non-expressing tissues. In parallel, we followed the distribution of histone marks of transcriptionally active H3 acetylated on lysine 9 (H3-K9) and methylated on lysine 4 (H3-K4), and of transcriptionally inactive chromatin trimethylated on lysine 27 (H3-K27). Chromatin immunoprecipitation revealed that the association of PcG proteins, and H3-K9 acetylation and H3-K27 trimethylation around Hoxb8 were distinct in tissues expressing and not expressing the gene. We show that developmental changes of these epigenetic marks temporally coincide with the misexpression of Hox genes in PcG mutants. Functional analyses, using mutant alleles impairing the PcG class 2 component Rnf2 or the Suz12 mutation decreasing H3-K27 trimethylation, revealed that interactions between class 1 and class 2 PcG complexes, mediated by trimethylated H3-K27, play decisive roles in the maintenance of Hox gene repression outside their expression domain. Within the expression domains, class 2 PcG complexes appeared to maintain the transcriptionally active status via profound regulation of H3-K9 acetylation. The present study indicates distinct roles for class 2 PcG complexes in transcriptionally repressed and active domains of Hoxb8 gene.