Embracing change to remain the same: conservation of polycomb functions despite divergence of binding motifs among species

Polycomb-group (PcG) proteins are evolutionarily conserved developmental regulators among species. In flies, PcG factors bind discrete DNA motifs known as the PREs (polycomb response elements); while in mammals they exhibit a marked preference for CpG islands (CGIs) that are unmethylated CpG-rich stretches in the genome. In this review, we discuss that despite the lack of similarities in DNA sequence both PREs and CGIs share many common properties and suggest that these could play a role for recruitment of PcG molecules into these loci. Further, in light of the antagonistic relationship between polycomb and DNA methylation pathways in mammals, we construct a model by compartmentalizing the genome into several representative categories and propose that the polycomb factors are the most suited to regulate development associated genes because of the robust yet reversible nature of PcG mediated transcriptional repression.