Polycomb in Transcriptional Phase Transition of Developmental Genes.

Combinatorial associations between distinct chromatin domains, namely promoters and cis-regulatory elements, determine transcriptional status. Developmental regulatory gene expression, mostly regulated by the Polycomb/Trithorax group of chromatin regulators, is often temporally and spatially specific, and sometimes changes repeatedly within the same cell lineage. Dysregulated expression of these genes causes morphological and/or functional disorganization of tissues and organs. Therefore, maintenance of both the active and negative states of transcription is equally important. In this review, we summarize the mechanisms of transition between transcriptional status of developmental regulators, including complex processes for enhancer activation and promoter-enhancer association. In particular, we propose testable models in which Polycomb group factors contribute to promoter-enhancer associations and thus proper gene expression.