The role of polycomb group proteins in cell cycle regulation during development.

Polycomb group (PcG) and trithorax group (trxG) proteins are evolutionarily conserved chromatin modifiers that have well known roles in the maintenance of silent and active expression states of homeotic genes. PcG proteins may also be involved in the control of cellular proliferation, as several PcG complexes have been shown to act either as proto-oncogenes or as tumor suppressors in vertebrates. In Drosophila, PcG factors associate with specific DNA regions termed PcG response elements (PREs), and a PRE was recently identified in the gene encoding Cyclin A. Still, it is not yet clear how PcG complexes could control cell cycle progression. Beyond acting as stable silencers of cell cycle genes during the differentiation process, PcG complexes might also be integrators and/or modulators of cell cycle checkpoints in dividing cells. Here, we discuss this dual aspect of PcG involvement in epigenetic cell cycle control.