Maintenance of cellular memory by the Polycomb-group genes.

The Poycomb-group genes (PcG) encode a group of repressors well known for their function in stably maintaining the inactive expression patterns of key developmental regulators, including homeotic genes. PcG genes are structurally and functionally conserved in Drosophila and Mammalians, and some homologues have been found in worms, yeast and plants. Their products act through different complexes and at least one of these complexes seems to induce histone deacetylation. In Drosophila: building of PcG complexes depends on both protein-protein interactions and recognition near target genes of specific DNA sequences called Polycomb-group response element (PRE). Together with the counteracting trithorax-group proteins, PcG products establish a form of cellular memory by faithfully maintaining transcription states determined early in embryogenesis. Here, we discuss several aspects of PcG functions: the composition of the different complexes, the establishment and the transmission of silencing to subsequent cell generations as well as the subnuclear localisation of the PcG products. (C) 2001 Academie des sciences/Editions scientifiques et medicales Elsevier SAS.