Maintenance of cellular memory by the Polycomb-group genes.

The Poycomb-group genes (PcG) encode a group of repressors well known for t heir function in stably maintaining the inactive expression patterns of key developmental regulators, including homeotic genes. PcG genes are structur ally and functionally conserved in Drosophila and Mammalians, and some homo logues have been found in worms, yeast and plants. Their products act throu gh 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 spec ific DNA sequences called Polycomb-group response element (PRE). Together w ith the counteracting trithorax-group proteins, PcG products establish a fo rm of cellular memory by faithfully maintaining transcription states determ ined early in embryogenesis. Here, we discuss several aspects of PcG functi ons: the composition of the different complexes, the establishment and the transmission of silencing to subsequent cell generations as well as the sub nuclear localisation of the PcG products. (C) 2001 Academie des sciences/Ed itions scientifiques et medicales Elsevier SAS.