Résumé
In Drosopbila the Polycomb group (PcG) and trithorax group (trxG) genes are required to maintain differential expression patterns of many important developmental regulatory genes. The PcG is responsible for heritable silencing throughout development. At target genes PcG response elements (PREs) attract PcG protein complexes and induce the formation of higher‐order chromatin structures. We have mapped the distribution of Polycomb and other PcG members at various target genes by using an improved formaldchyde cross‐linking and chromatin immunoprecipitation technique. We find that Polycomb spreads locally from PREs over several kilobases, thereby probably stabilizing the silencing complexes. Members of the trxG co‐localize at PREs. GAGA factor was found to be constitutively bound to PRRs independently of gene activity. PREs associated with active genes appear to have increased amounts of bound GAGA. We have developed a system capable of switching a PRE between the on/off modes. PREs and trxG‐regulated elements arc common chromosomal elements through which the proteins of the PcG/trxG exert their maintenance function on adjacent chromatin structures.