Résumé
The Polycomb group and trithorax group genes of Drosophila are required for maintaining the differential expression state of developmental regulators, such as the homeotic genes, in a stable and heritable manner throughout development. The Polycomb group genes have been suggested to act by regulating higher order chromatin and packaging repressed chromosomal domains in a heterochromatin‐like structure. We have mapped, at high resolution, the distribution of Polycomb protein on the bithorax complex of Drosophila tissue culture cells, using an improved formaldehyde cross‐linking and immunoprecipitation technique. Polycomb protein is not distributed homogeneously on the regulatory regions of the repressed Ultrabithorax and abdominal‐A genes, but is highly enriched at discrete sequence elements, many of which coincide with previously mapped Polycomb group response elements (PREs). Our results further suggest that Polycomb protein spreads locally over a few kilobases of DNA surrounding PREs, perhaps to stabilize silencing complexes. GAGA factor/Trithorax‐like, a member of the trithorax group, is also bound at those PREs which contain GAGA consensus‐binding sites. Two modes of binding can be distinguished: a high level binding to elements in the regulatory domain of the expressed Abdominal‐B gene, and a low level of binding to Polycomb‐bound PREs in the inactive domains of the bithorax complex. We propose that GAGA factor binds constitutively to regulatory elements in the bithorax complex, which function both as PREs and as trithorax group response elements.