Co-localization of Polycomb protein and GAGA factor on regulatory elements responsible for the maintenance of homeotic gene expression

Abstract

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.

The relationship between BCL6 bodies and nuclear sites of normal and halogenated DNA and RNA synthesis

“…All these activities are likely mechanistically linked and depend on a "basic" property shared by other POK proteins, namely the ability to self-aggregate and bring together "distant" pieces of DNA (Americo et al, 2002;Katsani et al, 1999;Mahmoudi et al, 2002;West et al, 2002). However, it is likely that such versatility also reflects different "states" of GAGA with respect to covalent modifications and/or interactions with particular protein partners or DNA sequences (Espinas et al, 2000;Horard et al, 2000;Strutt et al, 1997;Tsukiyama and Wu, 1995). It can, thus, be reasonably envisioned that the role of BCL6 might extend well beyond "classical" transcriptional repression.…”

Comparing active and repressed expression states of genes controlled by the Polycomb/Trithorax group proteins

“…A key to the understanding of the molecular mechanisms is to uncover the chromatin sites of action of the PcG/TrxG proteins and to correlate their binding patterns with the presence of histone modifications as well as with the expression level of their target genes. The advent of ChIP maps demonstrated an accumulation of PcG/TrxG proteins at PREs and some of the associated promoters (18,19). Interestingly, PcG proteins were found to be bound at PREs, even if the associated gene was active (20,21).…”

In situ dissection of a Polycomb response element in Drosophila melanogaster

“…PcG products act at the PREs in such constructs to repress or restrict expression of certain reporter genes (5)(6)(7)(8), and PRE transgenes can recruit the POLY-COMB protein to new chromosomal locations (6,(9)(10)(11). The binding of PcG proteins to PREs can also be assayed by formaldehyde cross-linking of nuclei from cultured cells, whole embryos, or imaginal discs (12)(13)(14)(15). These assays may define where PREs lie on the DNA sequence, but do not reveal how they function in their original context.…”