Chromatin compaction at the mononucleosome level

Using a previously described FRET technique, we measured the distance between the ends of DNA fragments on which nucleosomes were reconstituted from recombinant and native histories. This distance was analyzed in its dependence on the DNA fragment length, concentration of mono- and divalent counterions, presence of linker historic H1, and historic modifications. We found that the linker DNA arms do not cross under all conditions studied but diverge slightly as they leave the histone core surface. Historic H I leads to a global approach of the linker DNA arms, confirming the notion of a "stem structure". Increasing salt concentration also leads to an approach of the linker DNAs. To study the effect of acetylation, we compared chemically acetylated recombinant histories with histories prepared from HeLa cells, characterizing the sites of acetylation by mass spectroscopy. Nucleosomes from chemically acetylated histories have few modifications in the core domain and form nucleosomes normally. Acetylating all histories or selectively only H3 causes an opening of the nucleosome structure, indicated by the larger distances between the linker DNA ends. Selective acetylation of H4 distances the linker ends for short fragments but causes them to approach each other for fragments longer than 180 bp.