Histone acetylation reduces nucleosome core particle linking number change

Nucleosome core particles differing in their levels of histone acetylation have been formed on a closed circular DNA that contains a tandemly repeated 207 bp nucleosome positioning sequence. The effect of acetylation on the linking number per nucleosome particle has been determined. With increasing levels of acetylation, the negative linking number change per nucleosome decreases from -1.04 +/- 0.08 for control to -0.82 +/- 0.05 for highly acetylated nucleosomes. These results indicate that histone acetylation has the ability to release negative supercoils previously constrained by nucleosomes into a closed chromatin loop and in effect function as a eukaryotic gyrase.     

Opposing roles of H3- and H4-acetylation in the regulation of nucleosome structure—a FRET study

Using FRET in bulk and on single molecules, we assessed the structural role of histone acetylation in nucleosomes reconstituted on the 170 bp long Widom 601 sequence. We followed salt-induced nucleosome disassembly, using donor–acceptor pairs on the ends or in the internal part of the nucleosomal DNA, and on H2B histone for measuring H2A/H2B dimer exchange. This allowed us to distinguish the influence of acetylation on salt-induced DNA unwrapping at the entry–exit site from its effect on nucleosome core dissociation. The effect of lysine acetylation is not simply cumulative, but showed distinct histone-specificity. Both H3- and H4-acetylation enhance DNA unwrapping above physiological ionic strength; however, while H3-acetylation renders the nucleosome core more sensitive to salt-induced dissociation and to dimer exchange, H4-acetylation counteracts these effects. Thus, our data suggest, that H3- and H4-acetylation have partially opposing roles in regulating nucleosome architecture and that distinct aspects of nucleosome dynamics might be independently controlled by individual histones.     

Chemical acetylation, trypsinolysis and stability of nucleosomes

Gel electrophoretic analysis of the histone chemical acetylation in the nucleosome core particles with acetic andydride revealed availability of about 14 lysine residues of histone H2A, 15-21 of H2B, 8-11--H3 and 6-9--H4. Moderately lysine-rich histones H2A and H2B were found to be more susceptible to acetylation than arginine-rich H3 and H4. Chemical acetylation enhanced the rate of trypsin digestion in acetylated nucleosomes as evidenced by gel electrophoresis of histone fragments. A more pronounced trypsin digestion was evident at acetylation of only 3-5 histone amino groups per nucleosome. However, even heavily acetylated nucleosomes yielded in familiar trypsin limit digest pattern of histone fragments thus indicating persistence of histone octamer. Nucleosomes which were trace acetylated (up to 3-5 histone amino groups neutralized per nucleosome) and treated with trypsin to remove highly charged terminal histone regions revealed remarkable unfolding and partial dissociation when analyzed by gel electrophoresis. The same trace acetylated nucleosomes did not show such destabilization prior to trypsin digestion.