Comparative study on the thermal denaturation of different nucleosome preparations

Effect of DNA length and H4 acetylation on the thermal stability of reconstituted nucleosome particles

To examine the factors involved with nucleosome stability, we reconstituted nonacetylated particles containing various lengths (192, 162, and 152 base pairs) of DNA onto the Lytechinus variegatus nucleosome positioning sequence in the absence of linker histone. We characterized the particles and examined their thermal stability. DNA of less than chromatosome length (168 base pairs) produces particles with altered denaturation profiles, possibly caused by histone rearrangement in those core-like particles. We also examined the effects of tetra-acetylation of histone H4 on the thermal stability of reconstituted nucleosome particles. Tetra-acetylation of H4 reduces the nucleosome thermal stability by 0.8 degrees C as compared with nonacetylated particles. This difference is close to values published comparing bulk nonacetylated nucleosomes and core particles to ones enriched for core histone acetylation, suggesting that H4 acetylation has a dominant effect on nucleosome particle energetics.

Studies on the thermal denaturation of histone-H1-depleted chromatin

The multiphasic thermal denaturation profile of histone-H1-depleted chromatin was studied by using a nucleoprotein preparation which lacked the first high temperature transition at about 72 degrees C. Such a preparation was obtained by heating at 72 degrees C H1-depleted chromatin, the DNA of which was cross-linked with psoralen to ensure a complete renaturation of DNA upon cooling. When this nucleoprotein was redenatured, its melting profile was found to be significantly altered: only one high temperature peak centered at about 82 degrees C was observed in addition to the low temperature transition at about 53 degrees C. The kinetics of digestion of this material with micrococcal nuclease showed a limit digest equal to that found for the 'native' H1-depleted chromatin but the rate of hydrolysis was higher. The monomer particles prepared from this nucleoprotein were found similar to the 'native' monosomes in respect to protein:DNA ratio and size of DNA but showed an altered melting profile: the premelt area was broader, bigger, and centered at lower temperature; the main peak was reduced in size with no change in its melting temperature. On the basis of these data, the following conclusions were drawn: (a) the last two thermal transitions in H1-depleted chromatin most likely reflect the presence within each nucleosome of two regions with different stability of DNA; (b) DNA involved in the first high thermal transition of H1-depleted chromatin belongs to nuclease-accessible DNA, and (c) the main peak in the biphasic melting profile of the monomer particles reflects the denaturation of DNA regions which in the polymer nucleoprotein are involved in the two high temperature transitions.