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J. Biol. Chem., Vol. 261, Issue 19, 8784-8792, 07, 1986
BS Imai, P Yau, JP Baldwin, K Ibel, RP May and EM Bradbury
Recent studies report that the frictional resistance of partially
acetylated core particles increases when the number of acetyl
groups/particle exceeds 10 (Bode, J., Gomez-Lira, M. M. & Schroter, H.
(1983) Eur. J. Biochem. 130, 437-445). This was attributed to an opening of
the core particle though other explanations, e.g. unwinding of the DNA ends
were also suggested. Another possible explanation is that release of the
core histone N-terminal domains by acetylation increased the frictional
resistance of the particle. Neutron scatter studies have been performed on
core particles acetylated to different levels up to 2.4 acetates/H4
molecule. Up to this level of acetylation the neutron scatter data show no
evidence for unfolding of the core particle. The fundamental scatter
functions for the envelope shape and internal structure are identical to
those obtained previously for bulk core particles. The structure that gave
the best fit to these fundamental scatter functions was a flat disc of
diameter 11-11.5 nm and of thickness 5.5-6 nm with 1.7 +/- 0.2 turns of DNA
coiled with a pitch of 3.0 nm around a core of the histone octamer. The
data analysis emphasizes the changes in pair distance distribution
functions at relatively low contrasts, particularly when the protein is
contrast matched and DNA dominates the scatter. Under these conditions
there is no evidence for the unwinding of long DNA ends in the
hyperacetylated core particles. The distance distribution functions go to
zero between 11.5 and 12 nm which gives the maximum chord length in a
particle of dimension, 11 nm X 5.5 nm. The distance distribution function
for the histone octamer contains 85% of the vectors within the 7.0-nm
diameter of the histone core. 15% of the histone vectors lie between 7.0
and 12.0 nm, and these are attributed to the N-terminal domains of the core
histones which extend out from the central histone core. Histone vectors
extending beyond 7.0 nm are necessary to account for the measured radius of
gyration of the histone core of 3.3 nm. A similar value of 3.2 nm is
calculated for the recent ellipsoidal shape of 11.0 X 6.5 X 6.5 nm from the
crystal structure of the octamer. However, the nucleosome model based on
this structure is globular, roughly 11 nm in diameter, which does not
accord with the flat disc shape core particle obtained from detailed
neutron scatter data nor with the cross-section radii of gyration of the
histone and DNA found previously for extended chromatin in solution.
Hyperacetylation of core histones does not cause unfolding of nucleosomes. Neutron scatter data accords with disc shape of the nucleosome
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