Abstract
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. & Schröter, 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.
Highlights
Recent studies report that the frictional resisotfancwe ith the flat disc shape core particle obtained from partially acetylated core particles increases when thedetailed neutron scatter data nor witchrotshse-section number of acetyl groups/particleexceeds 10
Within the resolution of the neutron thatgavethebest fit tothesefundamentalscatter scatter data this solution structure is similar to the recent functions was a flat disc of diameter 11-11.5 nm and 0.7- ( 5 ) and 1.5-nm [6] x-ray crystal structures; in of thickness 5.5-6 nm with 1.7 f 0.2 turns ofDNA the 0.7-nm structure the calculatedR, for the histone core, if coiled with a pitch of 3.0 nm around a core of the it contains all of the histone polypeptide chains, is 2.55 nm histoneoctamer
Neutron changes in pair distance distribution functions at rel-diffractionstudies of core particlecrystals give asimilar atively low contrasts, when the proteiisn overall structure to the x-ray structure [7] except that they contrast matched andDNA dominates the scatter.Un- report regions of proteindensityextendingout from the der these conditions there is no evidence for the un- central histonecore into the DNAgyres
Summary
“half-nucleosomenonpairing” or the unwinding of the DNA ends, were suggested Another explanation is that the release of the core histone H3 and H4 N-terminal domains by acetylation would probably lead to an increase in the frictional resistance of the particle. P(R) becomes zero a t a value R., which corresponds to the largest vector that can be drawn in the particle, and it reaches a maximum a t a value R, the most probable length of the vectors which can be drawn in a particle.The monodispersity of the core particilne solution acetylation This increasein anisotropycan have several origins: (i) based on theeffect of acetylation on the interaction of the histone H4 peptide-(l-23) with DNA [21] acetylation would be expected to release the N-terminal domains of H3 can be demonstrated most by this procedure.
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