Nucleosome neutron scattering

H2b, H3 and H4, in some cases HI is replaced by another protein H5) and a considerable number of non-histone proteins, all in very small amounts. A repetitive structural subunit, the nucleosome, was isolated, crystallized, and investigated by X-ray diffraction (Finch et al., 1977) and neutron scattering (Pardon et al., 1977) (for a review see Felsenfeld, 1978). 

The radius of gyration of the nucleosome is 4.0 to 4.5 nm as determined by neutron scattering 26.37,38) x-ray scattering The model proposed for the nucleosome structure based on the histone octamer crystal structure differs from that resulting from the crystal structure studies of the nucleosome core particle This controversywill be resolved when higher resolution structures become available. 

Core particles and nucleosomes are globular particles that have been usefully examined by several groups by neutron and X-ray scattering [382-401]. Stuhrmann plots of the core particle 200,000 45% DNA) show large positive slopes, where Rq is 3.9-4.1 nm and a is 36-51 X 10 (Table 15) [382,387,390,395]. Nucleosomes likewise exhibit a Rq oi 4.0 nm and an a of 45 X 10 [385,393]. Further analysis shows that the DNA component of core particles has an Rq of 4.9 nm, while the protein component has an Rq of 3.3 nm. This shows that the DNA has a more elongated shape in the core particle. In summary, it was shown that the DNA is external to a protein core. Other work shows that 1.8 turns of superhelical DNA are wound around the octamer of core histones. Several workers have examined individual neutron scattering curves in H20 buffers or X-ray curves in H2O buffers... 

Fig. 5. Schematic model of the nucleosome, with histone HI shown as stabilizing the fold of the DNA molecule around the core histones [based on results of Sperling and Sperling (1978)]. The nucleosome dimensions are derived from X-ray (Finch et al., 1977) and neutron (Baldwin et al., 1975 Pardon et al., 1977 Suauet al., 1977) scattering experiments. The histone core dimensions are derived from electron microscopic and X-ray studies (Sperling and Amos, 1977 Wachtel and Sperling, 1979 Sperling and Wachtel, 1979). The regions of the DNA molecule indicated by dashed lines indicate those base pairs which are not present in nucleosome core particles.

Furthermore, there is a striking parallelism between these data and the neutron diffraction data from nucleosomes in 100% D 0 (Pardon et al., 1977 Suau et al., 1977), where scattering from the histone protein dominates, and from core protein in 2 M NaCl solution (Pardon et al., 1978). The above interference phenomenon may well be the explanation for the protein-dominated scattering maximum between 35 and 37 A observed for chromatin and nucleosomes in solution (Pardon et al., 1977 Suau et al., 1977). 

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