Neutron and x-ray scatter studies of the histone octamer and amino and carboxyl domain trimmed octamers

The structure of the nucleosome has been under intense investigation using neutron crystallography, x-ray crystallography, and neutron solution scattering. However the dimension of the histone octamer inside the nucleosome is still a subject of controversy. The radius of gyration 〈R_g〉 of the octamer obtained from solution neutron scattering of core particles at 63% ²H₂O, 37% ¹H₂O is 33 Å, and x-ray crystallography study of isolated histone octamer gives a R_g of 32.5 Å, while the reported values using x-ray crystallography of core particles from two individual studies are 29.7 and 30.4 Å, respectively. We report here studies of isolated histone octamer and trypsin-limited digested octamer using both neutron solution scattering and small angle x-ray scattering. The R_g of the octamer obtained is 33 Å, whereas that of the trimmed octamer is 29.8 Å, similar to the structure obtained from the crystals of the core particles. The N-terminal domains of the core histones in the octamer have been shown by high resolution nuclear magnetic resonance (Schroth, G. P., Yau, P., Imai, B. S., Gatewood, J. M., and Bradbury, E. M. (1990) FEBS Lett. 268, 117-120) to be mobile and flexible; it is likely that these regions are disordered and "not seen" by x-ray crystallography.