31P-NMR studies of DNA in nucleosome core particles.

Abstract

Abstract31P Nmr parameters (δ, T1, W1/2, and NOE) were measured for the DNA in nucleosome core particles at three frequencies and compared with similar data for the histone‐free DNA. An essentially linear relationship was found between the frequency of observation and line‐width for the single broad envelope of 31P resonances of the DNA in the nucleosome cores. We attributed this largely to chemical shift dispersion, with smaller contributions from chemical shift anisotropy and dipolar broadening. These results suggest the presence of different environments for phosphorus atoms in the core particles. However, within the accuracy of the method, no asymmetry in the resonance could be detected, which would tend to rule out any significant degree of DNA “kinking.” To investigate the interactions of the DNA and histones within the core particles we also studied transitions induced by urea and by temperature. Urea caused two stepwise increases in linewidth, which we attributed to conformational changes. A biphasic transition was also observed in the temperature profile, consistent with previous optical studies [Weischet et. al. (1978) Nucleic Acids Res. 5, 139]. Various models with different types of local mobility were examined by the relaxation theory. A model of isotropic motion having a broad distribution of correlation times gave a fairly good fit to the 31P‐nmr data.