The Crystal Structure of the Nucleosome Core Particle by Contrast Variation
The nucleosome core particle may be regarded as a basic structural unit of chromatin and thus of the chromosome. It is obtained by micrococcal nuclease digestion of chromatin and consists of about equal weights of DNA (146 base pairs) and protein (an octamer of two pairs each of the histones H2A, H2B, H3, and H4). Nucleosome structure has been reviewed by McGhee and Felsenfeld (6). Studies by small-angle neutron scattering using D2O/H2O contrast variation have given the overall dimensions of the nucleosome and have shown that the protein forms an inner core with the DNA located at the outer regions of the molecule (7,12). The x-ray crystal structure of the nucleosome core particle has been solved in projection to a resolution of about 25 Å (3). Although this has given a more detailed description of the overall shape of the molecule than has solution scattering, the distinction between protein and DNA is not made. In this paper we show how the principle of contrast variation can be used in low resolution neutron diffraction to achieve this end.
KeywordsStructure Factor Histone Core Contrast Variation Basic Structural Unit Scatter Length Density
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