The structure of DNA in a nucleosome.

We describe the application of the hydroxyl radical footprinting technique to examine the histone-DNA interactions of a nucleosome that includes part of the 5S ribosomal RNA gene of Xenopus borealis. We establish that two distinct regions of DNA with different helical periodicities exist within the nucleosome and demonstrate a change in the helical periodicity of this DNA upon nucleosome formation. In particular, we find that on average the helical periodicity of DNA in this nucleosome is 10.18 +/- 0.05 base pairs per turn. The same DNA, when bound to a calcium phosphate surface, has a periodicity of 10.49 +/- 0.05 base pairs per turn, similar to that of random sequence DNA. Modulations in minor groove width within the naked DNA detected by the hydroxyl radical are maintained and exaggerated in nucleosomal DNA. These features correlate with regions in the DNA previously suggested to be important for nucleosome positioning.