Singlet-singlet energy transfer studies of the internal organization of nucleosomes

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Abstract
The measurement of 2 specific protein to DNA distances in several conformational states of core nucleosomes by singlet-singlet energy transfer is reported. A distance of 50-53 .ANG. separates each DNA terminus from cysteine-110 of chicken erythrocyte histone H3 in the native nucleosome. This cysteine residue must be located very near the center of the nucleosome. H3-DNA distance remained nearly constant in several unfolded forms of the core particles, as found in very low salt, in 0.6 M NaCl and in high urea. Each DNA end lies within 32 .ANG. of cysteine-73 of Arbacia lixula sperm histone H4 in the compact and low-salt unfolded forms of the nucleosome. Because of the invariance of the 2 measured distances in the various conformational states of the nucleosome, the cysteine-containing C-terminal segments of histones H3 and H4 maintain a very strong and close association with the terminal positions of the 146 base pair nucleosomal DNA. This binding may provide the primary interactions necessary for folding of DNA into nucleosomes and for protection of 146 base pair nucleosomes from further nuclease digestion.