Preparation and physical characterization of a homogeneous population of monomeric nucleosomes from HeLa cells

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Abstract
We describe a method of isolating a homogeneous population of "trimmed" monomeric nucleosomes from Hela cells. These nucleoprotein particles contain a 140 +/- 5 base pair length of DNA and have a histone/DNA ratio of 1.2. They lack H1 and contain equal amounts of the four smaller histones. The DNA contains no single strand nicks. The particles sediment with an S20,w of 11S in D2O density gradients. After formaldehyde fixation, they band at a density of 1.4370 in neutral CsCl. Digestion of nucleosomes with either micrococcal nuclease or DNase I generates the same pattern of DNA fragments observed when intact nuclei are digested. Circular dichroism spectra indicate that the 280 nm positive ellipticity maximum of nucleosomes is about one-half that of chromatin. In the presence of 6 M urea, nucleosomes sediment with an S20,w of 6S, have a multiphasic thermal denaturation profile, and exhibit a circular dichroic spectrum nearly identical to that of B-form DNA. Our yield of purified nucleosomes (10-15% of the input DNA) is similar to the yields of other methods; our nucleosome population is substantially more homogeneous than those previously reported.