Micrococcal nuclease digestion of intact chicken erythrocyte nuclei is shown to result in the formation of core nucleoprotein particles containing about 140 base pairs of DNA. These core particles, which are almost entirely devoid of histones fI and f2c, are derived from transient nucleoprotein particles containing an average of approximately 180 base pairs of DNA. Oligomers of these latter particles may be isolated after brief nuclease digestion. (2). The latter has been proposed by Kornberg (9) to be the basic DNA repeating unit associated with an eight-histone complex. We will show that the eight-histone complex is associated with 140 base pairs of DNA, and that the remaining DNA in the subunit is especially nuclease-sensitive. MATERIALS AND METHODSIsolation and Digestion of Erythrocyte Nuclei. Blood was obtained from adult White Leghorn chickens by cardiac puncture in the presence of heparin. After centrifugation at 3000 X g for 10 min, the plasma and buffy coat were removed. The erythrocytes were washed twrice with an isotonic saline solution and frozen at -600 until needed. The frozen erythrocytes were thawed at 370 in an equal volume of 0.15 M NaCl, 0.015 M Na citrate, pH 7.2 (saline/citrate) and centrifuged at 3000 X g for 10 min; the nuclear pellet was resuspended in 0.25% Nonidet P-40 in saline/citrate. In some experiments 1 mM phenylmethane sulfonyl fluoride was added to inhibit protease action (15). The nuclei were repelleted, washed with saline/citrate and resuspended in 0.3 M sucrose, 0.75 mM CaCl2, 10 mM Tris-HCl, pH 7.2 at a concentration of 2 X 108 nuclei per ml. Digestion of the nuclei by micrococcal nuclease (Worthington) was carried out at 370 with 125 units of nuclease per ml of nuclei suspension. The digestion reaction was terminated by making the solution 10 mM in EDTA, 0.15 M NaCl, and 1% sodium dodecyl sulfate (NaDodSO4). After pronase treatment (0.5 mg/ml) for 4 hr at 370 the DNA was extracted by an NaDodSO4-phenol procedure (16).When the digested chromatin was to be fractionated on an agarose A-Sm column, the reaction was terminated by the addition of EDTA to 10 mM and cooling on ice. After centrifugation at 12,000 X g for 15 min, the nuclei were resuspended in 10 ml of 10 mM Tris-HCI, pH 7.5, 0.7 mM EDTA and disrupted by homogenization for about 1 min at a medium setting on a Virtis homogenizer. The nuclear debris was pelleted at 10,000 X g for 15 min. The supernatant was made 7% in sucrose and applied to a Bio-Rad A-Sm column, 90 X 2.5 cm, equilibrated with 10 mM 0

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