Characterization of Chromatin Subfractions Enriched in RNA from Lactating Mammary Gland of the Rat

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Mammary gland chromatin from lactating rats was fractionated according to the Mg²⁺-solubility method after mild digestion with DNase II. The chromatin properties were compared between the Mg²⁺-soluble (S₂) and the Mg²⁺-insoluble (P₂) fractions. The weight ratio of RNA to DNA was much greater in the S₂ fraction than in the P₂ fraction (S₂, 0.92; P₂, 0.03). The DNA repeat length of the nucleosome was very similar between the two fractions (S₂, 193±7; P₂, 196±8 nucleotide pairs). A significant difference was seen in electrophoretic pattern of H1 histone between the two fractions in the acid-urea gel system. Nonhistone proteins with molecular weights higher than about 40, 000 dalton were found to be enriched in the S₂ fraction.