Chicken erythrocyte nucleus contains two classes of chromatin that differ in micrococcal nuclease susceptibility and solubility at physiological ionic strength.

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Abstract
Inactive chromatin of the chicken erythrocyte nucleus consists of 2 distinct classes (I and S). I chromatin (.apprxeq. 60% of the total genome) is insoluble at > 0.1 M ionic strength whereas S chromatin (.apprxeq. 40% of the total genome) is soluble at all ionic strengths studied (0.01-0.3 M). These chromatins are released from nuclei upon digestion with micrococcal nuclease by 2 separate parallel processes that do not have a precursor-product relationship to each other. Isolated I-chromatin fragments show a progressive reduction in size from 250 to .apprxeq. 50 nucleosome equivalents with increasing digestion times at 0-2.degree. C. Prolonged digestion of nuclei at 37.degree. C results in conversion of I chromatin to mononucleosomes that are insoluble at > 30 mM NaCl. Isolated S-chromatin fragments show a constant size distribution, independent of digestion time, that peaks at .apprxeq. 35 nucleosome equivalents. Prolonged digestion of nuclei at 37.degree. C results in the conversion of S chromatin to mononucleosomes that are soluble at physiological ionic strength. Both I and S chromatins contain a full complement of histones with no nonhistone proteins.