A class of chromatin particles associated with nonhistone proteins

Unfixed nucleoproteins may be banded isopycnically in metrizamide (2(3-acetamido-5-N-methylacetamido-2,4,6-triiodobenzamido)-2-deoxy-D-glucose) according to the protein/nucleic acid ratio. Unsheared or lightly sheared chromatin bands sharply (p=1.2 g/ml); it has a protein/DNA ratio of 1.4. Chromatin sheared by sonication to approximately 350 base pairs of DNA contains two components with protein/nucleic acid ratios of approximately 1.3 (p=1,185 g/ml) and 2 (p=1.245 g/ml). When chromatin is digested exhaustively with staphylococcal nuclease, two density components are found, one with a protein/DNA ratio of 1.5 (p=1.21 g/ml), the other with a protein/DNA ratio of 2 (p=1.24 g/ml). In both instances the denser particle (p=1.24 g/ml) contains nearly all the nonhistone proteins, while both dense and light fractions contain histones in similar amounts. The base sequence complexity of DNA from the fractions is not distinguishable from that of total DNA and there is no evidence of any concentration of sequences complementary to polysomal polyadenylated RNA molecules.