| Abstract: | Chicken erythrocyte and liver nuclei, isolated and fixed in isotonic saline, contained compact chromatin fibers about 200 Å in diameter. Fibers very similar in dimension and appearance are usually visible in thin sections of fixed nuclei in situ and probably represent chromatin organization close to the native state. After suspension of isolated nuclei in a mildly hypotonic buffer, chromatin fibers extended, became reduced in diameter and apparently unraveled in places. Under such conditions, new detail was revealed suggestive of both helical structure and of subunit organization. The fibers extended and contracted reversibly, changing in diameter from 200 Å to about 100 Å when alternately exposed to isotonic saline and to distilled water. The 200 Å fibers were irreversibly lost, however, following extraction of nuclei with high concentrations of NaCl which selectively removed H1 from liver and H1 plus H5 from erythrocyte chromatin. After extraction of more tightly complexed histones, the residual chromatin consisted mainly of fine filaments less than 30 Å thick. These results suggest a model for native chromatin fibers in which sub-unit organization and coiled configuration are combined, and in which histones H1 and H5 play an integral role in the maintenance of structure. |
