Salt-dependent structural changes of chromatin in isolated chicken liver nuclei as visualized by scanning electron microscopy.

MgCl2- and KCl-dependent structural changes of chromatin in isolated chicken liver nuclei were examined by scanning electron microscopy. In the absence of salts, the chromatin appeared as granular or knobby fibrillar structures, 15-30 nm in diameter. At 0.2 mM MgCl2, the structures condensed into 30-60 nm fibers. At 2 mM MgCl2, most chromatin was present as condensed chromatin masses. Above 50 mM MgCl2, the condensed chromatin masses were loosened. At KCl concentrations higher than 25 mM, the chromatin appeared as discrete 30-60 nm fibers. However, no condensed chromatin masses were formed at any of the KCl concentrations examined in this study. These results indicate that the MgCl2- or KCl-dependent transitions from a lower to a higher level of chromatin organization in the isolated nuclei are comparable to those reported in isolated chromatin fragments, and that KCl is ineffective in the formation of condensed chromatin.