The organization of rat liver nuclei in vitro depends on the ionic milieu. Turbidity measurements of nuclear suspensions in the presence of varying concentrations of divalent cations have been correlated with nuclear ultrastructure. The concentration of MgCl2 (2 mM) at which turbidity of nuclear suspensions is maximal and chromatin condensation appears most extensive is the same concentration that reportedly (Gottesfeld et al., 1974, Proc. Natl. Acad. Sci. U. S. A. 71:2193-2197) precipitates "inactive" chromatin. Thus, a mechanism is suggested by which chromatin activity and ultrastructural organization within the nucleus may be mediated. The nuclear organizational changes attendant upon the decrease in divalent cation concentration were not entirely reversible.

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