[Effect of polyanions on the structure of rat liver nuclear chromatin]

Stepwise addition of polyanions (PA)-heparin and dextran sulfate-to interphase rat liver nuclei results in increased chromatin condensation (PA/DNA = 0.125) followed by its decondensation down to 10 nm fibrils (PA/DNA = 0.2) and, eventually, (PA/DNA > 0.3) to a new condensation of the material up to 30-50 nm fibrils and 0.15-0.5 M compact globular particles (GP). After subsequent addition of ammonium acetate (0.15-0.5 M) the whole nuclear material converts into an even network of GP connected by fibrils containing DNA. The GP diameter (40-70 nm) is unaffected in the PA/DNA range of 0.4 to 1.5. The GP-containing nuclei retain up to 85-90% of DNA and 50-70% of the protein. Treatment of the nuclei with staphylococcal nuclease reveals the absence of nucleosomal periodicity in the DNA structure. The nuclei treated with PA and the salt in the presence of EDTA retain all the histones of the nucleosomal core. When treated in the presence of Mg2+, the nuclei retain also histone H1. The GP obtained at pH 6.0-6.5 are the most compact ones. At pH 5.0 and 8.5 GP partly form tangles of approximately 10 nm granules linked by thin fibrils. GP can be reversibly unfolded into the same fibro-granular network when dyalized against low ionic strength solutions in the presence of EDTA. A more than 1.5-fold increase in the PA/DNA ratio or treatment of GP-containing nuclei with nucleases terminate in the failure of the fibrillar network between GP, giving rise to large globules (> 300 nm), presumably at the expense of GP fusion. In this case the nuclei lose DNA but retain no less than 70% of histones. The diameter of large globules depends on the concentration of PA. GP are suggested to arise by the aggregation of PA complexes with histone cores of nucleosomes and with other alkaline proteins of chromatin at the regularly spaced sites of partly deproteinated chromatin.