Aggregation of Mono- and Dinucleosomes into Chromatin-like Fibers

Three classes of chicken erythrocyte chromatin particles differing in their content of lysine-rich histones and/or spacer DNA have been studied in order to determine their ability to aggregate into complexes resembling those observed in native chromatin. The complexes have been obtained in the presence of MgCl2 and NaCl and studied by electron microscopy. Mononucleosomes, containing spacer DNA and histones H1 and H5, give rise to thick (about 70 nm) ellipsoidal particles in the presence of 0.5 mM MgCl2. These particles are disrupted by the addition of small amounts of NaCl (5-20 mM). On the other hand in 0.5 mM MgCl2 dinucleosomes give rise to regular fibrous complexes of about 40 nm in diameter which are very similar to native chromatin fibers. These complexes are much more stable when NaCl is added. We conclude that for the stability of nucleosomal aggregates, similar to native chromatin fibers, a continuity of DNA structure is not required, but the presence of divalent cations, spacer DNA and lysine-rich histones is essential.