Interaction of histone H1 with oligonucleosomes in vitro provides a convenient method for isolating mononucleosomes.

The interaction of histone H1 with oligonucleosomes was studied in 0.15 M NaCl. Exogenous calf thymus H1 was added to a mixture of oligonucleosomes, soluble in 0.15 M NaCl, and the partially precipitated chromatin was removed by centrifugation. A strong preference of H1 for the longer nucleosomes in the mixture was observed causing their precipitation and leaving the shorter oligonucleosomes preferentially soluble. Histone H1 could even discriminate in its binding between mono- and di-nucleosomes, binding to dinucleosomes and leaving the mononucleosomes preferentially soluble. On this basis a convenient method was developed for the rapid isolation of mononucleosomes. The fidelity of H1 reconstitution to the oligonucleosomes under these conditions was suggested employing zero-length carbodiimide cross-linking and radioiodinated H1. This fraction of mononucleosomes displays significantly higher levels in the more hydrophilic H3.2 and H3.3, as well as H2A.2 and H2A.3 variant histone forms, compared with bulk chromatin. The, presumably, lower affinity of histone H1 for this subfraction of mononucleosomes is discussed.