Presence of Non‐histone Proteins in Nucleosomes

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Abstract
Nucleosomes are made of histones and DNA fragments. This work sought to establish whether some non-histone proteins are also present in these chromatin subunits. Sucrose gradient analysis showed that nucleosome preparations [from rat livers] contain phosphorylated non-histone proteins and protein kinases. In order to establish whether these proteins are actually bound to nucleosomes or if they represent unbound or aggregated proteins, free non-histone proteins and proteins released from chromatin by [micrococcal] DNase overdigestion were analyzed by sucrose gradient centrifugation. No phosphoproteins but some phosvitin kinase activity was found in the part of the gradient which contained the nucleosomes, suggesting that part of the phosphoproteins are bound to nucleosomes. A digestion of nucleosomes with DNase I suppressed the phosvitin kinase activity in the 11-S region of the gradient. High ionic strength which extracted non-histone proteins, suppressed the phosvitin kinase activity in the nucleosome region. Part of the phosvitin kinase and nuclear phosphoproteins are therefore bound to nucleosomes and are released by nuclease digestion and by high ionic strength.