Non-histone Proteins in Mononucleosomes and Subnucleosomes

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Abstract
Nucleosomes and subnucleosomes [from mouse Ehrlich ascites tumor cells] separated by sucrose gradient ultracentrifugation or by polyacrylamide gel electrophoresis contain proteins incorporating [3H]tryptophan, i.e., non-histone proteins. The fractions of mononucleosomes MN3 and MN2 are enriched in these proteins as compared to the MN1 fraction. Two-dimensional gel electrophoresis of chromatin digests reveals a number of non-histone proteins comigrating with deoxyribonucleoprotein particles in the first direction (in non-dissociating conditions). A significant fraction of these proteins corresponds to basic non-histone proteins, so-called HMG (high-mobility-group) proteins. Two HMG proteins are present in mononucleosomes MN3 exclusively and 3 others in mononucleosomes MN3 and MN2. One is recovered also in subnucleosomes SN2 and another in SN3 subnucleosome fraction. At least 3 HMG proteins are rapidly released from the oligonucleosome fractions and from the insoluble DNA .cntdot. protein residue. They are located in chromatin readily available to DNAase action. Apart from HMG proteins a number of other non-histone proteins are present in mononucleosomes; their relative content in the oligonucleosome fraction is much higher. Many non-histone proteins, in particular HMG proteins, apparently interact with linker DNA in chromatin.