Subunit structures of different electrophoretic forms of nucleosomes

We have reported previously that five different electrophoretic forms of mononucleosomes (MI to MV) are produced upon treatment of mammalian chromatin with micrococcal nuclease. We show here that each of these mononucleosome classes possesses internal heterogeneity due to the presence of a variety of minor protein species. Defined subsets of mononucleosome classes MII to MV have been reconstituted by reassociating stripped nucleosomes with histone H1 and non-histone protein HMG-17. This procedure leads to the generation of the same five major electrophoretic forms of mononucleosomes found in native chromatin. From the results of one- and two-dimensional electrophoretic analyses on reconstituted samples, it is concluded that different mononucleosome classes possess the following subunit structures: MI, core histone octamer (8-mer); MII, 8-mer plus one copy of HMG-17; MIIIA, 8-mer plus one copy of histone H1; MIIIB, 8-mer plus two copies of HMG-17; MIV, 8-mer plus one copy each of histone H1 and HMG-17; and MV, 8-mer plus one copy of histone H1 and two copies of HMG-17. Equal numbers of HMG-14 molecules can substitute for HMG-17 and generate the same nucleosome components. Thus, mononucleosomes possess independent binding sites for at least 1 histone H1 molecule and 2 nonhistone chromosomal protein molecules. We show further that reassociated HMG-17 molecules can exhibit a rapid interchange between binding sites, even under conditions of low ionic strength.