Nucleosomes are assembled into discrete size structures by histone H1 in vitro

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Abstract
The involvement of histone H1 in the formation and maintenance of higher order chromatin structures in vitro was investigated biochemically. Addition of exogenous histone H1 to isolated calf thymus mononucleosomes in low ionic strength buffer resulted in the formation of electrophoretically distinct mononucleosome assemblies (supernucleosomes). The smaller super-nucleosomes were composed of about 12, 18, 24, or 30 nucleosomes and one to two molecules of histone H1 per nucleosome. It was difficult to determine accurately the size of the larger supernucleosomes, but their bands from native gels contained probably between 60 and 300 nucleosomes or more. Similar supemucleosome size classes were also obtained when oligonucleosomes instead of mononucleosomes were employed. When the assembly of mono- and oligo-nucleosomes with histone H1 took place in 0.15 M NaCl, discrete supernucleosomes containing only mono- or di-nucleosomes, but not a mixture of both, were formed. It is proposed that the small supernucleosomes containing oligomers of 6 nucleosomes may represent integral multiples of the second-order chromatin structural subunit, whereas the larger supernucleosomes containing about 60 to 300 or more nucleosomes may correspond to chromatin domains or third-order chromatin structures observed by other techniques.