Free DNA stretches in histone H1-depleted chromatin and their possible relation to chromomere structure.

Oligomers of chromatin subunits (oligonucleosomes) were prepared by a mild digestion of chromatin with staphylococcal nuclease followed by a purification of a high molecular weight material (hexanucleosomes and larger DNP particles) by gel chromatography. The main finding is that a mild removal of histone H1 from the oligonucleosome preparation by treatment with tRNA in the absence of any significant hydrodynamic shearing leads to the formation of free DNA molecules which constitute 5-6% of the total oligonucleosomal DNA. The size of nucleosome-free DNA stretches in H1-depleted hydrodynamically sheared chromatin is about 6000 base pairs and their content is apparently 10-12% of the total DNA. These and related findings are discussed in terms of the previously proposed "asymmetric hairpin" model of DNA packing in chromatin [1-4]. Different kinds of the asymmetric hairpin are considered and ambiguities in interpretations of experimental data are pointed out.