[Independence of nucleolysis of structural elements of chromatin of histone H1].

The type of nucleolysis and the size of DNA monomeric particles were studied at different steps of rat liver chromatin digestion by endogenous Ca2+, Mg2+-DNAase. At an early stage of nucleolysis (5-7% of monomers, 1-2% of acid-soluble material) the monomeric particles are represented by three electrophoretic classes containing all histones and differing in DNA size; the latter varies from 170 to 215 base pairs. The monomers with histone H1 and without it isolated at the stage of exhaustive fragmentation (85-90% of monomers, 12-15% of acid-soluble material) are represented by two electrophoretic classes. The mobility of these particles does not depend on the presence of histone H1 in them. A comparison of DNA sizes in the particles with and without histone H1 demonstrated that these sizes are identical and vary from 150 to 175 base pairs. The formation of histone H1-free monomers is not accompanied by the increase in exonucleolytic degradation of chromatin DNA. A comparison of electrophoretic patterns of monomeric particle DNA at different steps of nucleolysis showed that degradation of DNA in the structural chromatin unit occurs uninterruptedly from 215 to 150 nucleotides and in the region of 175-150 nucleotides is independent of histone H1. The data obtained suggest that histone H1 does not protect the DNA against nuclelysis at nucleotides 150-175 of the chromatin structural unit, which is in agreement with the existing concepts on protection of this nucleosomal region by histone octamer.