Isolation of high mobility group-containing mononucleosomes from avian erythrocyte nuclei and their sensitivity to DNase I.

Conditions have been established which have led to the isolation of mononucleosomes which contain the high mobility group (HMG) proteins, in particular HMG 14, from mature chicken erythrocyte nuclei after extended micrococcal nuclease digestion. This selective enhancement of HMG-containing mononucleosomes appears to be due to their preferential solubilization at a time when other mononucleosomes, i.e. those containing H5 and H1 which represent the bulk of the mononucleosomes, were no longer soluble. Isolation of "early" mononucleosomes and subsequent analysis of these mononucleosomes after DNase I digestion showed that the DNA of these "early" mononucleosomes was more accessible to DNase I and that those mononucleosomes which contained HMG 14 were more soluble when the DNA became extensively degraded by DNase I. The resulting pattern of single-stranded DNA fragments suggests that the NH2 termini of the core histones no longer bind strongly to the nucleosomal DNA of the "early" mononucleosomes, and thus enhance the rate of DNase I digestion, while the presence of HMG 14 increased the solubility of these mononucleosomes. These two properties are probably the basis for the increased DNase I sensitivity of the transcriptionally active chromatin.