The effects of salt concentration and H-1 depletion on the digestion of calf thymus chromatin by micrococcal nuclease

Abstract

We have removed histone H1 specifically from calf thymus nuclei by low pH treatment, and studied the digestion of such nuclei in comparison with undepleted nuclei. By a number of criteria the nuclei do not appear damaged. The DNA repeatlength in nuclear chromatin is found to be the same (192 † 4 bp) in the presence or absence of H1. These experiments demonstrate that the core histone complex of H2A, H2B, H3, and H4 can it self protect DNA sequences as long as 168 bp from nuclease. Our interpretation is that this represents an important structural element in chromatin, carrying two full turns of superhelical DNA. Depending on conditions of digestion this 168 bp fragment may be metastable and is normally rapidly converted by exonucleolytic trimming to the well-known " corep article" containing 145 bp. Larger stable DNA fragments observed in digestion of H-1 depleted nuclei appear to arise from oligomers assembled from 168 bp cores in close contact exhibiting trimming of 0-20 bp at the ends. Electrophorograms of undepleted nuclear digests reveal oligomer bands in several size classes, each corresponding to one or more combinations of 168 bp particles, H1-protected spacers of about 20 bp length, and particle s with ends trimmed to varying degrees. © 1979 Information Retrieval Limited.