[Structure of chromosomal deoxyribonucleoproteins. IX. Heterogeneity of chromatin subunits in vitro and location of histone H1]

Abstract

Chromatin subunits ("nucleosomes") isolated from a mild staphylococcal nuclease digest of chromatin by a sucrose gradient centrifugation have been studied. We found that such preparation contains nucleosomes of the two discrete types which can be separated from each other by a low-ionic-strength polyacrylamide gel electrophoresis. Nucleosome of the first type contains all five histones and a DNA fragment 170--180 base pairs long, whereas nucleosome of the second type lacks histone H1 and its DNA fragment is approximately 140 base pairs long. Purified dimer of the nucleosome (dinucleosome) can be fractionated by gel electrophoresis into three discrete bands which correspond to dinucleosomes, containing two, one and no molecules of H1 histone. Similar heterogeneity with respect to the content of histone H1 probably exists in the case of larger oligonucleosomes. These and related findings strongly suggest that the H1 molecule is bound to a short (30--40 base pairs) terminal stretch of the nucleosomal DNA segment which can be removed by nuclease (possibly in the form of H1--DNA complex) without any significant disturbance of the main structural features of the nucleosome.