Nucleosome Motion: Evidence and Models
Much of the voluminous literature concerning nucleosome arrangement in chromatin has implicitly assumed this to be a static structure, with the histone cores fixed in place upon the DNA. There exists, however, a growing body of evidence to indicate that under certain circumstances the cores can migrate along the DNA, both in vitro and in vivo. The first suggestion of such migration came from observations1–4 of anomalously close spacing of some histone cores in chromatin that had been depleted of the very lysine-rich histones. The presence of these “compact oligomers” was observed in the specific cases in which the very lysine-rich histones had been removed by extraction of the chromatin with salt solutions in the concentration range 0.5–0.65 M. This suggested that such structures might be generated under such conditions. This postulate was elegantly confirmed by Weischet5, who showed that nuclei depleted of H1 by a low pH, low salt extraction did not yield compact oligonucleosomes upon nuclease digestion, but that such structures could be found after the nuclei were incubated in salt concentrations greater than 0.3 M. That sliding, rather than nucleosome displacement and reassociation must be involved was indicated by the earlier studies of Germond et al.6, who had shown that histone cores could only be displaced from chromatin by salt concentrations in excess of 0.8 M.
KeywordsSalt Concentration Sucrose Gradient Core Particle Histone Core Sedimentation Coefficient
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