Nucleosome stability

Of important note, native nucleosomes are intrinsically heterogeneous both in terms of their DNA sequence and histone compositions and both of which can contribute to the stability of the complex. 

Enthalpic and entropic components of nucleosome core particle (NCP) free energies [277] 

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Physical Properties of DNA Affect the Nucleosome Stability and Dynamics... 

All this leads to the question of how DNA affects nucleosomal stability at the molecular level. DNA bendability has been repeatedly put forward as a candidate to play an important role [280-282]. This property is related to the persistence length of DNA [283]. A study on the characterization of nucleosomes reconstituted onto methylated DNA [poly(dG-m dC) poly (dG-m dC)] [284] provides support to this hypothesis. Poly(dG-m dC) poly (dG-m dQ DNA can be induced to change from its B to its Z conformation in the presence of millimolar amounts of divalent ions [285] such as MgCl2. The persistence length of poly(dG-m dC) poly (dG-m dC) in the Z form in high salt was found to be 208 nm (ca. 612 bp) [286], a value much lower than that of the same polymer in the B form (93.8 nm, 276 bp) at... 

From all the above it would be expected that only histone variability (variants or post-translational modification) affecting the histone fold or C-terminal domains would have a major effect on nucleosome stability in agreement with the experimental results that will be described in the following sections. 

A good example of the effects of histone variability on nucleosome stability is provided by the yeast histones, which are very divergent from their vertebrate counterparts [312]. Physical [313], biochemical [314], and crystallographic [315] analyses have produced evidence for a lower nucleosome stability resulting from the association of these histones with DNA. The lower stability conferred by yeast histones to the nucleosome when compared to vertebrate histones is in line with the more dynamic metabolic demand of this organism as most of its genome exists as euchromatin [316]. 

In the sections that follow, we are going to describe a few representative examples of how histone variability (histone variants and their post-translational modifications) can affect nucleosome stability and folding. This data supports the notion that while some of this variability may be exclusively used to provide an informational code [121,123,165] it can also have important implications for structural aspects involved in the highly dynamic nature of the chromatin fiber. 

Widlund, H.R., Vitolo, J.M., Thiriet, C., and Hayes, J.J. (2000) DNA sequence-dependent contributions of core histone tails to nucleosome stability Differential effects of acetylation and proteolytic tail removal. Biochemistry 39, 3835-3841. 

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