Chromatin fiber histone variability

Experimental results regarding the role of the histone tails indicate that these histone domains play a critical role in chromatin folding [358,365]. Removal as well as the modification (acetylation) of the lysine amino acids within these regions produces an imbalance of the electrostatic interactions, which results in a hierarchically impaired folding ability (H3/H4-H2A/H2B>H3/H4>H2A/H2B) of the chromatin fiber [358,366-369]. Therefore, sources of histone tail variability (histone variants and post-translational modifications other than lysine acetylation) are also likely to alter the extent of folding of chromatin. 

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. 

Nucleosomes are connected to one another by linker DNA of variable length and the linker-binding histone HI protein (Fig. 1) (7). These long arrays of nucleosomes spontaneously condense to form helical arrays of nucleosomes, termed the 30-nm fiber after its apparent diameter (Fig. 1) (8). Additional condensation and compaction of chromatin occur through intemucleosomal interactions. One important internucleosomal interaction required for chromatin fiber formation is the interaction of a highly acidic patch of histone H2A with the histone H4 tail (8). Ultimately, these internucleosomal interactions form interphase chromatin with an unknown architecture (Fig. 1) (9). 

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