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Subtle Changes Produce Big Differences in the Nucleosome

Structure of the hybrid macro-nucleosome core particle.

Histones are responsible for packing eukaryotic DNA into chromatin. The structure and function of chromatin, and ultimately the accessibility of nucleosomal DNA, can be regulated by the incorporation of non-allelic variants of histones. Most of these functional changes are caused by subtle differences in very specific regions within the structured parts of the histones, resulting in altered protein-protein or protein-DNA interactions.

In this Paper of the Week, Srinivas Chakravarthy and Karolin Luger present a detailed analysis of the incorporation of histone variant macro-H2A into nucleosomes. Macro-H2A contains a histone domain that is 64% identical to that of major H2A. Chakravarthy and Luger demonstrate that macro-H2A forms salt-stable octamers and that the basis of this stability lies in the four amino acid residues that comprise the L1 loop in the H2As. They make the surprising discovery that when mixed, major and macro-H2A-H2B dimers prefer to combine into "hybrid" nucleosomes. Finally, Chakravarthy and Luger solve the x-ray crystal structure of the hybrid nucleosome core particles and pinpoint the interactions that are likely responsible for the increased stability of the hybrid. These results demonstrate how relatively minor sequence and structural deviations can result in altered nucleosome dynamics.

FOOTNOTES

See referenced article, J. Biol. Chem. 2006, 281, 25522-25531

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