Molecular Biology

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Science  16 Apr 2010:
Vol. 328, Issue 5976, pp. 286-287
DOI: 10.1126/science.328.5976.286-c

To squeeze eukaryotic genomes into the cramped confines of the cell nucleus, DNA is packaged into nucleosomes, which are composed of octamers of histone proteins: two dimers of histones H3 and H4 and two dimers of H2A and H2B. Accessing the information stored in the genome requires that the nucleosomes be removed or shuffled out of the way and then later replaced. Assembly occurs via the interaction of dimers of H3-H4 with DNA to form a (H3-H4)2–DNA complex (the tetrasome), and then by the addition of two H2A-H2B dimers. Andrews et al. have studied the mechanism by which nucleosome assembly protein 1 (Nap1) acts to promote assembly and find that Nap1 does not affect tetrasome formation but instead binds to the H2A-H2B dimer and reduces its affinity for DNA. This is critical because histones are basic proteins and have a strong propensity for nonproductively interacting with DNA. Deletion of Nap1 in yeast results in increased levels of H2A and H2B in chromatin, without a corresponding increase in H3, supporting the idea that Nap1 protects against H2A-H2B dimers binding to DNA disobediently. The derangement of normal chromatin structure in the absence of Nap1 results in the misregulation of transcription, indicating that Nap1 chaperone activity is critical for the correct readout of information.

Mol. Cell 37, 834 (2010).

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