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Structure and Activity of the Histone H3/H4 Chaperone Asf1

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Together with non-histone proteins nucleosomes assemble the eukaryotic genome into higher order structures known as chromatin. Chromatin structure is dynamic, as it is continually assembled and disassembled for factors that carry out the processes of transcription, replication, DNA repair, and recombination to gain access to the DNA . The deposition of the histones H3/H4 onto DNA to give the tetrasome intermediate and the displacement of H3/H4 from DNA are thought to be first and last steps in nucleosome assembly and disassembly, respectively.

Anti-silencing function 1 (Asf1) is a chaperone of the H3/H4 dimer that functions in both of these processes. We have examined the molecular basis of the activity of Asf1 in nucleosome assembly and disassembly using a variety of methods, including X-ray crystallography, biophysical and biochemical methods. The structure of the globular domain of Asf1 bound to histones H3/H4 shows how Asf1 binds to the H3/H4 heterodimer, enveloping the C-terminus of histone H3, and physically blocks the formation of the H3/H4 heterotetramer.

Biophysical studies have revealed the ability of Asf1 to directly deposit H3/H4 dimers onto the DNA , but suggest that additional factors are required for their removal from DNA . Quantitative binding studies have characterized specific Asf1-H3/H4 interactions and provided insight into the hand-off of histones to other histone chaperones, providing insight into the molecular mechanisms of DNA accessibility that are essential and fundamental to all DNA -dependent cellular functions.

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