CENP-A-containing nucleosomes: easier disassembly versus exclusive centromeric localization.

CENP-A is a histone variant that replaces conventional H3 in nucleosomes of functional centromeres. We report here, from reconstitutions of CENP-A- and H3-containing nucleosomes on linear DNA fragments and the comparison of their electrophoretic mobility, that CENP-A induces some positioning of its own and some unwrapping at the entry-exit relative to canonical nucleosomes on both 5 S DNA and the alpha-satellite sequence on which it is normally loaded. This steady-state unwrapping was quantified to 7(+/-2) bp by nucleosome reconstitutions on a series of DNA minicircles, followed by their relaxation with topoisomerase I. The unwrapping was found to ease nucleosome invasion by exonuclease III, to hinder the binding of a linker histone, and to promote the release of an H2A-H2B dimer by nucleosome assembly protein 1 (NAP-1). The (CENP-A-H4)2 tetramer was also more readily destabilized with heparin than the (H3-H4)2 tetramer, suggesting that CENP-A has evolved to confer its nucleosome a specific ability to disassemble. This dual relative instability is proposed to facilitate the progressive clearance of CENP-A nucleosomes that assemble promiscuously in euchromatin, especially as is seen following CENP-A transient over-expression.

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