The nucleosome is the fundamental repeating unit by which DNA in eukaryotic cells is progressively condensed into chromatin, the final structure of chromosomes. The nucleosome is canonically assembled as an octamer containing two copies each of the histones H3, H4, H2A, and H2B. When in complex with the DNA, nucleosomes appear like beads on a string under an electron microscope (Fig. 1). A variant of histone H3, CenH3, is specifically localized to a specialized chromatin structure, the centromere, which is critical in facilitating chromosome segregation during cell division. CenH3 may associate with the centromere in the context of a nucleosome, much like H3, but this conjecture had yet to be shown. In this study, a combination of biochemical and biophysical techniques was employed to investigate CenH3-containing nucleosomes from Drosophila cells. Cross-linking experiments showed stabilized interactions of CenH3 in a tetrameric nucleosome, which was found by mass spectroscopy to be composed of CenH3, H4, H2A, and H2B. Compared to bulk chromatin nucleosomes, the CenH3 nucleosome was also found to protect a reduced amount of DNA (120 base pairs vs. 150 base pairs) during nuclease digestion and to migrate faster in a native polyacrylamide gel. Furthermore, electron and atomic force microscopy revealed a "bead on a string" appearance of purified centromeric nucleosomes, but they were only half as high as those from bulk chromatin (Fig. 1), confirming that the tertrameric histone assembly observed by cross-linking does indeed constitute the complete centromeric nucleosome.
|Original language||English (US)|
|Number of pages||2|
|State||Published - Mar 1 2008|
ASJC Scopus subject areas
- Molecular Biology