Nucleosomes and centromeric DNA packaging.

The eukaryotic chromosome is a conserved structure, with the DNA double-helix wrapping around octamers of histone proteins to form the chromatin, which is further packaged into chromosomes. The centromere defines the kinetochore, the region of spindle microtubule attachment that pulls the two replicated chromatids of each chromosome apart during cell division (Fig. 1), leading to fidelity in transmission of genetic information. Like the telomere, centromeres are well defined morphologically and functionally, but their DNA sequence shows no conservation between species, and their coiling into chromatin is still poorly understood. In PNAS, Zhang et al. (1) give unique insight into the centromeric CenH3-nucleosomes associated with the rice CentO satellite sequence and adjacent regions of DNA, having implications for centromere specification, activity, and evolution. Centromeres are readily observed as partial constrictions on the metaphase chromosome by light or electron microscopy (Fig. 1). However, few sequence-related characteristics of the DNA at the centromeres are conserved. In contrast, nucleosome proteins (2) and kinetochores linking centromeres and spindle microtubules with a multiprotein complex are highly conserved across all kingdoms (3). Typically, there are tandemly repeated satellite DNA sequences at the centromeres of chromosomes of animals and plants (4–6). Detailed work on the budding yeast Schizosaccharomyces pombe and brewer’s yeast Saccharomyces cerevisiae identified relatively short DNA sequences and binding protein counterparts that direct chromosome segregation (7, 8). The search was then on for critical “boxes” in plants and animals but, although some motifs were found to be involved in protein–DNA interactions, no motif shared across diverse phyla was found within the centromeric tandem repeats. As sequencing technology advanced, both variants of tandem repeats and almost any other class of DNA—retrotransposons, transposons, genes, transcription factors, and microsatellites—were revealed (9) underlying the kinetochore and microtubule attachment sites. Evidence from comparisons of species, mutants that still showed centromeric function, the lack of necessity for tandemly repeated satellite sequence, and the presence of A