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Defining a Functional Centromere

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The question of what makes a functional centromere remains one of the most important and intriguing unanswered questions in biology. The centromere boundary can be defined by identifying sequences that bind to CENH3, a centromere-specific histone H3 variant (called CENP-A in mammals; reviewed in Jiang et al., 2003). The centromeric and pericentromeric regions are also rich in heterochromatin, marked by massive amounts of repetitive elements. In Arabidopsis, centromeric regions contain megabase-sized arrays of a 178-bp satellite repeat, and cytological mapping of CENH3 on extended chromatin fibers has shown that only the middle portions of these repeat arrays are associated with CENH3 (Shibata and Murata, 2004). However, it is unknown what restricts CENH3 binding to the central core of centromeres.

In this issue, Zhang et al. (pages 25–34) investigate how CENH3 is targeted to the centromere core region through cytogenetic and sequence analysis of the centromeres of Arabidopsis and maize chromosomes and show that DNA methylation may play a role in this process. The authors show that the 178-bp repeats associated with the CENH3-containing chromatin (CEN chromatin) in Arabidopsis are hypomethylated compared with the same repeats located in the flanking pericentromeric regions (see figure), and a similar hypomethylation pattern in CEN chromatin was also observed in maize. The authors examined the distribution of CG and CNG sequence sites in Arabidopsis centromeres, since cytosine is often methylated at these sites. They found that the 178-bp repeats in CEN chromatin display a distinct distribution pattern of the CG and CNG sites, which may provide a basis for differential methylation. The authors proposed that DNA methylation in the centromere may play an important role in determining CENH3 binding and thus demarcating the core centromeric chromatin from flanking pericentromeric heterochromatin.

View larger version (39K): [in this window] [in a new window] [as a PowerPoint slide]   Cytological mapping of 5-methylcytosine (right) on meiotic pachytene chromosomes (left) of Arabidopsis thaliana. The hypomethylated central core of the centromere (arrows) is flanked by hypermethylated pericentromeric domains.

www.plantcell.org/cgi/doi/10.1105/tpc.108.200112

REFERENCES

Jiang, J., Birchler, J.A., Parrott, W.A., and Dawe, R.K. (2003). A molecular view of plant centromeres. Trends Plant Sci. 8: 570–575.[CrossRef][Web of Science][Medline]

Shibata, F., and Murata, M. (2004). Differential localization of the centromere-specific proteins in the major centromeric satellite of Arabidopsis thaliana. J. Cell Sci. 117: 2963–2970.[Abstract/Free Full Text]

Zhang, W., Lee, H.-R., Koo, D.-H., and Jiang, J. (2008). Epigenetic modification of centromeric chromatin: Hypomethylation of DNA sequences in the CENH3-associated chromatin in Arabidopsis thaliana and maize. Plant Cell 20: 25–34.[Abstract/Free Full Text]

Related articles in Plant Cell:

Epigenetic Modification of Centromeric Chromatin: Hypomethylation of DNA Sequences in the CENH3-Associated Chromatin in Arabidopsis thaliana and Maize

Wenli Zhang, Hye-Ran Lee, Dal-Hoe Koo, and Jiming Jiang
Plant Cell 2008 20: 25-34. [Abstract] [Full Text]  

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