PerspectiveMolecular Biology

How to Read the Chromatin Past

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Science  24 Aug 2012:
Vol. 337, Issue 6097, pp. 919-920
DOI: 10.1126/science.1227684

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It has become common parlance to refer to all histone modifications as “epigenetic,” meaning that they carry information about how to use the associated DNA sequences as a function of earlier events. However, some histone modifications, such as methylation of histone H3 at lysine 27 (H3K27), are more epigenetic than others in that they can self-renew from one cell cycle to the next, thereby establishing a “cellular memory” of earlier events. A series of articles in the past 3 years—including one by Yuan et al. (1) on page 971 of this issue—has revealed several subtle mechanisms by which the Polycomb repressive complex 2 (PRC2), which methylates H3K27, reads the preexisting chromatin state to ensure that it is faithfully maintained after cell division (27).