Maintenance of chromatin states: an open-and-shut case.

The traditional view of chromatin envisions two states: one is 'active' and accessible to nucleases, whereas the other is 'silent' and relatively inaccessible. Recent evidence that combinations of diverse histone tail modifications represent a spectrum of chromatin states challenges this simple view. Here, we examine inter-relationships between chromatin remodeling, histone modification, DNA methylation, RNA interference, and nucleosome assembly activities. We find that the two-state view can accommodate these new findings, and that nucleosome assembly pathways may ultimately maintain euchromatic and heterochromatic states.