Histones and histone modifications.

Histone variants, distinct patterns of posttranslational modifications of histones, and histone tail binding proteins all contribute to establishment of various ‘open’ or ‘closed’ chromatin domains that have specialized folding properties and biological functions. Some of these domains can be propagated through DNA replication and mitosis, guaranteeing the inheritance of chromatin states to progeny. Histone lysine methylation may play a central role in the stability of these chromatin states, as to date no enzymes are known that catalyze lysine demethylation. Furthermore, several nonhistone proteins, such as HP1 or the PRC1 polycomb complex, not only bind to methylated histone lysines, but also recruit the methylase itself, thus providing a means for templating new histone methylation events – for example, following replication fork passage – or for spreading the domain to adjacent nucleosomes.How ‘open’ states are propagated through cell divisions is not clear, especially as histone lysine acetylation or serine phosphorylation can be rapidly reversed by HDACs or histone phosphatases. Future studies will no doubt continue to identify the functional and biochemical properties of new chromatin domains as well as to elucidate the principles that govern their maintenance and propagation.