Making binding relationships: Histone methylation

The nucleosome is the fundamental unit of chromatin and its highresolution Xray crystal struc ture was solved in 19971. This remarkable achievement showed the elegant and highly ordered packing of histones within 1.75 turns of DNA and it hinted at ways in which histones may be involved in regulating higherorder chromatin structure. Perhaps most striking is the way in which histone Nterminal tails protrude from their own nucleosome and, in some cases, make contact with adjacent nucleosomes. Simple extrapolation from this observation has implicated these interactions in higherorder chromatin regulation and many considered it likely that PTM (posttranslational modification) of the tails would affect internucleosomal stability and hence DNA processes such as transcription. Histone tails actually support multiple PTMs, including acetylation (see the article by Colyn CraneRobinson in this issue), phosphorylation and methylation. In this brief overview, I will consider histone methylation and discuss what we understand concerning its role.