Histone covalent modifications that have been identified as functionally important in regulating transcription include acetylation (lysine residues), methylation (arginine and lysine residues), phosphorylation (serine and threonine residues), ubiquitination, sumoylation, ADP-ribosylation (lysine residues) and proline isomerization (Allis et al., 2007; Kouzarides, 2007). Of these, acetylation and phosphorylation of histones H3 and H4 most commonly activate transcription while sumoylation has been implicated in repression. Some modifications need to be interpreted in a context-specific manner, e.g. methylation, which can cause either activation or repression (Lachner, O’Sullivan, & Jenuwein, 2003). Lysine (K) residues are an important substrate for histone modifications since they undergo acetylation, methylation, ubiquitination and sumoylation. Furthermore, different patterns of modifications give rise to varying cellular outcomes. For example, lysine methylation at histone H3 residues K4, K36 and K79 (H3K4, H3K36 and H3K79) causes activation; however, methylation at H3K9, H3K27 and H4K20 causes transcriptional repression (Allis et al., 2007). Histone covalent modifications are dynamically regulated by the actions of deacetylases, serine-threonine phosphatases, lysine demethylases, deiminases and ubiquitin proteases that act to reverse the corresponding modifications. Thus, the epigenetic regulation of gene expression
