recruit other components of the transcriptional machinery, including RNA polymerase, which ultimately transcribes the gene [27]. As would be predicted from its biochemical role, genome-wide promoter analyses in a variety of cells, tissues, and species have identified strong correlations between histone H3 and H4 acetylation and highly transcribed genes [28, 29]. Thus, histone acetylation is almost universally thought of as a “mark” of activated chromatin. In addition, histone H3 acetylation at certain genes is coupled to phosphorylation of a neighboring serine residue, serine 10, on histone H3 (H3S10), a process known as phospho-acetylation [24]. Histone phospho-acetylation occurs in concert because H3S10 phosphorylation provides a docking site for the histone acetyltransferase (HAT), GCN5, on chromatin [30]. GCN5 then maintains a hyperacetylated state, which promotes gene activation as described above. Moreover, acetylation of lysine K9 on histone H3 prevents the methylation of this same residue, which is a mark of repressed chromatin, and the subsequent recruitment of HP1 (heterochromatin protein 1). These effects of lysine K9 acetylation further promote active transcription [24].
