One class of such proteins, histone acetyltransferase (HAT),89 catalyze the acetylation of selected amino acids, on the protruding histone tails, most commonly histone 3 (H3). Positively charged amino acids such as lysine and arginine are the common targets for acetylation. Histone acetylation modifies the histone-DNA relation. Acetylation of the lysine (K) residue on H3 neutralizes the positively charged histone, opening the histone-DNA relationship, and facilitating transcription factor binding to DNA. Thus, H3-K9 acetylation is a marker of active gene transcription. Many known transcriptional cofactors (proteins that enhance gene expression), such as CBP, are HATs. Interestingly, CBP is activated in hippocampal cell cultures in response to 5-HT or cAMP treatment. Histone acetylation is dynamic and is regulated by histone deacetylases (HDACs). HDACs block histone acetylation and suppress gene expression. Thus, chromatin structure can be viewed as dynamic and clearly subject to modification through intracellular signals that trigger either HATs or HDACs downstream.90-92 The study of histone acetylation provides a remarkable advance in our understanding of the dynamic and complex regulation of gene expression (see reference 88 for a review). Nevertheless, histone
