A key event in the regulation of eukaryotic gene expression is the post-translational modification of nucleosomal histones, which converts regions of chromosomes into transcriptionally active or inactive chromatin. The most common post-translational modification of histones is the acetylation of [unreadable] amino groups on conserved lysine residues in the histones' amino-terminal tail domains. Hyperacetylation of histones generally correlates with transcriptionally active chromatin, perhaps by increasing the accessibility of nucleosomal DNA to transcription factors, while hypoacetylation of histones correlates with transcriptional silencing. In addition to its effect on transcription, acetylation/deacetylation of histones may have important roles in many cellular processes including chromatin assembly, DNA replication and repair, recombination, and chromosome segregation. Swift and significant advances have been made in the last several years toward understanding histone acetylation and deacetylation. Currently, there are over twenty histone acetyltransferases identified and nearly the same number of cloned human histone deacetylase (HDAC) enzymes. In this proposal, the overall hypothesis is that HDACs play a pivotal role in gene regulation. The long-term goal of this project is aimed at obtaining a greater mechanistic understanding of how HDACs regulate gene expression. Particular emphasis will be devoted to a detailed molecular dissection of the human HDAC3 and HDAC4 enzymes, and the elucidation of the molecular mechanisms by which HDAC inhibitors alter gene transcription. These studies will ultimately provide important insights critical for a thorough understanding of the intricate mechanisms operating to orchestrate gene expression in mammalian cells.