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Histone deacetylases (HDACs) and histone acetyl-transferases (HATs) are universal regulators of eukaryotic transcriptional activity and emerging therapeutic targets for human diseases. Here we describe the generation of isotope-labeled deacetylation and acetylation reporters for simultaneous NMR readouts of multiple deacetylation and acetylation reactions at different histone H4 sites. The site preferences of two prototypic histone deacetylases (Sir2.1 and HDAC8) and two acetyl-transferases (HAT1 and p300/CBP) were studied in intramolecular competition assays. We identify a previously ill-defined acetylation site, lysine 20 of histone H4, as a preferred target of three of theses enzymes. In situanalyses of endogenous deacetylation reactions at H4 sites in HeLa nuclear extracts point to abundant HDAC activities in human cellular environments.