The human polybromo-1 protein is thought to localize the Polybroino, BRG1-associated factors chromatin-remodeling complex to kinetochores during mitosis via direct interaction of its six tandem bromodomains with acetylated nucleosomes. Bromodomains are acetyl-lysine binding modules roughly 100 amino acids in length originally found in chromatin associated proteins. Previous studies verified acetyl-histone binding by each bromodomain, but site-specificity, a central tenet of the histone code hypothesis, was not examined. Here, the acetylation site-dependence of bromodomain-histone interactions was examined using steady-state fluorescence anisotropy. Results indicate that single bromodomains bind specific acetyl-lysine sites within the historic tail with sub-micromolar affinity. Identification of duplicate target sites suggests that native Pb1 interacts with both copies of historic H3 upon nucleosome assembly. Quantitative analysis of single bromodomain-histone interactions can be used to develop hypotheses regarding the historic acetylation pattern that acts as the binding target of the native polybromo-1 protein. (c) 2007 Elsevier Inc. All rights reserved.