Histone proteins are subject to modifications, such as acetylation, methylation, phosphorylation, ubiquitination, glycosylation, and ADP ribosylation, some of which are known to play important roles in the regulation of chromatin structure and function. Here we report that histone H4 is modified by small ubiquitin-related modifier (SUMO) family proteins both in vivo and in vitro. H4 binds to the SUMO-conjugating enzyme (E2), UBC9, and can be sumoylated in an E1 (SUMO-activating enzyme)-and E2-dependent manner. We present evidence suggesting that histone sumoylation mediates gene silencing through recruitment of histone deacetylase and heterochromatin protein 1.C hromatin is composed of nucleosomes in which 146 bp of DNA are wrapped around a core histone octamer. The octamer consists of a (H3-H4) 2 tetramer associated with two H2A-H2B dimers. The C-terminal globular domains of core histones bind to each other to form the octamer whereas core histone N-terminal tails protrude from the nucleosome and are accessible to the enzymatic modification machineries that catalyze acetylation, methylation, phosphorylation, glycosylation, and ADP ribosylation (for reviews see refs. 1 and 2). Over the last several years, rapid progress has been made in the field of histone modification, owing largely to the discoveries that histone acetyltransferases and histone deacetylases function as transcriptional coactivators and corepressors, respectively (3). These discoveries were followed by analyses of the mechanism and effects of acetylation and the identification of other histone modifications including phosphorylation, methylation, and ubiquitination as regulators of transcription.Small ubiquitin-related modifier (SUMO) is the bestcharacterized member of a growing family of ubiquitin-like proteins involved in posttranslational modifications (for reviews see refs. 4-6). In mammals, there are three members of the SUMO protein family: SUMO-1, SUMO-2 (SMT3a), and SUMO-3 (SMT3b), which are implicated in partly overlapping, yet distinct functions (7,8). SUMO is covalently attached to other proteins through the activities of an enzyme cascade (E1-E2-E3) similar to that for ubiquitination. There is only one known SUMO-activating enzyme, E1 (a heterodimer of SAE1 and SAE2) and only one known SUMO-conjugating enzyme, E2 (UBC9). There appear to be a number of different SUMO ligases (E3s) in higher eukaryotes such as PIAS family proteins (9-11), RanBP2 (12), and the polycomb protein Pc2 (13). To date, dozens of proteins from different species have been identified as sumoylation substrates. Among these are the RanGTPase-activating protein, RanGAP1 (14), PML (15), I Bā£ (16), p53 (17, 18), and MDM2 (19). Unlike ubiquitination, sumoylation of proteins has not been linked to protein degradation. Proposed functions for sumoylation include regulation of protein-protein interaction and localization, inhibition of ubiquitin-mediated degradation, and enhancement of transcriptional activity.Histone proteins have been long known to be ubiquitinated (20),...