Histone lysine trimethyl states represent some of the most robust epigenetic modifications in eukaryotic chromatin. Using a candidate approach, we identified the subgroup of murine Jmjd2 proteins to antagonize H3K9me3 at pericentric heterochromatin. H3K27me3 and H4K20me3 marks are not impaired in inducible Jmjd2b-GFP cell lines, but Jmjd2b also reduces H3K36 methylation. Since recombinant Jmjd2b appears as a very poor enzyme, we applied metabolic labeling with heavy methyl groups to demonstrate Jmjd2b-mediated removal of chromosomal H3K9me3 as an active process that occurs well before replication of chromatin. These data reveal that certain members of the jmjC class of hydroxylases can work in a pathway that actively antagonizes a histone lysine trimethyl state. Histone lysine methylation is a central epigenetic modification with both activating and repressive roles in eukaryotic chromatin (Kouzarides 2002;Fischle et al. 2003;Lachner et al. 2004;Reinberg et al. 2004). There are five lysine residues in the histone N termini that are prominently methylated, with H3K4 and H3K36 methylation primarily transducing activating functions, whereas H3K9, H3K27, and H4K20 methylation is mainly associated with repressed chromatin. Histone lysine methylation can be presented as mono-, di-, or trimethyl states, where each distinct methyl state confers different biological read-outs (Santos-Rosa et al. 2002;Lachner et al. 2004). Histone lysine trimethyl states, particularly for the repressive functions, appear relatively robust, since they are stably propagated during several cell divisions Reinberg et al. 2004;Schotta et al. 2004) and resist reprogramming in early mammalian embryos (Santos et al. 2003).Until recently, no enzymatic mechanism had been described that would directly remove histone lysine methylation, although amine oxidation or radical attack by hydroxylation could destabilize the amino-methyl bond Bannister and Kouzarides 2005;Trewick et al. 2005). Indeed, a lysine-specific demethylase 1 (LSD1) has been shown to demethylate H3K4me2 (an active mark) (Shi et al. 2004) and, when associated with a different protein complex, also appears to convert H3K9me2 (a repressive mark) (Metzger et al. 2005). Based on the reaction mechanism, LSD1 cannot act on a lysine trimethyl state, and more potent mechanisms, involving attack by oxygen radicals via hydroxylases/dioxygenases, have been postulated (Clissold and Ponting 2001;Trewick et al. 2005). This could be inferred from the activities of the Escherichia coli AlkB enzyme to counteract alkylating damage of DNA or from other protein hydroxylases, such as the jmjC (jumonji) domain containing protein Factor Inhibiting Hypoxia (FIH) (Lando et al. 2002). In Schizosaccharomyces pombe, there is one silencing modifier, Epe1, which also carries a jmjC domain, and this factor antagonizes repressive H3K9me2 at centromeric regions (Ayoub et al. 2003). In addition, there are several jmjC proteins that have been shown to be important transcriptional/chromatin regulators (Takeuchi et al. 19...