Histone samples were prepared from wild-type ES cells fully labeled with lysine-8 ([13C6,15N2] heavy isotope-labeled L-lysine, abbreviated as K8,Supplementary information, Physique S3) and then mixed with equal amounts of histones extracted from G9a/ES cells labeled with regular L-lysine (K0). methyl-lysine analogues (MLA)6are compatible in biochemical reactions mediated by many histone methyltransferases, including Dot1L, HYPB, Suvar4-20 and Pr-Set77. We then extended this study to G9a. His-tagged H3 histones made up of K9C mutation were converted into His-H3Kc9me2 with the MLA reaction to mimic homogenously di-methylated H3K9. To our surprise, recombinant G9a displayed strong activity on nucleosomes put together Tirofiban Hydrochloride Hydrate with H3Kc9me2 (Physique 1A, left lane), which suggests that either G9a may be a trimethylase or may react with other H3 lysine residues on a Tirofiban Hydrochloride Hydrate nucleosomal substrate. Thus, we launched a K27A mutation into the His-tagged pre-methylated H3, which nearly completely abolished G9a’s activity (Physique 1A, right lane), indicating that G9a is mainly a dimethylase and also reacts with H3K27. == Physique 1. == G9a methylates H3K27in vitroand contributes to H3K27 methylationin vivo.(A)G9a methylates nucleosomal H3K27in vitro;(B)direct detection of H3K27me1 and H3K27me2 products by mass spectrometry on nucleosomal H3 reacted with G9a;(C)G9a/ES cells displayed reduced H3K27me1 levelsin vivo;(D)stable isotope labeling-based quantitative mass spectrometry revealed reduced levels of H3K27me1 and a subset of H3K27me2 in G9a/ES cells. To directly detect the reaction products, wild-type nucleosomes were reacted with G9a and S-adenosyl methionine. The reaction products were chemically propionylated8prior to mass spectrometry analysis to ensure the detection of H3K9 methylation marks in subsequent LC-MS (liquid chromatograph-coupled mass spectrometry) procedures. G9a-treated nucleosomal H3 samples contained H3K9me1, H3K9me2 signals and low amounts of H3K9me3 as expected (Supplementary information, Physique S1). Moreover, they also contained robust signals for H3K27me1 and H3K27me2 (Physique 1B). In addition, another histone methyltransferase GLP, a close homologue of G9a, can also methylate H3K27 in addition to H3K9in vitro(Supplementary information, Physique S2). The role of G9a as an H3K9 methyltransferase has been fully acknowledged4, but its contribution to H3K27 methylationin vivoremains obscure. This is likely due to the mind-boggling contribution of PRC2 in H3K27 methylation2,3, which may mask the contribution of G9a. Methylation status-specific antibodies were used to compare the levels of H3K27 methylation between wild-type and G9a/ES cells. Although no apparent changes of H3K27me2/3 levels were observed, H3K27me1 levels were clearly reduced in the G9a/ES cells (Physique 1C). To quantitatively compare the histone modification levels between the wild-type and G9a/ES Tirofiban Hydrochloride Hydrate cells, stable isotope labeling-based quantitative mass spectrometry analysis9,10was performed. Histone samples were prepared from wild-type ES cells fully labeled with lysine-8 ([13C6,15N2] heavy isotope-labeled L-lysine, abbreviated as K8,Supplementary information, Physique S3) and then mixed with equivalent amounts of histones extracted from G9a/ES cells labeled with regular L-lysine (K0). H3K27me1 levels in the G9a/ES cells were reduced to about 70% of the wild-type levels (Physique 1Dand1E). Comparison of H3K27me2 levels Tirofiban Hydrochloride Hydrate between wild-type and G9a/ES cells revealed a more complicated and interesting pattern. G9a/ES cells contained comparable levels (97%) of H3 transporting the combination of K27me2 and K36me2, reduced levels (75%) of H3 transporting the combination of K27me2 and K36me1, and further reduced levels (47%) of H3 transporting the combination of K27me2 and K36me0 (Physique 1Dand1E,Supplementary information, Splenopentin Acetate Physique S4). These results suggest a potential cross-talk between H3K27 and H3K36 methylations. Taken together, our results collectively suggest G9a’s contribution to H3K27 methylation in addition to its well-characterized role in H3K9 methylationin vivo. PRC2 complexes made up of Ezh2 and its closely Tirofiban Hydrochloride Hydrate related Ezh1 have been clearly shown to be the main contributors for H3K27me2/3in vivo3,4. Identification of G9a and possibly GLP as novel contributors to H3K27me1 and H3K27me2in vivoled to an intriguing question for further investigation: do G9a/GLP coordinate H3 K9 and K27 methylation at certain chromatin regions for a specific function? == Acknowledgments == We are grateful.