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ADP-ribosylation controls many processes, including transcription, DNA repair, and bacterial toxicity. ADP-ribosyltransferases and poly-ADP-ribose polymerases (PARPs) catalyze mono- and poly-ADP-ribosylation, respectively, and depend on a highly conserved glutamate residue in the active center for catalysis. However, there is an apparent absence of this glutamate for the recently described PARP6-PARP16, raising questions about how these enzymes function. We find that PARP10, in contrast to PARP1, lacks the catalytic glutamate and has transferase rather than polymerase activity. Despite this fundamental difference, PARP10 also modifies acidic residues. Consequently, we propose an alternative catalytic mechanism for PARP10 compared to PARP1 in which the acidic target residue of the substrate functionally substitutes for the catalytic glutamate by using substrate-assisted catalysis to transfer ADP-ribose. This mechanism explains why the novel PARPs are unable to function as polymerases. This discovery will help to illuminate the different biological functions of mono- versus poly-ADP-ribosylation in cells.

Eukaryotic genomes are organized into active (euchromatic) and inactive (heterochromatic) chromatin domains. Post-translational modifications of histones (or 'marks') are key in defining these functional states, particularly in promoter regions. Mutual regulatory interactions between these marks--and the enzymes that catalyse them--contribute to the shaping of this epigenetic landscape, in a manner that remains to be fully elucidated. We previously observed that asymmetric di-methylation of histone H3 arginine 2 (H3R2me2a) counter-correlates with di- and tri- methylation of H3 lysine 4 (H3K4me2, H3K4me3) on human promoters. Here we show that the arginine methyltransferase PRMT6 catalyses H3R2 di-methylation in vitro and controls global levels of H3R2me2a in vivo. H3R2 methylation by PRMT6 was prevented by the presence of H3K4me3 on the H3 tail. Conversely, the H3R2me2a mark prevented methylation of H3K4 as well as binding to the H3 tail by an ASH2/WDR5/MLL-family methyltransferase complex. Chromatin immunoprecipitation showed that H3R2me2a was distributed within the body and at the 3' end of human genes, regardless of their transcriptional state, whereas it was selectively and locally depleted from active promoters, coincident with the presence of H3K4me3. Hence, the mutual antagonism between H3R2 and H3K4 methylation, together with the association of MLL-family complexes with the basal transcription machinery, may contribute to the localized patterns of H3K4 tri-methylation characteristic of transcriptionally poised or active promoters in mammalian genomes.