Glutamine methylation in histone H2A is an RNA-polymerase-I-dedicated modification

A description of a new histone modification, methylation of glutamine, on histone H2A in yeast and human cells. Post-translational modifications of histones are important for many DNA-templated processes. Here, Tony Kouzarides and colleagues identify a novel histone modification, methylation of glutamine, on histone H2A in yeast and human cells. The modification is exclusive to the nucleolus, being enriched over rDNA, and the enzyme responsible is a previously described rRNA methyltransferase. Glutamine methylation facilitates rDNA transcription by regulating binding of the histone chaperone complex FACT to nucleosomes, and it seems to have evolved as a modification dedicated to ribosomal biosynthesis. The finding that a protein can catalyse the methylation of both proteins and RNA raises the possibility that many other enzymes may have such dual specificity. Nucleosomes are decorated with numerous post-translational modifications capable of influencing many DNA processes1. Here we describe a new class of histone modification, methylation of glutamine, occurring on yeast histone H2A at position 105 (Q105) and human H2A at Q104. We identify Nop1 as the methyltransferase in yeast and demonstrate that fibrillarin is the orthologue enzyme in human cells. Glutamine methylation of H2A is restricted to the nucleolus. Global analysis in yeast, using an H2AQ105me-specific antibody, shows that this modification is exclusively enriched over the 35S ribosomal DNA transcriptional unit. We show that the Q105 residue is part of the binding site for the histone chaperone FACT (facilitator of chromatin transcription) complex2. Methylation of Q105 or its substitution to alanine disrupts binding to FACT in vitro. A yeast strain mutated at Q105 shows reduced histone incorporation and increased transcription at the ribosomal DNA locus. These features are phenocopied by mutations in FACT complex components. Together these data identify glutamine methylation of H2A as the first histone epigenetic mark dedicated to a specific RNA polymerase and define its function as a regulator of FACT interaction with nucleosomes.