The carboxy-terminal domain (CTD) of RNA polymerase II (RNAPII) in mammals undergoes extensive posttranslational modification, which is essential for transcriptional initiation and elongation. Here, we show that the CTD of RNAPII is methylated at a single arginine (R1810) by the coactivator-associated arginine methyltransferase 1 (CARM1). Although methylation at R1810 is present on the hyperphosphorylated form of RNAPII in vivo, Ser2 or Ser5 phosphorylation inhibits CARM1 activity toward this site in vitro, suggesting that methylation occurs before transcription initiation. Mutation of R1810 results in the misexpression of a variety of small nuclear RNAs and small nucleolar RNAs, an effect that is also observed in Carm1−/− mouse embryo fibroblasts. These results demonstrate that CTD methylation facilitates the expression of select RNAs, perhaps serving to discriminate the RNAPII-associated machinery recruited to distinct gene types.

SMN (Survival motor neuron protein) was characterized as a dimethyl-arginine binding protein over ten years ago. TDRD3 (Tudor domain-containing protein 3) and SPF30 (Splicing factor 30 kDa) were found to bind to various methyl-arginine proteins including Sm proteins as well later on. Recently, TDRD3 was shown to be a transcriptional coactivator, and its transcriptional activity is dependent on its ability to bind arginine-methylated histone marks. In this study, we systematically characterized the binding specificity and affinity of the Tudor domains of these three proteins quantitatively. Our results show that TDRD3 preferentially recognizes asymmetrical dimethylated arginine mark, and SMN is a very promiscuous effector molecule, which recognizes different arginine containing sequence motifs and preferentially binds symmetrical dimethylated arginine. SPF30 is the weakest methyl-arginine binder, which only binds the GAR motif sequences in our library. In addition, we also reported high-resolution crystal structures of the Tudor domain of TDRD3 in complex with two small molecules, which occupy the aromatic cage of TDRD3.

SUMMARY Developmental specification of germ cells lies at the core of inheritance as germ cells contain all of the genetic and epigenetic information transmitted between generations. The critical developmental event distinguishing germline from somatic lineages is the differentiation of primordial germ cells (PGCs)1,2, precursors of sex specific gametes that produce an entire organism upon fertilization. Germ cells toggle between uni- and pluripotent states as they exhibit their own “latent” form of pluripotency. For example, PGCs express a number of transcription factors (TFs) in common with embryonic stem cells (ESCs), including OCT4, SOX2, NANOG and PRDM142–4. A biochemical mechanism by which these TFs converge on chromatin to produce the dramatic rearrangements underlying ESC- and PGC-specific transcriptional programs remains poorly understood. Here, we discover a novel co-repressor protein, CBFA2T2, that regulates pluripotency and germline specification. Cbfa2t2−/− mice display severe defects in PGC maturation and epigenetic reprogramming. CBFA2T2 forms a biochemical complex with PRDM14, a germline-specific transcription factor. Mechanistically, CBFA2T2 oligomerizes to form a scaffold upon which PRDM14 and OCT4 are stabilized on chromatin. Thus, in contrast to the traditional “passenger” role of a co-repressor, CBFA2T2 functions synergistically with TFs at the crossroads of the fundamental developmental plasticity between uni- and pluripotency