Expanding the CTD code: methylation of non-consensus Lysine residues marks early transcription in mammalian cells. Mus musculus

Dynamic post-translational modification of RNA polymerase II (RNAPII) coordinates the co-transcriptional recruitment of enzymatic complexes that regulate chromatin states and co-transcriptional processing of nascent RNA. Extensive phosphorylation of serine residues occurs at the structurally-disordered C-terminal domain (CTD) of the largest RNAPII subunit, which is composed of multiple heptapeptide repeats with consensus sequence Y1-S2-P3-T4-S5-P6-S7. Serine-5 and Serine-7 phosphorylation mark transcription initiation, whereas Serine-2 phosphorylation coincides with productive elongation. In vertebrates, the CTD has eight non-canonical substitutions of Serine-7 into Lysine-7, which can be acetylated (K7ac). Here, we describe for the first time mono- and di-methylation of CTD Lysine-7 residues (K7me1 and K7me2). K7me1 and K7me2 are observed during the earliest transcription stages and precede or accompany Serine-5 and Serine-7 phosphorylation. Overall design: Genome wide mapping of 2 novel RNAPII post-translational modifications (CTD-K7me1 and CTD-K7me2) in mouse ES cells.

RNA聚合酶II（RNA polymerase II，RNAPII）的动态翻译后修饰，可协同调控共转录阶段染色质调控酶复合物的招募，以及新生RNA的共转录加工过程。RNAPII最大亚基的结构无序C端结构域（C-terminal domain，CTD）存在大量丝氨酸残基磷酸化修饰，该结构域由多个共有序列为Y1-S2-P3-T4-S5-P6-S7的七肽重复单元构成。其中丝氨酸-5与丝氨酸-7的磷酸化可标记转录起始过程，而丝氨酸-2的磷酸化则与转录有效延伸阶段相契合。在脊椎动物中，CTD存在8处非经典的丝氨酸-7向赖氨酸-7的取代位点，该赖氨酸残基可发生乙酰化修饰（K7ac）。本研究首次报道了CTD赖氨酸-7残基的单甲基化与二甲基化修饰（K7me1与K7me2）。K7me1与K7me2可在转录早期阶段被检测到，且早于或伴随丝氨酸-5与丝氨酸-7的磷酸化过程。实验整体设计：在小鼠胚胎干细胞（mouse ES cells）中，对两种新型RNAPII翻译后修饰（CTD-K7me1与CTD-K7me2）开展全基因组范围的定位分析。