Site-specific methylation and acetylation of lysine residues in the C-terminal domain (CTD) of RNA polymerase II

Dynamic modification of the carboxy-terminal domain (CTD) of RNA polymerase II (RNAPII) regulates transcription-coupled processes in eukaryotes. The CTD in mammals is composed of 52 heptad-repeats with the consensus sequence Y1-S2-P3-T4-S5-P6-S7. Repeats in the distal part of CTD deviate from the consensus sequence and have frequently replaced serine at position 7 by lysine residues (K7). Mass spectrometry analysis revealed modification of K7 residues in heptad-repeats 39, 42, and 47/49 by acetylation and in heptad-repeats 38, 39, 40, 42, and 47/49 by mono-, di-, or trimethylation. Notably, acetylated as well as di- and tri-methylated K7 residues were found exclusively in phosphorylated CTD peptides, while mono-methylated K7 residues occurred also in non-phosphorylated CTD peptides. Methylation of K7 residues was further studied with the monoclonal antibody (mAb) 1F5, which recognizes mono- and di-methylated K7 in CTD. ChIP experiments revealed high levels of K7 methylation at the transcriptional start site of genes. The low levels of K7 methylation in the body of genes results from impairment of epitope recognition in hyper-phosphorylated CTD by mAb 1F5. In agreement with this notion, phosphatase treatment of hyper-phosphorylated CTD or treatment of cells with kinase inhibitor flavopiridol restored the reactivity of mAb 1F5 towards methylated K7 residues in CTD. We conclude that methylation and acetylation of K7 residues further expand the mammalian CTD code and potentially contribute to regulation of gene expression. Overall design: This study was performed in a human Raji cell line. It contains ChIP-seq data for CTD Lysine mono- and di-methylated (one replicate). Other samples used in this study can be found at GSE52914.