Role of the C-terminal domain of RNA polymerase II in expression of small nuclear RNA genes

Pol II (RNA polymerase II) transcribes the genes encoding proteins and non-coding snRNAs (small nuclear RNAs). The largest subunit of Pol II contains a distinctive CTD (C-terminal domain) comprising a repetitive heptad amino acid sequence, Tyr¹-Ser²-Pro³-Thr⁴-Ser⁵-Pro⁶-Ser⁷. This domain is now known to play a major role in the processes of transcription and co-transcriptional RNA processing in expression of both snRNA and protein-coding genes. The heptapeptide repeat unit can be extensively modified in vivo and covalent modifications of the CTD during the transcription cycle result in the ordered recruitment of RNA-processing factors. The most studied modifications are the phosphorylation of the serine residues in position 2 and 5 (Ser² and Ser⁵), which play an important role in the co-transcriptional processing of both mRNA and snRNA. An additional, recently identified CTD modification, phosphorylation of the serine residue in position 7 (Ser⁷) of the heptapeptide, is however specifically required for expression of snRNA genes. These findings provide interesting insights into the control of gene-specific Pol II function.