Abstract
RNA polymerase II (RNAPII) is one of the major nuclear enzymes found in all eukaryotic cells affecting its viability directly or indirectly. Eukaryotic RNAPII not only synthesizes mRNA but also coordinates transcription related processes. It catalyzes the transcription of DNA to synthesize precursors of mRNA, most snRNA and microRNA. The largest subunit of RNAPII contains, an unusual and unique extension appended to its C terminus known as C terminal domain (CTD), serves as a flexible binding scaffold for a large number of nuclear factors, enzymes and protein/RNA-binding factors. The CTD consists of consists of tandem repeats of the heptapeptide consensus Y1S2P3T4S5P6S7 (Tyrl-Ser2-Pro3-Thr4-Ser5-Pro6-Ser7). It is the protein domain that is extensively involved in the initiation elongation and termination of transcription, operating through capping of the nascent RNA transcript, RNA splicing, Cis-Trans isomerisation, Histone modifications (methylation and deacetylation), nucleocytoplasmic transport of mRNA and DNA damage and repair etc. Dynamic modification of the CTD enables changes in the tethered activities as RNAPII proceeds through the transcription cycle. The dynamic remodelling of the CTD modifications most notably phosphorylation by kinases, phosphatases, isomerases, and other enzymes introduce changes in the CTD structure. These changes serve as structural switches that spatially and temporally regulate the binding of processing factors which modulates its activities in the above processes. Therefore, understanding the nature of CTD modifications, its function and role in transcription regulation, is essential to get a clear insight the mechanisms that control gene expression.