The "CTD"
is the carboxyl-terminal domain of the largest subunit of RNA polymerase
II. In humans it is over 350
amino acids long and is comprised of a tandemly repeated 7 amino acid motif
whose consensus is tyr-ser-pro-thr-ser-pro-ser. This domain is central to the mechanism by which post-transcriptional
events are coupled to transcription. For example, early in transcription the
cleavage & polyadenylation factors, CPSF and CstF, bind to the CTD where
they wait for the poly(A) signal to emerge.
Capping and splicing factors also bind to the CTD. All of this traffic is modulated by dynamic
phosphorylation and dephosphorylation of the CTD, especially at its serine
residues.