The Mechanism: Termination
Termination
The CTD is connected to the rest of Rpb1 by a linker protein region (12). When in its compacted form, this linker region is also located where the mRNA leaves RNAPII through groove 1. 5’ mRNA capping occurs once 25 residues of the mRNA are exposed to the outside of the enzyme. When the CTD has been phosphorylated, it recruits and binds capping enzymes to perform their function. As stated above, this modification of the transcript is necessary for mRNA transport out of the cell’s nucleus.
Additionally, a specific sequence known as the cleavage signal, is located at the end of the ORF on the DNA (2). When this signal is transcribed onto the mRNA transcript it becomes exposed to the outside of the enzyme. The CTD in its phosphorylated state can recognize the cleavage signal and cause the recruitment and binding of cleavage factors such as CPSF and CstF. These cleavage factors are necessary to cut the mRNA and release it from the enzyme. Transcription is completed when the enzyme releases the bound DNA so that it can repeat the cycle at a different location.