Just as transcription initiation and capping are coupled, transcription termination and the 3’ cleavage and polyadenylation reactions are functionally linked. The Cleavage/Polyadenylation reaction is carried out the polyadenylation complex, a large multi-protein complex assembled from CPSF, CStF, CF1, and CF2. Some pre-mRNA’s have multiple poly(a) signals, and cleavage and polyadenylation at these different signals can include or exclude various 3′ terminal exons.
| Step |
Overview |
| Upstream Binding |
Upstream AAUAAA bound by Cleavage and Polyadenylation Specificity Factor (CPSF; 4 subunits). |
| Downstream Binding |
Downstream G/U rich element is bound by the Cleavage Stimulatory Factor (CStF; 3 subunits). |
| Additional Factors |
RNA cleavage requires Cleavage Factors (CF1 and 2); the endonuclease is the 73kd subunit of CPSF. |
| Binding of PAP |
The PolyA polymerase (PAP) binds, activating cleavage of the 3′ end. |
| Fragment Digested |
The free cleaved 3’ piece is rapidly degraded by 5’-3’ exonucleases. |
| Start of Poly(A) Tail |
Poly(A) Polymerase slowly adds ~12 adenosine residues to the 3’ end of the 5’ product RNA. |
| Poly(A) Binding Prtn |
The 12 adenosine residues are a binding site for PolyA Binding Protein (PABPII). |
| PAP Stimulation |
Bound PABPII stimulates PAP to quickly add ~250 adenosines, forming a poly(A) tail bound by many PABPII’s. |
Coupling 3′ Cleavage & Transcription Termination
| Theory |
Overview |
| Antitermination Model |
The polyA sequence is recognized as it leaves the polymerase and causes loss of elongation factors and/or gain of termination factors. |
| Torpedo Model |
The free 5’ end after cleavage of the polyA site recruits exonucleases that rapidly degrade the RNA and induce release from the template when they reach the polymerase. |
