Symplekin and CPSF73 are directly involved in Pol II termination. (A) The first step in Pol II termination involves dephosphorylation of the CTD (YSPTSPS). This is achieved by the phosphatase SSU72, whose activation requires Symplekin, which is associated with CPSF. P in circle: phosphate. (B) The second step involves the endonucleolytic cleavage by the endonuclease CPSF73, which exposes the unprotected 5′ phosphate of the downstream cleavage product, providing a substrate for the 5′ to 3′ exonuclease XRN2. Lightning bolt: endonucleolytic cleavage. (C) Finally, XRN2 degrades the downstream cleavage product (dashed line) and facilitates the displacement of Pol II from the DNA template. The upstream cleavage product is polyadenylated.
