Figure 7.

Hybrid model for coupling termination with 3′-end processing. This model incorporates aspects of both the antiterminator/allosteric and torpedo models. Cooperative association of Rat1 and cleavage/polyadenylation factors exemplified by Pcf11 with the CTD Ser2 phosphorylated pol II elongation complex is shown at the poly(A) site (AATAAA). This interaction is proposed to result in an allosteric change in pol II (designated by a change from green to red) that favors termination. (Blue Ps) Ser5-PO₄; (red Ps) Ser2-PO₄. Nascent RNA downstream of the poly(A) cleavage site (blue line) is degraded by both Xrn1 and Rat1. This degradation could facilitate termination; however, it is not sufficient to cause termination.