Figure 4.

The CTD functions in 3′ processing of both polyadenylated and nonpolyadenylated RNAs. (A) At 3′ ends of polyadenylated mRNA, Ser2-P serves to recruit Pcf11, a component of CFII (human nomenclature is used for all factors). Other 3′ end factors, such as CstF50 and AAUAAA-binding factor CPSF-160 (Yhh1 in yeast), also bind the CTD, whereas Ssu72, with the aid of Pin1 (Ess1), must dephosphorylate Ser5-P. The AAUAAA element and G/U elements are bound by CPSF and CstF, respectively. Loading of some factors, including CPSF and CstF, may occur upstream, perhaps at the promoter (see the text). (B) The 3′ end of histone pre-mRNA contains a stem–loop motif bound by SLBP and a downstream element recognized by U7 snRNP. A complex containing CPSF73, CPSF100, and Symplekin is recruited for 3′ cleavage. Thr4-P facilitates this process, likely through a yet-to-be-identified factor. (C) The 3′ end of snRNA genes contains a 3′ box that interacts with the Integrator complex. RPAP2 binds to Ser7-P on the CTD and to recruit the Integrator, and Int 11, an Integrator subunit, cleaves the RNA.