Capping, splicing, and 3′ processing are independently stimulated by RNA polymerase II:: different functions for different segments of the CTD

Capping, splicing, and cleavage/polyadenylation of pre-mRNAs are interdependent events that are all stimulated in vivo by the carboxy-terminal domain (CTD) of RNA Pol II. We show that the CTD independently enhances splicing and 3 ' processing and that stimulation of splicing by enhancers is facilitated by the CTD. We provide evidence that stimulation of 3 ' processing by the CTD requires contact with the 50-kD subunit of the cleavage stimulation factor, CstF. Overexpression of the CTD-binding domain of CstF p50 had a dominant-negative effect on 3 ' processing without disrupting the CstF complex. The CTD comprises 52 heptad repeats. The CTD carboxyl terminus including heptads 27-52 supported capping, splicing, and 3 ' processing but the amino terminus supported only capping. We conclude that the CTD independently stimulates ail three major pre-mRNA processing steps and that different regions of the CTD can serve distinct functions in pre-mRNA processing.