Eukaryotic mRNA precursors (pre-mRNAs) 1 are synthesized and processed in the nucleus prior to their export to the cytoplasm where they serve as templates for protein synthesis. Transcription is coupled spatially and temporally to the capping of the pre-mRNA at the 5Ј-end, splicing, and 3Ј-end formation. The mature 3Ј-ends of most eukaryotic mRNAs are generated by endonucleolytic cleavage of the primary transcript followed by the addition of a poly(A) tail to the upstream cleavage product (for reviews see Refs. 1 and 2). In mammals, these reactions are catalyzed by a large multicomponent complex that is assembled in a cooperative manner on specific cis-acting sequence elements in the pre-mRNA. The cleavage and polyadenylation specificity factor (CPSF) (3) recognizes the highly conserved hexanucleotide AAUAAA, whereas the cleavage stimulation factor (CstF) (4) binds a more degenerate GUor U-rich element downstream of the poly(A) site. It has been suggested that in vivo CPSF and CstF may become associated with each other prior to pre-mRNA binding, recognizing the two elements in a concerted manner (5). In addition, the cleavage reaction requires mammalian cleavage factor I (CF I m ), cleavage factor II m (CF II m ), and poly(A) polymerase (PAP). After the first step of 3Ј-end processing, CPSF remains bound to the upstream cleavage fragment and tethers PAP to the 3Ј-end of the pre-mRNA (6). In the presence of the nuclear poly(A)-binding protein 1 (PABPN1), PAP elongates the poly(A) tail in a processive manner (6). These factors are both necessary and sufficient to reconstitute cleavage and polyadenylation in vitro. However, the other proteins involved in either transcription, such as the C-terminal domain of RNA polymerase II, or capping (nuclear cap-binding complex) and splicing (U2AF65) have been shown to greatly enhance the efficiency of the first step of the reaction (7-9).Three major polypeptides of 25, 59, and 68 kDa and one minor polypeptide of 72 kDa copurify with CF I m activity from HeLa cell nuclear extract (10). Reconstitution of CF I m activity with recombinant proteins suggests that CF I m is a heterodimer consisting of the 25-kDa subunit and one of the larger polypeptides (11). All of the three larger proteins appear to be highly related in their amino acid sequence. Moreover, all of the CF I m subunits are only present in metazoan organisms. The primary sequence of the 25-kDa polypeptide contains a NUDIX motif (12). The amino acid composition of the 68-kDa protein has a domain organization that is reminiscent of spliceosomal SR proteins. Members of the SR family of splicing factors contain one or more N-terminal RNA recognition motifs (RRMs) that function in sequence-specific RNA binding and a C-terminal domain rich in alternating arginine and serine residues, referred to as RS domain, which is required for proteinprotein interactions with other RS domains (13). In the 68-kDa protein, the RRM and the RS-like domain are separated by a