Cleavage and polyadenylation specificity factor (CPSF), a key component of the mammalian RNA 3'-end processing machinery, consists of four subunits of 160, 100, 73, and 30 kD. Here we report the isolation and characterization of a cDNA encoding the 30-kD polypeptide. Antibodies raised against this protein inhibit cleavage and polyadenylation and coimmunoprecipitate the other CPSF subunits. The protein sequence contains five C3H-zinc-finger repeats and a putative RNA-binding zinc knuckle motif at the carboxyl terminus. Consistent with this observation, the in vitro translated 30-kD protein binds RNA polymers with a distinct preference for poly(U). In addition, an essential S. cerevisiae gene, YTH1, was cloned which is 40% identical to CPSF 30K at the protein level. Extracts prepared from a conditional yth1 mutant have normal cleavage activity, but fail to polyadenylate the upstream cleavage product. Efficient polyadenylation activity can be restored by the addition of purified polyadenylation factor I (PF I). We demonstrate that Yth1p is a component of PF I that interacts in vivo and in vitro with Fip1p, a known PF I subunit.