The 3′ ends of most eukaryotic messenger RNAs are generated by internal cleavage and polyadenylation. In mammals, there is a strict dependence of both reactions on the sequence AAUAAA, which occurs upstream of polyadenylation [poly(A)] sites and which is recognized by CPSF. In contrast, cis-acting signals for yeast 3′-end generation are highly divergent from those of mammals, suggesting that trans-acting factors other than poly(A) polymerase would not be conserved. The essential yeast protein Brr5/Ysh1 shows sequence similarity to subunits of mammalian CPSF and is required for 3′-end processing in vivo and in vitro. These results demonstrate a structural and functional conservation of the yeast and mammalian 3′-end processing machineries despite a lack of conservation of the cis sequences.