The cleavage/polyadenylation factor (CPF) of Saccharomyces cerevisiae is thought to provide the catalytic activities of the mRNA 3′-end processing machinery, which include endonucleolytic cleavage at the poly(A) site, followed by synthesis of an adenosine polymer onto the new 3′-end by the CPF subunit Pap1. Because of similarity to other nucleases in the metallo-β-lactamase family, the Brr5/Ysh1 subunit has been proposed to be the endonuclease. The C-terminal domain of Brr5 lies outside of β-lactamase homology, and its function has not been elucidated. We show here that this region of Brr5 is necessary for cell viability and mRNA 3′-end processing. It is highly homologous to another CPF subunit, Syc1. Syc1 is not essential, but its removal improves the growth of other processing mutants at restrictive temperatures and restores in vitro processing activity to cleavage/ polyadenylation-defective brr5-1 extract. Our findings suggest that Syc1, by mimicking the essential Brr5 C-terminus, serves as a negative regulator of mRNA 3′-end formation.