Polyadenylation of premessenger RNAs occurs posttranscriptionally in the nucleus 
of eukaryotic cells by cleavage of the precursor and polymerization of adenosine 
residues. In the yeast Saccharomyces cerevisiae, the mature poly(A) tail ranges 
from 60 to 70 nucleotides. 3'-end processing can be reproduced in vitro with 
purified factors. The cleavage reaction requires cleavage factors I and II (CF I 
and CF II), whereas polyadenylation involves CF I, polyadenylation factor I 
(PFI), and poly(A) polymerase (Pap1p). CF I has recently been separated into two 
factors, CF IA and CF IB. We have independently purified CF IA and found that 
five polypeptides cofractionate with the activity. They include Rna14p, Rna15p, 
Pcf11p, a new protein called Clp1p, and remarkably, the major poly(A)-binding 
protein Pab1p. Extracts from strains where the PAB1 gene is mutated or deleted 
are active for cleavage but generate transcripts bearing abnormally long poly(A) 
tracts. Complementation with recombinant Pab1p not only restores the length of 
the poly(A) tails to normal, but also triggers a poly(A) shortening activity. In 
addition, a monoclonal Pab1p antibody prevents the formation of poly(A) tails in 
extracts or in a reconstituted system. Our data support the notion that Pab1p is 
involved in the length control of the poly(A) tails of yeast mRNAs and define a 
new essential function for Pab1p in the formation of mature mRNAs.