THE MAJOR YEAST POLY(A)-BINDING PROTEIN IS ASSOCIATED WITH CLEAVAGE FACTOR IA AND FUNCTIONS IN PREMESSENGER RNA 3'-END FORMATION

Polyadenylation of premessenger RNAs occurs posttranscriptionally in t he nucleus of eukaryotic cells by cleavage of the precursor and polyme rization of adenosine residues, In the yeast Saccharomyces cerevisiae, the mature poly(A) tail ranges from 60 to 70 nucleotides. 3'-end proc essing 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 pol y(A) polymerase (Pap1p), CF I has recently been separated into two fac tors, CF IA and CF IB, We have independently purified CF IA and found that five polypeptides cofractionate with the activity, They include R na14p, 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 t ranscripts bearing abnormally long poly(A) tracts, Complementation wit h recombinant Pab1p not only restores the length of the poly(A) tails to normal, but also triggers a poly(A) shortening activity, In additio n, 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 forma tion of mature mRNAs.