A MULTISUBUNIT 3'-END PROCESSING FACTOR FROM YEAST CONTAINING POLY(A)POLYMERASE AND HOMOLOGS OF THE SUBUNITS OF MAMMALIAN CLEAVAGE AND POLYADENYLATION SPECIFICITY FACTOR

Polyadenylation is the second step in 3' end formation of most eukaryo tic mRNAs. In Saccharomyces cerevisiae, this step requires three trans -acting factors: poly(A) polymerase (Pap1p), cleavage factor I (CF I) and polyadenylation factor I (PF I), Here, we describe the purificatio n and subunit composition of a multiprotein complex containing Pap1p a nd PF I activities, PF I-Pap1p was purified to homogeneity by compleme ntation of extracts mutant in the Fip1p subunit of PF I, In addition t o Fip1p and Pap1p, the factor comprises homologues of all four subunit s of mammalian cleavage and polyadenylation specificity factor (CPSF), as web as Pta1p, which previously has been implicated in pre-tRNA pro cessing, and several as yet uncharacterized proteins, As expected for a PF I subunit, pta1-1 mutant extracts are deficient for polyadenylati on in vitro, PF I also appears to be functionally related to CPSF, as it polyadenylates a substrate RNA more efficiently than Pap1p alone, P ossibly, the observed interaction of the complex with RNA tethers Pap1 p to its substrate.