POLY(A) TAIL LENGTH CONTROL IN SACCHAROMYCES-CEREVISIAE OCCURS BY MESSAGE-SPECIFIC DEADENYLATION

We report that newly synthesized mRNA poly(A) tails are matured to pre cise lengths by the Pab1p-dependent poly(A) nuclease (PAN) of Saccharo myces cerevisiae, These results provide evidence for an initial phase of mRNA deadenylation that is required for poly(A) tail length control . In RNA 3'-end processing extracts lacking PAN, transcripts are polya denylated to lengths exceeding 200 nucleotides, By contrast, in extrac ts containing PAN, transcripts were produced with the expected wild-ty pe poly(A) tail lengths of 60 to 80 nucleotides, The role for PAN in p oly(A) tail length control in vivo was confirmed by the finding that m RNAs are produced with longer poly(A) tails in PAN-deficient yeast str ains. Interestingly, wild-type yeast strains were found to produce tra nscripts which varied in their maximal poly(A) tail length, and this m essage-specific length control was lost in PAN-deficient strains. Our data support a model whereby mRNAs are polyadenylated by the 3'-end pr ocessing machinery with a long tail, possibly of default length, and t hen in a PAN-dependent manner, the poly(A) tails are rapidly matured t o a message-specific length. The ability to control the length of the poly(A) tail for newly expressed mRNAs has the potential to be an impo rtant posttranscriptional regulatory step in gene expression.