Three-layered control of mRNA poly(A) tail synthesis in .

Biogenesis of most eukaryotic mRNAs involves the addition of an untemplated polyadenosine (pA) tail by the cleavage and polyadenylation machinery. The pA tail, and its exact length, impacts mRNA stability, nuclear export, and translation. To define how polyadenylation is controlled in , we have used an in vivo assay capable of assessing nuclear pA tail synthesis, analyzed tail length distributions by direct RNA sequencing, and reconstituted polyadenylation reactions with purified components. This revealed three control mechanisms for pA tail length. First, we found that the pA binding protein (PABP) Nab2p is the primary regulator of pA tail length. Second, when Nab2p is limiting, the nuclear pool of Pab1p, the second major PABP in yeast, controls the process. Third, when both PABPs are absent, the cleavage and polyadenylation factor (CPF) limits pA tail synthesis. Thus, Pab1p and CPF provide fail-safe mechanisms to a primary Nab2p-dependent pathway, thereby preventing uncontrolled polyadenylation and allowing mRNA export and translation.