Birth of a poly(A) tail: mechanisms and control of mRNA polyadenylation

During their synthesis in the cell nucleus, most eukaryotic mRNAs undergo a two‐step 3′‐end processing reaction in which the pre‐mRNA is cleaved and released from the transcribing RNA polymerase II and a polyadenosine (poly(A)) tail is added to the newly formed 3′‐end. These biochemical reactions might appear simple at first sight (endonucleolytic RNA cleavage and synthesis of a homopolymeric tail), but their catalysis requires a multi‐faceted enzymatic machinery, the cleavage and polyadenylation complex (CPAC), which is composed of more than 20 individual protein subunits. The activity of CPAC is further orchestrated by Poly(A) Binding Proteins (PABPs), which decorate the poly(A) tail during its synthesis and guide the mRNA through subsequent gene expression steps. Here, we review the structure, molecular mechanism, and regulation of eukaryotic mRNA 3′‐end processing machineries with a focus on the polyadenylation step. We concentrate on the CPAC and PABPs from mammals and the budding yeast, Saccharomyces cerevisiae, because these systems are the best‐characterized at present. Comparison of their functions provides valuable insights into the principles of mRNA 3′‐end processing.