Polyadenylation is the covalent linkage of a polyadenylyl moiety to a messenger RNA molecule. In eukaryotic organisms, polyadenylation is the mechanism by which most messenger RNA (mRNA) molecules are terminated at their 3' ends. The poly(A) tail aids in mRNA stability by protecting it from exonucleases. Polyadenylation is also important for transcription termination, for export of the mRNA from the nucleus, and for translation. Some prokaryotic mRNAs also are polyadenylated, although the poly(A) tail's function is different from that in eukaryotes.
Polyadenylation occurs during and immediately after transcription of DNA into RNA. After transcription has been terminated, the mRNA chain is cleaved through the action of an endonuclease complex associated with RNA polymerase. The cleavage site is characterized by the presence of the base sequence AAUAAA near the cleavage site. After the mRNA has been cleaved, 80 to 250 A residues are added to the free 3' end at the cleavage site. This reaction is catalyzed by polyadenylate polymerase.