ca. 200 adenylate residues generally tails mature mRNAs of eukaryotes. This is not the end of the primary transcript; transcription may continue by a thousand or more nucleotides beyond the end of the gene. Polyadenylation requires that the transcript be cut by an endonuclease and then a poly-A RNA polymerase attaches to this poly-A tail, which is probably required for stabilization of the mRNA. Several of the histone protein genes do not have, however, a poly-A tail. Other histone mRNAs, which are not involved with the mammalian cell cycle and histone mRNAs of yeast and Tetrahymena, are polyadenylated. The common post-transcriptional polyadenylation is signaled generally by the presence near the 3′-end an AATAAA sequence, which is followed two dozen bases downstream by a short GT-rich element. Polyadenylation takes place within the tract bound by these two elements. Within most gene tracts, AATAAA occur at more upstream locations but they are not used for poly-A tailing. Several genes may have alternative polyadenylation sites, however, and thus can be used for the translation of different molecules, e.g., for membrane-bound or secreted immunoglobulin, respectively. The polyadenylation signal may also have a role in signaling the termination of transcription, no matter how much further downstream that takes place. In the nonpolyadenylated histone genes, there is a 6-base pair palindrome that forms a stem for a 4 base loop near the 3′-end of the mRNA, and it is followed further downstream by a short polypurine sequence. The latter may pair with a U7 snRNP that facilitates termination. The U RNA transcripts of eukaryotic RNA polymerase II are not polyadenylated, either. The formation of an appropriate 3′-tail requires that it would be transcribed from a proper U RNA promoter and the transcript would have the 5′ trimethyl guanine cap. The 3′-end of U1 and U2 RNAs is formed by the signal sequence GTTN_0–3AAAPUAGA [PU any purine, PY any pyrimidine] near the end.  transcription termination,  decapping,  polyadeylation signal,  polyA polymerase; Hilleren P et al 2001 Nature [Lond]:413:538.