polyadenylation

Polyadenylation is a stage of RNA processing in which the 3’ end of the pre-mRNA is cleaved before a stretch of adenosines (mol) are added to the end of the molecule. Employment of alternative polyadenylation sites can result in the insertion or deletion of sequences that control the stability of the mRNA, and thus the level of protein expression. The polyA tail is also involved in initiation of translation.

Eubacterial poly(A) tails in RNA: short.
Archaeal poly(A) tails in RNA: short (avg. 12 bases in length).
Eukaryotic poly(A) tails in RNA : long.

Table  Comparisons of Eubacteria, Archaea, and Eukaryotes :

MOLECULAR BIOLOGY: ON TRANSCRIPTION TERMINATION: "A defining feature of mRNAs is a tail composed of a long string of adenosine nucleotides -- the poly(A) tail. This is not encoded by the gene but is added following cleavage of the nascent RNA transcript. Molecular factors that recognize the cleavage site, and cut the RNA, bind to a regulatory region of the transcribing RNA polymerase II called the carboxy-terminal domain (CTD). This interaction is important for recruiting the factors to the nascent transcript. In turn, these factors must be off-loaded from the polymerase onto the RNA at their site of action for transcription to be terminated(4,5). In fact, recognition of the cleavage site has been believed to be the key step in termination -- but the new data(1-3) show that this isn't the whole story."