The addition of poly(A)-tails to RNA is a phenomenon common to almost all 
organisms. Not only homopolymeric poly(A)-tails, comprised exclusively of 
adenosines, but also heteropolymeric poly(A)-rich extensions, which include the 
other three nucleotides as well, have been observed in bacteria, archaea, 
chloroplasts, and human cells. Polynucleotide phosphorylase (PNPase) and the 
archaeal exosome, which bear strong similarities to one another, both 
functionally and structurally, were found to polymerize the heteropolymeric 
tails in bacteria, spinach chloroplasts, and archaea. As phosphorylases, these 
enzymes use diphosphate nucleotides as substrates and can reversibly polymerize 
or degrade RNA, depending on the relative concentrations of nucleotides and 
inorganic phosphate. A possible scenario, illustrating the evolution of RNA 
polyadenylation and its related functions, is presented, in which PNPase (or the 
archaeal exosome) was the first polyadenylating enzyme to evolve and the 
heteropolymeric tails that it produced, functioned in a 
polyadenylation-stimulated RNA degradation pathway. Only at a later stage in 
evolution, did the poly(A)-polymerases that use only ATP as a substrate, hence 
producing homopolymeric adenosine extensions, arise. Following the appearance of 
homopolymeric tails, a new role for polyadenylation evolved; RNA stability. This 
was accomplished by utilizing stable poly(A)-tails associated with the mature 3' 
ends of transcripts. Today, stable polyadenylation coexists with unstable 
heteropolymeric and homopolymeric tails. Therefore, the heteropolymeric 
poly(A)-rich tails, observed in bacteria, organelles, archaea, and human cells, 
represent an ancestral stage in the evolution of polyadenylation.