<P> Polyadenylate polymerases are not as ancient . They have separately evolved in both bacteria and eukaryotes from CCA - adding enzyme, which is the enzyme that completes the 3' ends of tRNAs . Its catalytic domain is homologous to that of other polymerases . It is presumed that the horizontal transfer of bacterial CCA - adding enzyme to eukaryotes allowed the archaeal - like CCA - adding enzyme to switch function to a poly (A) polymerase . Some lineages, like archaea and cyanobacteria, never evolved a polyadenylate polymerase . </P> <P> Poly (A) polymerase was first identified in 1960 as an enzymatic activity in extracts made from cell nuclei that could polymerise ATP, but not ADP, into polyadenine . Although identified in many types of cells, this activity had no known function until 1971, when poly (A) sequences were found in mRNAs . The only function of these sequences was thought at first to be protection of the 3' end of the RNA from nucleases, but later the specific roles of polyadenylation in nuclear export and translation were identified . The polymerases responsible for polyadenylation were first purified and characterized in the 1960s and 1970s, but the large number of accessory proteins that control this process were discovered only in the early 1990s . </P>

What is 5 cap and poly a tail