Poly (A) Polymerase is very selective for the addition of ATP to the 3' end of RNA. However, Poly (U) Polymerase (M0337) will add all four ribo NTPs to RNA. The enzyme is most active with UTP, but it will also add other NTPs at slower rates. However, please note, that Poly (U) Polymerase can exhibit some activity bias based on the 3’ sequence of the RNA. In some of those instances the enzyme will not add the alternative NTPs efficiently.