Primer Specificity of Ribosome‐Associated Poly(A) Polymerase from Ehrlich Ascites Tumour Cells

The reaction product of the ribosomal poly(A) polymerase [ATP(UTP):RNA ucleotidyltrasferase] is analyzed. Two systems are used in vitro: (a) isolated polyribosomes with endogenous enzyme and RNA primer and (b) purified enzyme with total polyribosomal RNA as primer. In the polyribosome system about 50% of the [³H]AMP label is in poly(A)‐containing mRNA. This RNA displays a heterogeneous size ditribution in the range of 8–30 S with a maximum at about 14 S. Upon denaturation the maximum is shifted towards the 10–S zone. The poly(A) polymerase catalyzes the addition of 12–18 adenylate residues to pre‐existing mRNA poly(A) sequences of 40–160 residues. The [³H]AMP incorporated into poly(A)‐lacking RNA is mainly in a fraction with an electrophoretic mobility corresponding to 4‐S RNA. In the purified enzyme system, specificity towards poly(A)‐containing mRNA is lost to a considerable extent. Only 10% of the [³H]AMP label is retained by oligo(dT)‐cellulose. The bulk of the product is in 18‐S rRNA and heterogeneous small molecular weight RNA. We conclude that the ribosome‐associated poly(A) polymerase is most likely the enzyme responsible for the cytoplasmic polyadenylation of poly(A)‐containing mRNA in vivo.