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

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