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 nucleotidyltransferase] 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.