Selective inhibition of initial polyadenylation in isolated nuclei by low levels of cordycepin 5"-triphosphate.

The effect of cordycepin 5'-triphosphate on poly(A) synthesis was investigated in isolated rat hepatic nuclei. Nuclei were incubated in the absence and presence of exogenous primer in order to distinguish the chromatin-associated poly(A) polymerase from the "free" enzyme (Jacob, S.T., Roe, F.J. and Rose, K.M. (1976) Biochem. J. 153, 733--735). The chromatin-bound enzyme, which adds adenylate residues onto the endogenous RNA, was selectively inhibited at low concentrations of cordycepin 5'-triphosphate, 50% inhibition being achieved at 2microng/ml. At least 80 times more inhibitor was required for 50% reduction in the "free" nuclear poly(A) polymerase activity. Inhibition of DNA-dependent RNA synthesis also required higher concentrations of the nucleotide analogue. These data not only offer a mechanism for the selective inhibition of initial polyadenylation of heterogeneous nuclear RNA in vivo by cordycepin, but also provide a satisfactory explanation for the indiscriminate effect of the inhibitor on partially purified or "free" poly(A) and RNA polymerases.