The sensitivity of RNA polymerases I and II from Novikoff hepatoma (N1S1) cells to 3′-deoxyadenosine 5′-triphosphate

The synthesis of ribosomal precursor RNA in Novikoff hepatoma (NISI) cells is very sensitive to cordycepin (3′-dA). The synthesis of hnRNA, however, is resistant to inhibition by concentrations of 3′-dA that completely block the synthesis of 45S ribosomal RNA precursor. We have examined the RNA polymerases present in these cultured cells with regard to their sensitivity to cordycepin 5′-triphosphate (3′-dATP) in an effort to explain the differential inhibition of RNA synthesis observed in vivo. RNA polymerases I and II were characterized on the basis of their chromatographic behavior on DEAE-Sephadex, as well as the response of their enzymatic activities to ionic strength, the divalent metal ions Mn2$ and Mg2$, and the toxin α-amanitin. For both enzymes the inhibition of in vitro RNA synthesis by 3′- dATP was competitive for ATP. The Km values for ATP and the Ki values for 3′-dATP for the two enzymes were quite similar. RNA polymerase II, the enzyme presumed responsible for hnRNA synthesis, was actually slightly more sensitive to 3′-dATP than RNA polymerase I, the enzyme presumed responsible for ribosomal precursor RNA synthesis. Similar data were obtained when the RNA polymerases were assayed in isolated nuclei. These results indicate that the differential inhibition of RNA synthesis caused by 3′-dA in vivo cannot be simply explained by differential sensitivity of RNA polymerases I and II to 3′-dATP.