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Selective inhibition of rat liver nuclear RNA polymerase II by actinomycin D in vivo.

It is well known that actinomycin D, a carcinogen, inhibits DNA-dependent RNA synthesis. The interpretation of this inhibition has been that this carcinogen binds specifically to the deoxyguanosine moiety of a DNA molecule, and thus blocks the template function. This paper presents evidence which suggests that this single mechanistic interpretation of actinomycin D action may not be adequate in eukaryotic cells. Thirty minutes after actinomycin D injection (250 microg/100 g body weight), rat liver nuclear RNA synthesis was inhibited by 81% and nucleolar RNA synthesis was inhibited by 98%. In order to determine whether this inhibition is due to an inhibition of DNA template function or of the RNA polymerase activity, the total nuclear free and engaged RNA polymerases were solubilized and the individual RNA polymerase species were partially purified by DEAE-Sephadex column chromatography. It was found that while the overall enzyme activities of RNA polymerase I and III were not affected, there was a selective inhibition of RNA polymerase II activity (42%). This result suggests that actinomycin D, like aflatoxin B1 and N-hydroxy-2-acetylaminofluorene, inhibits nuclear RNA synthesis at multiple sites; it inhibits nucleolar RNA synthesis by blocking the nucleolar DNA template function, and it inhibits messenger RNA synthesis by inhibiting at least partially the enzyme RNA polymerase II activity per se.