A RNA-dependent RNA-nucleotidyl transferase (RNA-polymerase) has been demonstrated in chick fibroblasts infected with fowl plague virus. This enzyme was not found in uninfected cells. It needed Mg-ions and all four nucleoside triphosphates for its activity. Pretreatment with RNase or addition of RNase prevented the incorporation of [H]GTP into acid insoluble material. DNase had no effect. The enzyme's activity was only slightly inhibited by actinomycin or proflavine. The incorporation of [H]GTP could be stimulated by RNA isolated from rat liver microsomes. It was strongly inhibited by small doses of polyvinyl sulphate or dextran sulphate, and by larger amounts of DNA and protamine sulphate. The enzyme was found in the microsomal and—at a lower activity—in the nuclear fraction of infected cells. enzymic activity could not be detected until 1 hr after infection; it then increased steadily with increasing time. The rate of synthesis, however, resembled that of viral RNA synthesis. The appearance of the enzyme could be inhibited by actinomycin, actidione, or -fluorophenylalanine. If actinomycin was added 2 hr after infection to the cells, viral RNA synthesis ceased completely at 6 hr after infection, in spite of the fact that large amounts of RNA polymerase could be then detected after homogenization.


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