The early steps in vaccinia virus infection were studied in HeLa cells which had been treated with actinomycin D (1 μg/ml) and then incubated for several hours in fresh medium prior to infection. Initiation of infection occurred in such cells even though the synthesis of cellular ribonucleic acid and deoxyribonucleic acid (DNA) was severely depressed. Thymidine kinase was synthesized in amounts that exceeded those found in untreated, infected cells. The breakdown of viral “cores” to liberate viral DNA and the synthesis of viral specific DNA-polymerase also occurred but were somewhat delayed. A deoxyribonuclease resembling an exonuclease was made by the infected, pretreated cells. The time course for these events suggested that the genetic code for synthesis of thymidine kinase can be expressed before “cores” are broken down, but the DNA-polymerase can be synthesized only after liberation of the viral DNA. The amount of viral specific DNA-polymerase which was made after infection was proportional to the total number of virus synthesizing sites even beyond the point where all the cells were infected with one infectious particle. A similar relationship was observed for the amount of thymidine kinase formed and for the rate of viral DNA synthesis from 3H-thymidine.
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