Irradiation by γ-rays of purified FV3 led to a dissociation between the capacities for infectivity, for nuclear-associated (replicating) DNA, and for cytoplasmic DNA replication in chick embryo fibroblasts. When the amount of DNA synthesis induced by irradiated virus was expressed as a percentage of that induced by nonirradiated virus, the inactivation of the capacity for DNA replication (nuclear as well as cytoplasmic fraction) followed a simple exponential law. The rate of inactivation of the capacity for nuclear-associated DNA replication (<tex-math>${\rm D}_{37}=8\times 10^{5}\ {\rm R}$</tex-math>) was half that of the capacity for cytoplasmic DNA replication (<tex-math>${\rm D}_{37}=4\times 10^{5}\ {\rm R}$</tex-math>). This indicates that the synthesis of DNA which appears in the cytoplasm requires more cistrons than the synthesis of the nuclear-associated DNA, and that the passage from cytoplasmic DNA to the virions requires further protein synthesis. Irradiation with γ-rays did not affect the capacity of FV3 for inhibiting host-cell DNA replication.

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