SUMMARYMouse fibroblasts arrested in Go by isoleucine deprivation were inoculated with the autonomous parvovirus minute virus of mice (MVM). Infected cells were released from the Go block by transfer to complete medium and their progression to and through the S phase was monitored. The onset of viral and cellular DNA synthesis coincided, suggesting that cellular factor(s) required for MVM DNA replication became available as soon as cells entered the S phase. Cellular DNA synthesis was reduced to about 60~ by MVM infection. However, this inhibition did not decrease significantly the overall rate of DNA replication in infected cells because it was compensated by concomitant viral DNA synthesis. MVM infection delayed the movement of the cells out of S phase by at least 5 h. At any time post-infection, more than 95~ of both viral and cellular DNA synthesis was sensitive to inhibition by aphidicolin. Since this drug is highly specific for cellular DNA polymerase ~, the data are consistent with a major role of this enzyme in the in vivo DNA replication of autonomous parvovirus. The assembly of 95 ~ of virus progeny particles was concomitant with a late phase of viral DNA replication which accounted for 30 ~ of the total viral DNA synthesized. The inhibition of this residual viral DNA replication by aphidicolin reduced dramatically the size of the burst of infectious particles; this observation concurs with other evidence to suggest that encapsidation is driven by a late replication event sensitive to this drug.

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The incorporation of thymidine into the DNA of eukaryotic cells is markedly depressed, but not completely inhibited, by aphidicolin, a highly specific inhibitor of DNA polymerase alpha. An electron microscope autoradiographic analysis of the synthesis of nuclear and mitochondrial DNA in vivo in Concanavalin A stimulated rabbit spleen lymphocytes and in Hamster cell cultures, in the absence and in the presence of aphidicolin, revealed that aphidicolin inhibits the nuclear but not the mitochondrial DNA replication. We therefore conclude that DNA polymerase alpha performs the synchronous bidirectional replication of nuclear DNA and that DNA polymerase gamma, the only DNA polymerase present in the mitochondria, performs the "strand displacement" DNA synthesis of these organelles.

A radioautographic examination of nuclear DNA synthesis in unirradiated and u.v.-irradiated HeLa cells, in the presence and in the absence of aphidicolin, showed that aphidicolin inhibits nuclear DNA replication and has no detectable effect on DNA repair synthesis. Although the results establish that in u.v.-irradiated HeLa cells most of the DNA repair synthesis is not due to DNA polymerase alpha, they do not preclude a significant role for this enzyme in DNA repair processes.

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