Polyoma virus-induced release of inhibition of cellular DNA synthesis caused by iododeoxyuridine

Interactions of polyoma and mouse DNA's. II. Polyoma-induced mouse DNA replication and pseudovirion formation

Secondary mouse embryo (ME) cultures which had been grown prior to infection in the presence of 5-bromodeoxyuridine (BUdR) and 5-fluorodeoxy-uridine were found to be permissive for polyoma virus (16). The DNA extracted from the progeny virus yielded two bands on CsCl isopycnic centrifugation. The light band (LL) contained supercoiled circular (polyoma DNA I), open circular (polyoma DNA II), and linear (polyoma DNA III) molecules, as was seen by electron microscopy. The hybrid band (HL) contained exclusively linear molecules. This DNA was pure, density - labeled, pseudovirion DNA, i.e., fragmented HL mouse DNA. The quantitative comparison of HL and LL polyoma DNA III from six different virus preparations always revealed an excess of HL DNA, the ratio of HL/LL being between 1.2 and 2.2. These results led to the conclusion that in BUdR-prelabeled, polyoma-infected ME cells pseudovirion DNA is excised both from unreplicated and newly replicated regions of mouse DNA.

Interactions of polyoma and mouse DNAs. I. Lytic infection of bromodeoxyuridine-prelabeled mouse embryo cells

On CsCl isopycnic centrifugation of the DNA extracted from secondary mouse embryo (ME) cultures grown in the presence of 5-bromodeoxyuridine (BUdR) and 5-fluorodeoxyuridine (FUdR) for 40 h, 10 to 25% of the DNA was found to be unsubstituted, 70 to 80% was bromouracil-hybrid DNA, and 5 to 10% was heavy DNA. These results together with cell number determinations, autoradiography, and Feulgen microspectrophotometry revealed three types of cells in these cultures: (i) 60 to 80% of the cells replicated their DNA once, divided, and then stopped mitotic activity, (ii) 5 to 10% were going through a second round of DNA replication; whereas (iii) 10 to 30% did not replicate DNA during the BUdR-FUdR exposure. After the transfer of these cultures to normal medium (without BUdR-FUdR), up to 20% of the cells resumed DNA synthesis asynchronously within 60 h, but no increase in cell number was observed. BUdR-FUdR-treated cultures, which were infected with polyoma virus in the absence of the thymidine analogues, supported a lytic infection to the same extent as did untreated ME cultures. This was concluded from the similar number of cells, which were induced to synthesize DNA, from the similar replication rate of the viral DNA, from the similar number of cells containing polyoma capsid proteins, and from the similar yields of progeny virus determined by hemagglutination and plaque formation. Thus, BUdR-prelabeled ME cultures are suitable for the investigation of interactions of the polyoma and mouse genomes during the lytic infection.

Relationship between replication of simian virus 40 DNA and specific events of the host cell cycle

The relationship between replication of simian virus 40 (SV40) DNA and the various periods of the host-cell cycle was investigated in synchronized CV(1) cells. Cells synchronized through a double excess thymidine procedure were infected with SV40 at the beginning or the middle of S, or in G(2). The first viral progeny DNA molecules were in all instances detected approximately 20 h after release from the thymidine block, independent of the time of infection. The length of the early, prereplicative phase of the virus growth cycle therefore depended upon the period of the cell cycle at which the cells were infected. Infection with SV40 was also performed on cells obtained in early G(1) through selective detachment of cells in metaphase. As long as the cells were in G(1) at the time of infection, the first viral progeny DNA molecules were detected during the S period immediately following, whereas if infection took place once the cells had entered S, no progeny DNA molecule could be detected until the S period of the next cell cycle. These results suggest that the infected cell has to pass through a critical stage situated in late G(1) or early S before SV40 DNA replication can eventually be initiated.