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

Interactions of Polyoma and Mouse DNAs III. Mechanism of Polyoma Pseudovirion Formation

In primary mouse kidney cell cultures infected with polyoma virus, the processes leading to virion and pseudovirion formation were studied. By photometric DNA quantitation, we followed the kinetics of mouse and polyoma DNA synthesis and the formation of low-molecular-weight fragmented mouse DNA (mouse f-DNA). Virus was harvested at different times and analyzed for its proportion of pseudovirions. The following correlations between the intracellular events and the production of virions and pseudovirions were found. (i) Syntheses of cellular and viral DNA were closely linked, both in time and in rates of synthesis. (ii) An increase of mouse f-DNA could only be detected several hours after the onset of mouse and polyoma DNA replication; its formation coincided in time with the appearance of progeny virus. (iii) The proportion of pseudovirions was not dependent on the amount of mouse f-DNA formed, but seemed to be inversely related to the amount of viral DNA synthesized. This was borne out by experiments in which DNA synthesis was partially inhibited by mitomycin C or after a synchronized onset of DNA replication. Under these conditions, virus preparations with a two- to threefold increased proportion of pseudovirions were obtained as compared with those from uninhibited cultures. Virus isolated from the remaining monolayer always had a higher proportion of pseudovirions than virus isolated at the same time from the supernatant medium only; also, the proportion of pseudovirions increased slightly with time after infection. Thus, according to the experimental conditions used, polyoma virus preparations with a low (10 to 20%) or a high (60 to 80%) proportion of pseudovirions can be obtained.

Polyomavirus major capsid protein VP1 is capable of packaging cellular DNA when expressed in the baculovirus system

Using the p2Bac dual multiple cloning site transfer vector, the polyomavirus major capsid protein gene VP1 was cloned for expression in the baculovirus-insect cell expression system. The 5-day-infected cellular lysate from this recombinant preparation was purified by cesium chloride density gradient centrifugation. Capsid-like particles were observed in the resulting preparation. The purified particle preparation was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and was shown to have accurately expressed the polyomavirus VP1 protein as cloned. It was found that the preparation revealed the presence of host histones in the stained gels, which is indicative of DNA packaging. To determine if cellular DNA was being packaged in the particles, Sf9 insect cells were prelabeled with [3H] thymidine. The label was removed, and the cells were subsequently infected with a recombinant Autographa californica multiple nuclear polyhedrosis virus (AcMNPV) carrying the polyomavirus VP1 gene. Upon purification through three cesium chloride gradients and DNase I treatment, capsid-like particles, containing [3H]thymidine-labeled DNA, were isolated which were found to coincide with hemagglutination activity. Studies have indicated that the AcMNPV appears to have the ability to fragment Sf9 cellular DNA. When infected with the recombinant AcMNPV carrying the VP1 gene of polyomavirus, these host DNA fragments are being packaged by the VPI major capsid protein; further, these DNA fragments have been shown to be approximately 5 kb in size, which corresponds to the size of the native polyomavirus genome. These studies demonstrate that the recombinant polyomavirus VP1 protein has the ability to package DNA in the absence of the minor structural proteins VP2 and VP3 and independently of the polyomavirus T antigens.

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.