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.