Treatment with 5-bromodeoxyuridine (BrdU) of nonproducer BALB 3T3 cells (KA31 cells) transformed by the Kirsten strain of murine sarcoma virus (Ki-MSV) induced the synthesis of DNA polymerase-containing particles which band in sucrose gradients at a density of 1.16 and utilize poly(I) (dC)12-18, a template-primer specific for RNA-directed DNA polymerase. In order to establish optimal conditions for virus induction, the induction process was studied by analysis of the culture fluid for polymerase activity and the induced cells for viral proteins by immunofluorescence and virus particles by electron microscopy. By varying the concentration and time of exposure to BrdU, it was found that the maximal level of polymerase activity was induced when cells were treated with 20 μg of BrdU for a 24-hr period, 24 hr after seeding. Under these conditions about 15% of the cells were induced to synthesize viral antigens detected with anti-Moloney MSV rat serum. The level of polymerase activity in the culture fluid and viral antigen in the cells became maximal at 3 days after BrdU treatment and decreased rapidly. Electron microscopy showed a large increase in the proportion of cells containing incomplete virus particles and in the number of particles per cell at 2 and 3 days after BrdU treatment. At 3 days after treatment, 82% of the cells contained incomplete virus particles within dilated vesicles of the endoplasmic reticulum as compared to 18% for untreated cultures. A high level of virus production could be maintained for at least 15 days by continuous maintenance of cells on 4 μg/ml of BrdU. It is not clear whether the mechanism involves continuous reinduction of virus or overcoming the normal inhibition of virus multiplication in these cells. Under these conditions, BrdU reversed the rounded, transformed appearance of KA31 cells to a flat fibroblastic morphology.
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