Induction of polyomavirus DNA replication by carcinogens in polyomavirus-transformed rat cells: evidence that the viral enhancer is not the primary target in the induction pathway.

In the polyomavirus (Py)-transformed rat cell line designated LPT, replication of the integrated Py DNA can be induced by exposure of the cells to carcinogens. In view of the observation that enhancer elements are essential components of the Py origin of replication, it appeared plausible that the induction is triggered by synthesis or modification of an enhancer-binding protein which is required for activation of the viral origin. To test this hypothesis, we have used a plasmid containing a modified Py origin (test plasmid), in which the Py enhancer has been replaced with five repeats of the yeast GAL4 upstream activating sequence, and a plasmid encoding the GAL4 transcriptional activator protein. Previous studies in which these two plasmids were cotransfected into mouse cells that are permissive for Py showed that the GAL4 protein can transactivate the modified Py origin and cause replication of the test plasmid. When similar cotransfection assays were performed in LPT cells, no replication of the test plasmid was observed unless the cells were exposed to the carcinogen mitomycin C subsequent to the transfection, in which case replication of the test plasmid was induced. Control experiments showed that even though the GAL4 protein was required for the induction, its concentration was not affected by the exposure to mitomycin C. These results indicate that the primary target in the induction pathway is not an enhancer-binding protein; instead, the induction appears to be triggered by changes in other components of the replication initiation complex which may be associated with the origin core.