Amplification of integrated viral DNA sequences in polyoma virus-transformed cells.

Polyoma virus (Py) transformation of rat cells requires integration of viral genomes into the host DNA, which generally occurs in a partial or full head-to-tail tandem arrangement. The instability of this structure was previously demonstrated by the high rate of loss of integrated Py genomes in the presence of viral large tumor (T) antigen. We now show that integrated Py DNA sequences can also undergo amplification. We studied two rat cell lines transformed by the ts-a Py mutant, which codes for a thermolabile large T antigen. In a derivative of the ts-a H6A cell line, we have observed loss of full-length Py DNA molecules from the integrated tandem ("curing"), accompanied by the creation of new tandem repeats of two segments of viral DNA corresponding to 38% and 10% of the viral genome, each containing the origin of DNA replication. In the ts-a H3A cell line, which contains an integrated partial tandem of about 1.3 viral genomes with three distinct deletions, propagation at 33 degrees C resulted in the generation of full tandem repeats of a 94% Py DNA "unit" (including two 3% deletions), an 85% "unit" (including a 3% and the 12% deletion), or both. Amplification of integrated viral DNA was not observed in cells propagated at 39.5 degrees C, the nonpermissive temperature for large T antigen function. Amplification of integrated Py DNA sequences thus requires an active large T antigen and can generate a full tandem of integrated viral DNA molecules long after the initial integration event.