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We have characterized the viral sequences integrated in a polyomavirus-transformed mouse cell line, Py-3T3 (clone Py-6), and followed their excision and packaging upon superinfection. The polyomavirus sequences contained in Py-6 cells are present as a single insert of nonidentical tandem copies which includes, in addition to a normal middle T-antigen-coding region, some very rearranged sequences. Infection of Py-6 cells with polyomavirus strains encoding a normal large T antigen leads to the reproducible recovery in the resulting viral stock of specific defective viral genomes. The defective genomes contain a wild-type coding region for middle and small T antigens and intact viral origin and enhancer sequences. The remainder of the viral genome is rearranged or lost, so that there is no capacity to code for large T antigen or viral capsid proteins. The recovered defective sequences are also found integrated in Py-6 genomic DNA. Presumably, in infections of Py-6 cells, large T antigen, provided by the superinfecting virus, amplifies and excises the integrated viral sequences. The superinfecting helper virus must also produce viral capsids for packaging of the defective viral DNA and thus provides a means to shuttle the defective sequences from the mouse cells into other hosts, such as rat cells. In the latter host, the defective sequences are able to induce transformation.

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A mutation in polyomavirus large T antigen which affects viral DNA synthesis was discovered in strain NG59RA (RA). The effect was most visible in nonpermissive cells. Although a substantial yield in DNA synthesis is normally observed in infections of Fischer rat cells when these are maintained at 33°C (D.