The roles of the simian virus 40 tumor antigens in transformation of Chinese hamster lung cells.

Simian virus 40 mutants and deletions between 0.54 and 0.59 map units direct the synthesis of defective 20K t antigens (Crawford et al., 1978). These deletion mutants transformed actively growing CHL cells nearly as efficently as did wild-type virus, in either the focus formation assay or the growth in soft agar assay. In contrast, when CHL cells were in a resting state during infection, the transformation frequency of the mutants relative to wild-type dropped approximately 50 fold. The presence of the phorbol ester, TPA, diminished this difference. CHL cell lines transformed by the deletion mutants and selected by the focus assay grew almost as efficiently in soft agar as lines transformed by wild-type SV40. Both produced tumors in nude mice. The function of the 20K t antigen is discussed.