"Rescued" SV40: increased transforming efficiency in mouse and human cells.

Plaque-size mutants of simian virus 40 (SV40) have been characterized with respect to their ability to transform mouse and human cells in vitro. Large plaque mutants of SV40 were extremely inefficient at transforming mouse or human cells, requiring approximately 50 times more infectious particles per transformation event than did the small plaque mutants. The minute plaque SV40 was the most efficient, requiring less than 10(3) plaque-forming units per transformation in mouse Balb/3T3 cells and only 1.5 x 10(4) plaque-forming units per transformation unit in the "susceptible" human strains. Both small and minute plaque virus were unable to help the large plaque virus to transform. Virus "rescued" from transformed 3T3 cells by cocultivation with monkey kidney cells was more efficient at transformation than the parental type. The increased efficiency could be shown for each plaque type and in human as well as in mouse cells. The property of "high efficiency transformation" is stable through at least two passages in monkey kidney cells.