Mutants of SV40 with an altered small t protein are reduced in their ability to transform cells.

Mutants of SV40 with deletions of various sizes mapping between 0.54 and 0.59 on the genome grow at a rate equal to or slightly slower than that of wild-type virus, in a range of host cells. Their ability, however, to induce transformation in several mouse, rat and rabbit cell lines is impaired. The extent of transformation observed is dependent upon the assay used to measure it, but in general, the ability of the mutants to transform falls as the size of the deletion increases. In addition, rat embryo fibroblasts transformed by deletion mutants have fewer of the characteristics of a fully transformed phenotype (for example, growth in low serum, increased saturation density, growth in semi-solid medium) than those transformed by wild-type virus. During lytic infection, immunoprecipitable T antigen produced by the deletion mutants is of the same size as that seen during infection with wild-type virus, and is present at a similar level. Mutant virus-coded small t protein, however, is reduced in size compared with that from wild-type virus. For each mutant, the reduction in protein size is dependent upon the amount of DNA deleted, but not on the relative position of the deletion in the genome. These results demonstrate that the DNA sequences mapping between 0.54 and 0.59 on the viral genome code for the small t protein, and that SV40-induced transformation is at least partially dependent upon the expression of this protein.