Growth control in simian virus 40-transformed rat cells: temperature-independent expression of the transformed phenotype in tsA transformants derived by agar selection.

Fisher rat fibroblasts (FR 3T3), transformed with the tsA30 mutant of simian virus 40 and selected by colony formation in soft agar, maintained the transformed phenotype at high temperature, whereas most transformants isolated from foci were found to undergo a phenotypic reversion toward the normal state in their saturation density, ability to grow in soft agar, and rate of 2-deoxyglucose transport. The temperature-independent phenotype observed in agar-selected transformants was not due to a reversion of the viral mutation. These results, similar to those previously obtained with polyoma virus tsa mutants, further suggest that two distinct mechanisms may operate in both cases for maintaining the transformed phenotype. Immunofluorescence studies suggested a different regulation of T antigen synthesis in these two classes of transformants.