mKSA207 cells, a BALB/c mouse kidney line transformed by a A mutant of SV40, are temperature-dependent for the expression of the ‘standard transformed phenotype’. At the permissive temperature (33.5 °C), the mKSA207 cells resembled wild-type (wt) SV40 transformants; they contained the intranuclear SV40 T antigen, grew to high saturation density in monolayer culture in either 10% or 0.5% serum, and also in methylcellulose suspension culture and became multinucleate in cytochalasin B. At the nonpermissive temperature (39.8 °C), the mKSA207 cells lost some of their transformed properties; they grew only to low density in 10% serum, hardly grew at all in 0.5% serum or in methylcellulose suspension culture, and remained mono- or binucleate in cytochalasin B. At 40 °C in low serum, mKSA207 cells lost the intranuclear T antigen and when fed 10% serum at 39.8 °C, accumulated large amounts of T antigen in the cytoplasm. Derivatives of mKSA207 have been selected at 39.8 °C in liquid medium and methylcellulose suspension culture. The heat-adapted lines, like wt SV40 transformants, exhibited the standard transformed phenotype at both 33.5 and 39.8 °C. It is unlikely that acquisition of temperature-independence for the transformed phenotype was due to reversion of the A gene to wild-type because the heat-adapted cell lines displayed the cytoplasmic T antigen at 39.8 °C, characteristic of the parental mKSA207 cells and SV40 rescued from one of the heat-adapted lines was temperature sensitive for growth. The T antigen levels (complement fixation units per 10 cells) of heat-adapted lines grown at 39.8 °C were comparable to those of mKSA207 cells grown at 33.5 or 39.8 °C.


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