T antigen and initiation of cell DNA synthesis in a temperature-sensitive mouse line transformed by an SV40ts a mutant and in heterokaryons of the transformed cells and chick erythrocytes

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Abstract
The role of SV40 gene A product in initiation of cellular DNA synthesis was investigated, using a mouse kidney line [mKSA207] transformed by SV40ts A207. mKSA207 cells were temperature sensitive for growth, lost SV40 T antigen (Tag) when incubated in low serum at 40°C, and accumulated Tag in the cytoplasm when fed 10% serum and incubated at the nonpermissive temperature (39.7°C). Following serum addition, the percentage of mKSA207 cells synthesizing DNA was essentially the same at nonpermissive (39.7°C) and permissive temperatures (33.5°C). The cells entered S phase asynchronously at both temperatures, but most cells entered S within 16 h, and before Tag accumulated. mKSA207 synchronized by a double thymidine block also synthesized DNA at 39.7°C and entered a second S phase. Tag-depleted or Tag-synchronized mKSA207, when fused with chick erythrocytes (CE), activated CE DNA synthesis. At nonpermissive temperatures (39.7°C), 40% of CE nuclei in heterokaryons with Tag-depleted mKSA207 displayed3H-thymidine-labeled nuclei 28–40 h after fusion, when only 12% of CE nuclei were Tag+. The experiments indicate that SV40 gene A product probably does not have a direct role as initiator of cellular DNA synthesis.