Regulation of viral transciption and tumor antigen expression in cells transformed by simian virus 40

We have studied the expression of simian virus 40 (SV40) specific tumor antigen (T-antigen) and viral RNA in SV40-transformed mouse 3T3 cells that are temperature-sensitive for the expression of the transformed phenotype (ts SV3T3). Although transformed by wild-type SV40, ts SV3T3 cells at 32 degrees behave like standard transformants, while at 39 degrees they became arrested in G1 after reaching saturation density or under conditions of serum starvation. ts SV3T3 cells at 32 degrees or exponentially growing at 39 degrees are uniformly T-antigen positive. However, after G1 arrest at 39 degrees the majority of the cells becomes T-antigen negative. Induction of proliferation in the resting cultures results in the reappearance of T-antigen in most of the cells, concomitant with the induction of DNA synthesis. The reason for the disappearance of T-antigen from ts SV3T3 cells arrested in G1 seems to reside in a transcriptional control operating on the integrated viral DNA, since these cells contain no appreciable amounts of SV40 specific RNA. Viral RNA can be easily detected in cells growint at 32 degrees or at 39 degrees. The results suggest that transcription of the viral genome in SV40-transformed cells is cell-cycle-dependent.