Deeper 'Go' phase in rat fibroblasts affects the lag time required for T antigen induced DNA synthesis, but not the lag time required for SV40 T antigen expression

In density-arrested rat 3YltsG125 cells, the time required for entry into S phase after serum stimulation is prolonged with increase in duration of the arrest. When these cells were infected with simian virus 40, the time required for expression of T antigen did not change, but the time required for entry into S phase was prolonged with increase in duration of the arrest. The extent of prolongation of the interval between T antigen expression and entry into S phase correlated well with the extent of the prolongation detected with serum stimulation. These observations suggest that: (1) expression of T antigen was not affected by the level of cellular preparedness for entry into S phase, i.e., it did not depend on cellular functions directly involved in the preparedness, and (2) T antigen induced cellular DNA synthesis, at least in part, by a mechanism similar to that of serum-induced DNA synthesis.