Non-specific elongation of cell cycle phases by cycloheximide in rat 3Y1 cells, and specific reduction of G1 phase elongation by simian virus 40 large T antigen.

Partial inhibition of protein synthesis by cycloheximide caused prolongation of G1, S and G2 phases in rat 3Y1 fibroblasts. In cells expressing simian virus 40 (SV40) large T antigen, by infection with SV40 in the previous generation, the prolongation of G1 phase in the presence of cycloheximide was suppressed. However, the prolongation of S and G2 phases in the presence of cycloheximide was not suppressed in cells expressing large T antigen, by infection with SV40 in the current generation. Similarly, when density-arrested cells (cells in G0 phase) were infected with SV40 (either wild-type strain or a mutant deleted in the unique coding region for small t antigen) and reseeded sparsely in the presence of cycloheximide, the cycloheximide-induced delay of entry into S phase was suppressed. In this case, the reduction in [35S]methionine incorporation, that in protein accumulation and that in cell volume increase, were not surmounted by SV40 infection. In T-antigen-negative cells, all the regions in G1 phase seemed to be sensitive to cycloheximide, i.e. they suffered elongation. These results suggest that, in comparison with cells that enter S phase by the action of growth factors, cells expressing large T antigen can enter S phase more efficiently through a quite different process.