Serum-induced stimulation of nucleoplasmic and nucleolar transcription in mouse 3T3 fibroblasts revisited.

Transition of 3T3 mouse fibroblasts from a quiescent to a growing state was induced by the addition of 10% fresh bovine serum to the culture medium. The number of DNA-synthesizing cells began to increase 10-11 h after the addition of serum and reached a maximum of 70-80% around 24 h. In quiescent cells, maintained in 0.5% serum, residual RNA synthesis represented mainly nucleoplasmic transcription (hnRNA, 5-S RNA and tRNA). Synthesis of tRNA was 2-3-fold increased by 1-2 h after addition of serum; however, a significant stimulation of hnRNA, 5-S RNA and 45-S pre-rRNA synthesis could only be observed around 4h. The experimental data also revealed a close temporal relationship between the onset of serum-stimulated hnRNA and overall protein synthesis. Determined colorimetrically, the cellular RNA and protein content began to increase by 4-5 h and had doubled by 24 h. Virtually the same results on RNA and protein synthesis were obtained when the experiments were performed in the presence of cytosine arabinoside, an inhibitor of DNA synthesis. From our results we concluded that serum-stimulated overall RNA and protein synthesis preceded by several hours serum-induced S phase and was independent of DNA replication.