Abstract
The onset and rate of semiconservative DNA replication were measured in stimulated cultured rat fibroblasts and their Rous sarcoma virus‐transformed derivatives after a period of serum deprivation. Rat‐1 (tsLA24/RSV) cells initiated DNA synthesis following a shift to the permissive temperature or addition of serum at the non‐permissive temperature. Their rate of DNA replication was unaffected by the presence of serum at the permissive temperature, however, there was a serum requirement at the non‐permissive temperature. The transition probability was less at the permissive temperature, independent of serum, than at the non‐permissive temperature in the presence of serum. The amount of DNA induced to replicate by addition of serum at the non‐permissive temperature or by a shift to the permissive temperature was similar. Using the untransformed Rat‐1 cells and these cells transformed by wild‐type RSV (Rat‐1 (wt/RSV)), it was confirmed that the rate of entry into S phase (transition probability) was always lower in the transformed cell line at both 39° and 35°. In both cell lines the rate of DNA replication was independent of temperature, but the onset was delayed at the lower temperature. These results indicate that in the cell lines examined, (1) serum was able to commit the cells to replicate DNA (alter the transition probability) in both transformed and untransformed cells, but the transforming function was able to supplant a serum‐dependent process during G~1~ necessary for the initiation of DNA replication, and (2) the effects of the transforming function and serum factor(s) on the alteration of the transition probability are not additive, suggesting that the transforming function initiates a process which acts at the level of the commitment to DNA replication which may render the normal serum‐related control mechanisms ineffective in the regulation of growth.