Effects of withdrawal of a serum stimulus on the protein-synthesizing machinery of cultured fibroblasts.

3T6 cells resting in medium containing 0.5% serum were stimulated to prepare for multiplication by the addition of medium containing 10% serum. After a number of hours, when the rate of preribosomal RNA synthesis, total RNA content (mainly ribosomal), and the cytoplasmic content of poly A (a measure of poly A(+) mRNA) were considerably elevated, the serum-rich medium was withdrawn, and the original medium replaced. The rate of preribosomal RNA synthesis began to drop within 30 minutes, but required a much longer time to fall to a new resting level. When the serum-rich medium was withdrawn after 12 hours of stimulation, the total RNA content required 12-18 hours to fall to the resting level, whereas cytoplasmic poly A content and the rate of protein synthesis declined more rapidly, reaching a new resting level within eight hours. During the 12 hours following withdrawal of the serum-rich medium an appreciable fraction of the cells initiated DNA synthesis. Presumably, the cellular preparations for DNA synthesis cannot be immediately reversed because of the inertial factors related to the protein-synthesizing machinery.