Histone protein and DNA synthesis in HeLa cells after thermal shock

Exposure of suspension-cultured HeLa cells to a 45 degrees thermal shock resulted in cell inactivation and inhibition of both protein and DNA synthesis. DNA synthesis was inhibited in a biphasic manner with a more sensitive (Do = 7 min) and a less sensitive (Do = 20 min) phase. The less sensitive process was demonstrated to be DNA chain elongation. Transport of thymidine into intracellular pools was significantly less sensitive to thermal shock (Do in excess of 200 min). When HeLa cells were heated at 45 degrees for 15 min there was an 80% inhibition of incorporation of precursors into both DNA and protein with little effect on precursor transport into cellular pools. While the rate of synthesis of whole cell and histone protein (H2a, H2b, H3, and H4) and DNA chain elongation recovered by 6 h after cell heating, total precursor incorporation into DNA was only 0.4 of control levels. The long-term depression of the DNA synthetic rate could not be explained by a cell cycle redistribution, a depression in the total fraction of S phase cells synthesizing DNA, or by a depression in the rate of DNA chain elongation. We conclude that thermal shock results in a long-term depression in the fraction of cell replicons involved in DNA replication.