Gamma-irradiation induces in HeLa cells radioresistant DNA synthesis

Cells of a suspension HeLa culture at the logarithmic phase of growth were exposed to 60Co-gamma-rays (5 Gy), incubated in the nutritious medium, and in 4 h subjected to repeated irradiation: the dose-response function and the dynamics of DNA synthesis inhibition were determined. It was shown that DNA synthesis was inhibited to a lesser extent after preirradiation, in other words, DNA synthesis was radioresistant. A correlation between this synthesis and reproductive cell death is discussed.     

Correlation between synthesis and methylation of DNA in HeLa cells

For the whole cell cycle the methylation of DNA was studied in synchronized $\text{HeLa}$ cells and in nuclei isolated from them. In the intact cells the methylation of DNA cytosine runs parallel to DNA synthesis. The pattern of DNA cytosine methylation by the isolated nuclei is almost identical to that obtained with the whole cells. Since the isolated nuclei do not synthesize DNA, it is shown that DNA methylation continues for at least 30 min after DNA synthesis is over. No DNA minor thymine is found in the isolated nuclei.     

Histone protein and DNA synthesis in HeLa cells after thermal shock

Exposure of suspension-cultured HeLa cells to a 45° 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 (D₀ = 7 min) and a less sensitive (D₀ = 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 (D₀ in excess of 200 min). When HeLa cells were heated at 45° 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.     

Effects of cordycepin,hydroxyurea and cycloheximide on histone mRNA synthesis in synchronized HeLa cells

The effects of cordycepin, hydroxyurea and cycloheximide on the synthesis of histone mRNA in synchronized HeLa cells were studied by quantitating the RNA, using translation in a rabbit reticulocyte cell-free system. It was found that biologically active histone mRNA is synthesized in the presence of cordycepin. This result strengthens the findings by others that histone mRNA does not contain poly(A). Hydroxyurea and cycloheximide, when used at concentrations that inhibit cellular DNA synthesis, had different effects on histone messenger activity. While polyribosomal messenger activity rapidly declined after addition of hydroxyurea it was not impaired by cycloheximide. These results might help to elucidate regulatory mechanisms involved in the coupled synthesis of DNA and histones in HeLa cells.     

The dependence of DNA synthesis on protein synthesis in HeLa S3 cells

The rate of DNA synthesis in HeLa S3 cells, as measured by incorporation of C¹⁴-labeled thymidine, is strongly dependent on protein synthesis at all times during the S phase. The relation between the rate of DNA synthesis and the rate of protein synthesis is linear when measured two or three hours after reducing the rate of protein synthesis with either puromycin or cycloheximide. The effect is manifested rapidly, is found in both random and synchronized cultures, and is independent of the method of synchronization.     

Effect of n-butyrate on DNA synthesis in chick fibroblasts and HeLa cells

n-Butyrate in low concentrations stops reversibly the proliferation of chick embryonic fibroblasts and of HeLa cells, shutting off DNA synthesis. Extensive acetylation of histones is seen at the same time as inhibition of DNA synthesis. Nuclei from n-butyrate-treated HeLa cells remain inactive in control cytosol; control nuclei are strongly inhibited by cytosol from treated cells.