Time-dependent changes in H1 content, H1 turnover, DNA elongation, and the survival of cells blocked in early S phase by hydroxyurea, aphidicolin, or 5-fluorodeoxyuridine

Abstract

Cells were synchronized in G1 by isoleucine deprivation and then released into medium containing 1 mM hydroxyurea (HU), 5 .mu.g mL-1 aphidicolin (APC), or 1 .mu.g mL-1 5-fluorodeoxyuridine (fl5dU). Coulter volume, content of histone H1 per unit DNA, turnover of histone H1, the extent of DNA elongation, and the survival of cells were measured as functions of time after release into the presence of the drugs. Among the concentrations used in the experiments, the new drugs differ in their toxicity (fl5dU > HU > APC), induction of unbalanced cell growth, and the distribution of new DNA fragment sizes allowed during block, but they all (1) allow cells to enter S phase, (2) cause similar time-dependent losses of histone H1 per unit DNA which begin as synchronized G1 cells begin to enter S phase, (3) retard DNA elongation beyond replicor size, and (4) retard the turnover of histone H1. The results indicate that loss of histone H1, inhibition of histone turnover, the retarded ligation of newly replicated DNA into bulk chromatin, and chromatin structural changes may be part of the cell''s general response to inhibition of DNA replication. Since transient S phase block increases the frequencies of gene amplification [Mariani, B.D., and Schimke, R.T. (1984) J. Bio. Chem. 259, 1901-1910] and sister chromatid exchanges (SCE) [Rainaldi, G., Sessa, M.R., and Mariani, T. (1984) Chromosoma 90, 46-49], the observed changes in H1 content and chromatin organization may also be essential features of gene amplification and SCE.