Nonrandom assembly of chromatin during hydroxyurea inhibition of DNA synthesis.

Incubation of MSB-1 chicken lymphoblastoid cells with hydroxyurea leads to a rapid 25-fold decrease in the incorporation of [3H]thymidine into DNA and a 5-fold decrease [3H]lysine into the nucleosome core histones. I have investigated whether the distortion in the normal proportion of histone-DNA synthesis results in alterations in the nucleosome assembly process and find that neither the stoichiometry of new histone synthesis nor the deposition is appreciably changed during hydroxyurea incubation. Protein cross-linking and micrococcal nuclease digestion show that the histones synthesized during hydroxyurea treatment form octamer structures and are assembled into typical nucleosome particles. Minor nucleosome subpopulations are found which exhibit altered sensitivity to nuclease digestion and which are depleted in new histones H3 and H4. When MSB-1 cells incubated in hydroxyurea are pulsed briefly with density-labeled amino acids and [3H]lysine, the radiolabeled core histone octamers formed are as dense as individual monomer histones. These results suggest that the newly synthesized histone octamers are uniformly dense and do not contain mixtures of new and old histones. Thus, histones synthesized during hydroxyurea incubation are deposited nonrandomly and do not exchange with preexisting histones.