Conservative segregation of nucleosome core histones.

Density labelling studies have shown that nascent histones are not mixed with parental histones during the assembly of nucleosome cores. However, experiments in other laboratories, examining histone deposition with respect to newly synthesized DNA, have been interpreted as suggesting that a substantial proportion of core histones (greater than 15%) are randomized at each chromatin replication. The data presented here support our previous results in showing that conservatively assembled nucleosome core histone octamers are conservatively segregated over successive cell generations. It is also shown that the nucleosome cores assembled during 1-beta-D-arabinofuranosylcytosine inhibition of DNA synthesis are conservatively segregated for a minimum of five or six cell generations. These results suggest that the nonrandom assembly of nucleosome cores is not merely a coincidence of the mechanism of histone transport into the nucleus and that the conservative mode of nucleosome segregation is a fundamental feature of chromatin replication, one which is stable to modulations in chromatin packaging.