Chromatin assembled in the presence of cytosine arabinoside has a short nucleosome repeat.

Incubation of MSB cells with cytosine arabinoside (1-beta-D-arabinofuranosylcytosine, ara-C) inhibits 3H-thymidine incorporation into nascent DNA while nucleosome core histone synthesis proceeds in molar stoichiometry at about 20% of control rates. The excess nascent histone is incorporated into chromatin and nucleosome cores are assembled normally on the small amount of DNA which is synthesized at submaximal levels of ara-C. This DNA becomes packaged into a shortened nucleosome repeat, however. These results indicate that the nucleosome core is a strongly conserved unit of chromatin replication and suggest that the stoichiometry of nascent histone to DNA may be one factor influencing the establishment of the nucleosome repeat length. It cannot be the only factor, however, since the closely packed nucleosomes made in the presence of ara-C begin to return to their normal spacing within six hours after reversal.