Nucleosome segregation in chromatin replicated in the presence of cycloheximide

Ehrlich ascites tumour cells and L cells were grown in the presence of ¹⁴C]thymidine to label DNA replicated under normal conditions and were then cultured in the presence of cycloheximide and ³H]thymidine to label DNA replicated in the absence of histone synthesis, Chromatin from these cells was digested with micrococcal nuclease and with restriction endonuclease BspRI (an isoschizomer of HaeIII). The rates of digestion of the ¹⁴C-labelled and of the ³H-labelled DNA, and the size and buoyant density of the BspRI-generated chromatin fragments showed that: (1) chromatin replicated in the presence of cycloheximide contained half the normal amount of histones; (2) it did not contain long stretches of naked DNA; and (3) it was organized in nucleosomes distributed along DNA in groups of several particles separated by relatively short stretches of histone-free DNA. Control experiments showed that this could not be the result of a long-distance sliding of nucleosomes.These data suggest a bilateral mode of nucleosome segregation during DNA replication.