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Histones have been both radiolabeled and density-labeled with amino acids in vivo to determine the dynamics of histone-DNA and histone-histone interactions at the replication fork and on active genes. Proteins were uniformly labeled and subsequently chased for three cell generations. During the chase period, H3,H4 tetramers dissociated from the H2A,H2B dimers to re-form nucleosomes with the corresponding nondense histones synthesized during the chase period. These data suggest that the prereplicative nucleosomes are dissolved during advancement of the replication fork with release of associated histones in the form of the H3,H4 tetramers and H2A,H2B dimers. Experiments that involve density labeling of cells in the presence of actinomycin D indicate that the dynamic exchange of H2A,H2B that has been previously described Jackson, V. (1987) Biochemistry 26, 2315-2324 is partially dependent on RNA polymerase movement. These results provide indirect evidence that nucleosome dissolution occurs during transcription. When deposition during replication and transcription is inhibited by simultaneous treatment of cells with cytosine arabinoside and actinomycin D, the majority of the newly synthesized histones are unable to deposit into nucleosome structure. The low level of deposition that is observed has characteristics similar to the deposition of uH2A and uH2B, and it is proposed that conjugation of H2A and H2B by ubiquitin occurs when these proteins are in a free pool within the nucleus. The new H3,H4 tetramers and new H2A,H2B dimers when prevented from depositing are not stable. New and old H3 and H4 intermix to form hybrid tetramers, and a similar intermixing is observed for the H2A,H2B dimers. A model is presented to describe the dynamics of histone-DNA interactions during replication and transcription.
Vaughn Jackson (Tue,) studied this question.