Temporal relationships of chromatin protein synthesis, DNA synthesis, and assembly of deoxyribonucleoprotein.

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Abstract
Chromatin assembly was investigated using human HeLa S-3 cells in terms of the sites on DNA where newly synthesized chromatin proteins associate. Chromatin from cells labeled with [14C]-BrdUrd [5''-bromodeoxyuridine] and [3H]lysine was fixed with formaldehyde and resolved in CsCl gradients. By varying the spacing of the labeling intervals of the 2 isotopes so as to encompass all possible periods in S-phase, the association of labeled, newly synthesized proteins on newly synthesized (BrdUrd-substituted) or preexisting chromatin DNA was determined. Newly synthesized chromatin proteins predominantly associated with nonreplicating DNA. Possible mechanisms by which cells recycle preexisting chromatin proteins to restore the protein content of newly synthesized DNA are discussed.