Preferential association of newly synthesized H3 and H4 histones with newly replicated DNA.

The assembly of newly synthesized histones into chromatin during replication of MH-134SC cells was studied. Cells pulse-labeled with iododeoxyuridine and [3H]lysine were mixed with an equivalent number of normal cells labeled with [14C]lysine. Nuclease chromatin obtained from pooled cells was fractionated by buoyant-density centrifugation in a gradient containing Metrizamide and 3-iodo-1,2-propanediol. Histones extracted from heavy and normal chromatin regions of the gradient were fractionated by acid-urea gel electrophoresis, and 3H/14C ratios of individual histones were compared. The results showed highly preferential association of newly synthesized H3 and H4 with newly replicated DNA.