Conservation of the dimeric unit of H2A and H2B histones during the replication cycle

The behavior of H2A and H2B histones during the replication cycle has been investigated. H2A-H2B dimer fractions obtained from MH-134SC cells labeled with suitable precursors were fractionated by rate zonal centrifugation in sucrose gradients containing 2 M NaCl. Labeling for one round of replication cycle with an amino acid mixture enriched with dense isotopes and [3H]lysine as a radioactive marker yielded a distinct peak of dense dimer that sedimented faster than the normal dimer. Similar density labeling of cells did not cause measurable alteration in the sedimentation profile of the preexisting normal dimer marked with [14C]lysine. The data suggest conservation of the bulk of the dimeric unit during the replication cycle. Biological significance of the dimeric behavior of H2A and H2B is discussed in favor of the partially conservative model of histone octamer assembly.