Heterogeneous assembly of nascent core histones to form nucleosomal histone octamers in mouse FM3A cells

We investigated the assembly of newly synthesized histones into nucleosomal histone octamers using an isopycnic centrifugation analysis of these proteins of mouse FM3A cells that had been labeled by culture with heavy amino acids. Cross-linked histone octamers or non-cross-linked core histones were separated electrophoretically from other proteins on sodium dodecyl sulfate/polyacrylamide gels before being banded to equilibrium in cesium formate/guanidinium chloride density gradients. The density of core histones rapidly came to equilibrium after culture of cells in the medium containing heavy amino acids for a time as short as 30 min, indicating that the density of histone octamers in the density-labeled cells reflects the content of these new (dense) core histones relative to old (light) ones in the octamer assembly. Cross-linked histone octamers from these cells were of heterogeneous densities as judged from a broad band in a density gradient. The position and width of the band was essentially unaffected, though the peak position of the density distribution within the band moved progressively from a less to a more dense area, when the time of culture was prolonged from 2 h to 16 h. It is concluded, therefore, that new and old histones are assembled into histone octamers in heterogeneous ratios in contradiction to a conservative or semi-conservative assembly model and it is also suggested that incorporation into chromatin of new histones is not necessarily restricted to newly replicated chromatin regions.