Mode of deposition of the histone subtypes during replication.

By using various approaches, we have observed in cycling mouse erythroleukemia cells a characteristic sequential deposition of newly synthesized histones. Control experiments confirmed that this deposition is replicative and is not associated with a process of histone replacement. Interestingly, each of the newly synthesized variants of histones H3, H2B, and H2A was found to deposit simultaneously. This observation indicates that the mechanisms governing the nucleosome assembly do not discriminate among the variants and thus supports the idea that they have equal biological significance. In contrast, a different mode of deposition was found for the regular subtypes H1A and H1B, suggesting a structural and functional difference between these two histones.