Histone dimers on the move

Nucleosome assembly onto DNA was presumed to be an exclusively replication-coupled (RC) process. Mounting evidence has revealed that histones are highly dynamic, with extensive replication-independent (RI) nucleosome assembly occurring throughout the cell cycle. In metazoans, high rates of transcription result in the eviction of histone H3 and its replacement with its variant H3.3. We examined whether H2A and its variant H2AV followed a similar mode of behavior and uncovered very different roles for the two histones. We found that H2A mobility is much greater than previously suggested. A cytological approach in cultured cells and in vivo showed that H2A exchanges rapidly between a soluble pool and chromatin at hundreds of sites throughout the Drosophila genome in an RI manner. Only a small fraction of this exchange is the result of transcription. RNAi in cultured cells implicated the FACT complex and the Ino80 ATPase in transcription-independent H2A exchange. We propose that nucleosome packaging is relaxed in euchromatic regions of the genome by rapid and continuous exchange of H2A/H2B dimers. The resulting mobility and flexibility may poise these domains for future transcription. Though H2A is exchanged at transcribed loci, it is not replaced with H2AV. In fact, H2AV demonstrates a behavior that is unique among histones studied thus far. Expression and deposition of H2AV display a developmental delay and the localization of H2AV varies by cell type. A pattern of enrichment at sites adjacent to HP1 in polytene chromosomes implies a cell type-specific modification and a potential role in heterochromatin function. However we do not observe a direct relationship between the histone and the formation of heterochromatin. The mislocalization of phosphorylated H2AV in the absence of the domino ATPase implies a role for the remodeller in positioning a second sub-population of the histone. We propose that differentially- modified H2AV may be used to delineate specific genomic regions during development.