Nucleosomes are forced out from the promoters for gene expression, and later reassembled for repression. Disagreement between studies on histone replacement dynamics in Drosophila, Saccharomyces and Physarum, lead this group to address this two questions at a genomic scale: (1) Is there evidence for general transcription-dependent H3 turnover? and (2) Are there additional mechanisms for histone turnover? To this end, they compared the ratios of two systems carrying histone H3 protein fusions: A constitutively Myc-H3, and an inducible Flag-H3. First, turnover rates were measured in arrested G1 cells using ChIP-on-chip of the fusions, and hybridized to an array covering 4% of the genome (the turnover rate was defined as the number of H3 replacements per unit of time). They found that nucleosomes over protein- coding regions were "coldest" (meaning that they rarely turn over), whereas promoter region nucleosomes were "hotter" (with faster replacement rate). These results were confirmed by repeating the experiment in unsynchronized yeast cells. Analysis of RNA polymerase II occupancy of coding regions was found to be correlated with the variation of histone replacement rates in protein- coding regions; although authors mentioned that it was unlikely that this turnover is only a result of the polymerase activity. Altogether, they concluded that "erasure of histone marks ... by rapid turnover delimits the spread of chromatin states"
Dion MF. et al. 2007. Science 315:1405-8
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