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Reexamining the role of linker histones beyond 30 nm fibers in a complex chromatin environment


Eukaryotic cells store DNA in the cell nucleus in the form of chromatin. Chromatin is composed of nearly equal parts proteins and DNA. It is both highly compacted and organized into discrete domains within the nucleus. However, the manner in which chromatin is compacted, and domains are organized, remains elusive. The primary players in chromatin compaction are core histones, which bind DNA to form the nucleosome and the basis for 10 nm fibers. Linker histones also play an important role in chromatin compaction. Previous work showed that linker histones are important for the formation of 30 nm structures. 30 nm structures were long held to be folding intermediates for repressive chromatin domains. However, there is little evidence for these structures in most eukaryotic cell types. Instead, chromatin appears to be composed of an interdigitated 10 nm fibers in both repressive and accessible chromatin types. The role of linker histones in 10 nm fibers is not well characterized. Previous work showed that linker histones stabilized 30 nm structures, rendering them inaccessible to binding by additional proteins. In the following, we investigate the behavior of linker histones in an interdigitated 10 nm fiber environment. We use an in vitro model called "condensates" to mimic the formation of 200 nm chromatin domains. We find that linker histones stabilize these condensates by cross-linking chromatin fibers. Importantly, we show that the presence of linker histones does not preclude binding by additional proteins. Linker histones readily bind condensates in ratios above an expected one linker histone per nucleosome. Additional binding by linker histones suggests that 10 nm fibers provide a complex environment in which linker histones dynamically interact with both nucleosomes and linker DNA.  


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linker histones
30 nm fibers


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