Nucleosomes are the fundamental subunits of eukaryotic chromatin. They are not static entities, but can undergo a number of dynamic transitions including spontaneous repositioning along DNA. Since nucleosomes are spaced close together within genomes it is likely that on occasion they approach each other and or collide. Here we have used a dinucleosomal model system to show that the 147bp DNA territories of two nucleosomes can overlap extensively. In the situation of an overlap by 44 bp or 54 bp one histone dimer is lost and the resulting complex can condense to form a compact single particle. We propose a pathway in which adjacent nucleosomes promote DNA unraveling as they approach each other and that this permits their 147bp territories to overlap. These may represent early steps in a pathway for nucleosome removal via collision.
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