Nucleosomes can invade DNA territories occupied by their neighbors

Nucleosomes can be closely spaced in vivo , suggesting that they may on occasion approach one another or even meet. Using in vitro dinucleosomal model systems, positioned nucleosomes, as well as nucleosomes in the process of being repositioned, are now shown to overlap, forming single, compact particles, with one histone dimer ejected in the process. The potential relevance to remodeling processes is discussed. 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. As nucleosomes are spaced close together within genomes, it is likely that on occasion they approach each other and may even collide. Here we have used a dinucleosomal model system to show that the 147-base-pair (bp) 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 147-bp territories to overlap, and we suggest that these events may represent early steps in a pathway for nucleosome removal via collision.