摘要：

This chapter describes the salient features of chromatin structure involved in positioning, reviews examples of non-random location of nucleosomes, examines mechanisms that have been proposed for positioning and experimental tests of several of these, and discusses some recent efforts to ascertain whether positioning could and/or does have any effect on the function of DNA in chromatin. There is no experimental evidence for, and sound theoretical arguments against, positioning of nucleosomes for most of any eukaryotic genome. However, strong evidence is accumulating for positioning of some nucleosomes. Almost any mechanism that has been postulated for the positioning has found experimental support in some system. It seems clear that there will be hierarchies in the strength of positioning signals and mechanisms that may lead to variability in the location of nucleosomes in different cell types. However, a positioned nucleosome is located in a precise site relative to DNA sequence in all cells of a given population. In this situation, any particular DNA sequence in the region of the positioned nucleosome would always lie in the same relationship to histones-in the linker or in the core particle, in the central or the peripheral region of the core particle, etc.

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