Effects of sequence alterations in a DNA segment containing the 5 S RNA gene from Lytechinus variegatus on positioning of a nucleosome core particle in vitro.

Reassociation of a 260-base pair cloned fragment of Lytechinus variegatus DNA with core histones has been shown to give rise to a uniquely positioned nucleosome (Simpson, R. T., and Stafford, D. W. (1983) Proc. Natl. Acad. Sci. U. S. A. 80, 51-55). In an attempt to define the features that dictate the unique positioning of the nucleosome, we have constructed a number of mutants of this DNA sequence. The ability of these mutants to form positioned nucleosomes was analyzed by DNase I digestion of the DNA after reassociation with chicken erythrocyte core histones. While all the mutants were efficiently incorporated into core particles, not all of these modified sequences were capable of forming a positioned nucleosome. Of the 13 mutants examined, 7 fell into a class that gave rise to nucleosomes in which no unique positioning could be demonstrated. While no specific feature of the DNA sequences has been identified as the critical factor in allowing, or dictating, the formation of positioned nucleosomes, our results do indicate that the region 20-30 bases either side of the center of the core particle appears to contain the major elements necessary for positioning. Additionally, these studies clearly show that differences in the digestion of naked and core particle DNA are related to specific interactions of the DNA and histones rather than to an altered specificity of the enzyme induced by the presence of the proteins.