Mapping nucleosome locations on the 208-12 by AFM provides clear evidence for cooperativity in array occupation.

Concatameric sea urchin 5S rDNA templates reconstituted with histones provide very popular chromatin models for many kinds of in vitro studies. We have used AFM to characterize the locational aspects of nucleosome occupation on one such array, the 208-12, by determining the internucleosomal- and end-distance distributions for arrays reconstituted to various subsaturating levels with nonacetylated or hyperacetylated HeLa histones. A simulation analysis of the experimental distributions confirms the qualitative conclusions and provides quantitative parameter values for the identified features. For nonacetylated arrays, the end-distance data demonstrate the nucleosome positioning ability of the 5S sequence and detect an enhanced preference for nucleosomes to bind at DNA termini. The internucleosomal-distance data provide clear evidence for cooperativity in nucleosome location on these templates, detectable even at subsaturated loading levels. Hyperacetylated arrays show no change in the preference of nucleosomes to bind at termini and a slight change in nucleosome positioning behavior but, most strikingly, little or no evidence for cooperativity in nucleosome location. Thus, acetylation of the N-terminal histone tails abolishes the cooperativity.