Atomic force microscopy sees nucleosome positioning and histone H1-inducedcompaction in reconstituted chromatin

We addressed the question of how nuclear histones and DNA interact and form a nucleosome structure by applying atomic force microscopy to an in vitro reconstituted chromatin system. The molecular images obtained by atomic for ce microscopy demonstrated that oligonucleosomes reconstituted with purifie d core histones and DNA yielded a 'beads on a string' structure with each n ucleosome trapping 158 +/- 27 bp DNA. When dinucleosomes were assembled on a DNA fragment containing two tandem repeats of the positioning sequence of the Xenopus 5S RNA gene, two nucleosomes were located around each position ing sequence. The spacing of the nucleosomes fluctuated in the absence of s alt and the nucleosomes were stabilized around the range of the positioning signals in the presence of 50 nM NaCl. An addition of histone H1 to the sy stem resulted in a tight compaction of the dinucleosomal structure, (C) 199 9 Federation of European Biochemical Societies.