Dynamics of unfolding and refolding controlled by the nucleosome repeat length and the linker and core histones

Chromatin is composed of genomic DNA and histones, forming a hierarchial architecture in the nucleus. The chromatin hierarchy is common among eukaryotes despite different intrinsic properties of the genome. To investigate an effect of the differences in genome organization, chromatin unfolding processes were comparatively analyzed using Schizosaccaromyces pombe, Saccharomyces cerevisiae, and chicken erythrocyte. NaCl titration showed dynamic changes of the chromatin. 400-1000 mM NaCl facilitated beads with similar to 115 nm in diameter in S. pombe chromatin. A similar transition was also observed in S. cerevisiae chromatin. This process did not Involve core histone dissociation from the chromatin, and the persistence length after the transition was similar to 26 nm for S. pombe similar to 28 nm for S. cerevisiae, indicating a salt-induced unfolding to beads-on-a-string fibers. Reduced salt concentration recovered the original structure, suggesting folding-unfolding process. On the other hand, the linker histone was extracted from chicken chromatin at 400 mM NaCl, and AFM observed the beads-on-a-string fibers around a nucleus. Unlike yeast chromatin, therefore, this unfolding was irreversible because of linker histone dissociation. These results indicate that the chromatin unfolding and refolding depend on the presence and absence of the linker histone, and the length of the linker DNA. (c) 2007 Wiley Periodicals, Inc.