Studies on roles of histone H1 in nucleosome condensation

Abstract

Genomic DNA in eukaryotic cells is packaged with associated proteins into a set of compact chromosomes. The first level of DNA packaging involves the formation of nucleosomes in which chromosomal DNA wraps around core histone proteins. The binding of linker histone H1 organizes the nucleosomal arrays into a more condensed chromatin fibre. In this study we investigated the effect of histone H1 on DNA topological properties, and attempted to elucidate the inherent relationship between the superhelical path of nucleosomal DNA and the supercoiling of linker DNA segments. Atomic force microscopy was used to visualize the formation and disintegration of histone-induced nucleosome macroaggregates. The results obtained suggest the existence of negative DNA supercoiling within the linker DNA. Additional electrophoretic studies were performed to quantitatively determine the degree of DNA supercoiling generated by the binding of histone H1. In order to disclose the structural origin of these supercoils, our FRET-based studies revealed that upon the formation of chromatosomes, two linker DNA segments orient themselves towards each other, which is opposite to the directions of nucleosomal DNA paths on cylindrical histone octamer. This right-handed solenoidal wrap of linker DNA was expected to constrain local positive supercoiling inside chromatosome particle, and be compensated with global negative supercoils introduced in overall chromatin fibres.