Linker histone-dependent DNA structure in linear mononucleosomes

Abstract

We have examined the binding of the linker histone H5 (LH) to mononucleosomes. Mononucleosomes reconstituted on short DNA fragments display a series of discrete bands on a gel corresponding to various nucleosome positions along the DNA. When a series of engineered H5s with differing extents of the C-terminal tail are bound to these mononucleosomes, the electrophoretic mobilities of the resulting complexes are altered. Not only is there a general increase in mobility upon complex formation, but there is a reduction in the differences in mobility of the most distal nucleosomes. The complexes were also visualized by electronmicroscopy. From these two complementary studies, we conclude the following. (1) Entering and exiting DNAs are uncrossed in the LH-free particles, despite a DNA wrapping of 1.65 to 1.7 turns around the histone core. This results from a bending of the entering and exiting DNA away from each other and the histone surface, presumably as a consequence of electrostatic repulsion. This confirms and extends conclusions derived from our recent examination of the same particles in 3D through cryo-electron microscopy. (2) Binding of the globular domain of H5 increases DNA wrapping to 1.8 to 1.9 turns, but fails to induce a crossing due to an accentuation of the bends. (3) The C-terminal tail of H5 bridges entering and exiting DNAs together into a four-stranded stem over a distance of about 30 bp. The occurrence of such a stem may introduce constraints on models of the 30 nm chromatin fiber.