Structure of an H1-Bound 6-Nucleosome Array Reveals an Untwisted Two-Start Chromatin Fiber Conformation.

Summary Chromatin adopts a diversity of regular and irregular fiber structures in vitro and in vivo . However, how an array of nucleosomesfolds into and switches between different fiber conformations is poorly understood. We report the 9.7 A resolution crystal structure of a 6- nucleosomearray bound to linker histone H1determined under ionic conditions that favor incomplete chromatin condensation. The structure reveals a flat two-start helix with uniform nucleosomalstacking interfaces and a nucleosomepacking density that is only half that of a twisted 30-nm fiber. Hydroxyl radical footprinting indicates that H1 binds the array in an on-dyad configuration resembling that observed for mononucleosomes. Biophysical, cryo-EM, and crosslinking data validatethe crystal structure and reveal that a minor change in ionic environment shifts the conformational landscape to a more compact, twisted form. These findings provide insights into the structural plasticity of chromatin and suggest a possible assembly pathway for a 30-nm fiber.