Fig. 7: Structural insights and an assembly model of the H5-chromatin fiber.

a An illustrated cartoon diagram displaying the structural insights into H5-bound dodecanucleosomal fiber. The structural asymmetries of H2B-NTD (purple), H2A-CTD (yellow), and the interplay between the extended H2A C-tails with either H5-GD (magenta) or H5-CTD (green) are highlighted by different colors, conferring a polarity to the chromatin fiber. N1 and N3 show an opposite polarity as indicated by red arrows. H5 (NTD: orange, GD: magenta, CTD: green) binds to the nucleosome off-dyad in a three-contacts mode, while the H5 of two adjacent stacked nucleosomes are arranged with a “head-to-head” polarity within the tetranucleosome unit. The tetranucleosomal units are connected by a pair of interactions between H4 N-tails (green) and their adjacent acidic patches (red dots), and twisted through the H5–H5 interactions between tetranucleosome units (not shown for viewing purposes, see b). b A model illustrating an assembly pathway from the open nucleosomal arrays into the compacted chromatin fiber with a two-start helical structure twisted by tetranucleosomal units. The switch from on-dyad binding (orange) to off-dyad binding (magenta) of H1/H5 (as shown in Fig. 2c) plays a critical role in the assembly of the chromatin fiber. c A density map of the chromatin fiber with 48 nucleosomes and 177-bp nucleosome repeat lengths (left) and its corresponding atomic model (right), built by directly stacking the cryo-EM structure of the dodecanucleosomal H5-chromatin fiber on top of each other to form a continuous fiber.