Fig. 2.

Subtomogram average of chicken erythrocyte fibers. (A) A depiction of the locations and mutual positions of the slices and isosurface visualized in B–H. (B) Computational projection image parallel to the fiber axis through the subtomogram average. Six radially oriented elliptical densities are visible that surround a central density. The size of one of the radially placed densities corresponds to the dimensions of an individual nucleosome. (C) A computational projection image calculated perpendicular to the fiber axis through the periphery of the fiber shows a checkerboard pattern of nucleosomes. (D) 0.6 nm thick longitudinal slice showing the off-superhelical-axis nucleosome juxtapositioning present in the fiber. (E and F) Two 0.6-nm-thick slices, 10 nm apart, showing radially arranged nucleosomes. (G) Isosurface of the subtomogram average shown in perspective view. Two gyres of the fiber are visualized. The fiber axis and nucleosome spacing are indicated. (H) Isosurface of the subtomogram average contoured at two threshold levels to indicate the center of the nucleosome positions at the higher threshold and the appearance of the 30-nm fiber at the lower threshold. The isosurface at the higher threshold shows distinct densities in elliptical form, depicting the center of the positions of the individual nucleosomes. The isosurface at lower threshold has been selected in order to faithfully represent a complete nucleosome and is shown as transparent, pseudocolored according to the cylindrical radius. Red indicates positions at the outer radius of the fiber, while green-blue positions are toward the center of the fiber. Positions of the nucleosomes are well defined.