PAGE 7 OF 15 Nucleic Acids Research, 2007, Vol. 35, No. 16 e106 Figure 3. Selected conformations of star and polygon ﬁbers with repeated b sequences. (A) A star ﬁber with b sequence (11)n showing collisions between every ﬁfth nucleosome. (B–I) Polygon ﬁbers. (B and C) (22)n, (C) in a compact form as seen from the side and from the end. (D) A coil formed by (44)n. (E and F) A polygon ﬁber with the approximate sequence (222)n, as shown in side-on view from two angles diﬀering by 908 (E), and in a more compact form (F). (G–I) The ﬁber (1111)n as seen in side view from two angles and from the end (G). One and the same nucleosome is marked by an asterisk. (H) Model of the ﬁber in (G, F) with partially overlapping hexanucleosomal loops. The ﬁrst four nucleosomes ( 1 1 1 1) in each repeat are painted in same color in the sequence red, green, blue, yellow and red (top to bottom) and numbered 1–4. (I) A loop formed by b sequence (3333)n. a = 36 and 1088 in the star- and polygon ﬁbers, respectively, except in (E, F) where a = 908. Bar = 30 nm. of a. Thus, even in a ﬁlament with as little variation in b as (11)n every ﬁfth nucleosome is touching each other (Figure 3A). Regular helices are formed by polygon ﬁbers with alternating b(+)- and b( )-linkers, which make the ﬁlament shift repeatedly between forward and backward direction. Fibers with b sequence (22)n and (33)n can be compacted into short tubes by very small changes in a and b (Figure 3B and C), while the coiled ribbon at (44)n cannot be compacted (Figure 3D). Another type of helix, which can be circumscribed by an ellipsoid cylinder, is formed by the triple repeat (222)n (approximate values). This ﬁber can be slightly condensed by minor adjustments of the size of Z1 (Figure 3E and F). The shift between forward and backward direction in the polygon ﬁber, caused by b(+) and b( ), leads to a diﬀerent type of ﬁber when the sign of b is changed at every second linker, as shown in Figure 3G. The occurrence of b( ) linkers in the repeated b sequence (1111)n thus causes the ﬁlament to change direction at every second nucleosome, forming a ﬁber with a 3-fold axial symmetry and partially overlapping loops of six nucleosomes ( 1 1 1 1 1 ) (numbered 1-2-3-4-1-2 in Figure 3H). Only a single nucleosome in each loop