PAGE 9 OF 15 Nucleic Acids Research, 2007, Vol. 35, No. 16 e106 Figure 5. Four types of chromatin double ﬁbers formed by polygon ﬁbers (red and white). (A) Two ﬁlaments with b sequence (333)n and a = 1088 (as shown in Figure 4B) align and interlock like a zip, in which nucleosomes in one ﬁber ﬁt spatially into the space provided by a swivel-linker between two trinucleosomes in the other ﬁber (framed). (B) Two ﬁbers with closed, negative linker conﬁguration, b sequence (3033)n, a = 1088, coil right-handed around each other forming a double helix. The swivel-linkers are parallel to the axis of the ﬁber, with alternating positions of the red and the white ﬁlament on the outside of the ﬁber. (C) Two ﬁbers coil left-handed around each other by intercalation of every second nucleosome, a varying between 85 and 1208 and b between 72 and 1608. (D) The positions of the intercalated core particles in the ﬁber in (C). (E–G) Two nucleosomal ﬁlaments with b(5) (1808) placed one on top of the other. (E) Nucleosomes stacking face-to-face with every second linker (red and green) touching each other at two crossover points (arrows), which are symmetrically positioned relative to the midpoint of the linker. (F and G) Parallel staggering of the ribbons along the ﬁber axis causes one crossover to be displaced toward the middle of the linker and the other toward the nucleosomal DNA of every second nucleosome, which is located at one edge of the ribbon (arrows). (H and I) A left-handed double helix formed by two ﬁlaments with alternating negative and positive linker conﬁguration folded from a ﬁlament with open linker conﬁguration; 908 < a < 1088; b(+3). (I) The positions of the core particles in (H) with negative (red and white) and positive linker conﬁguration (mauve and gray). (J) Folding of a nucleosomal ﬁlament leading to the conformation in (H). The ﬁrst nucleosome is marked by an asterisk. Asterisks in (C, D, H, I) indicate the ﬁrst and the last nucleosome in a sequence of intercalated nucleosomes. Bar = 30 nm (A–I). Double ﬁbers can also be formed by intercalation of every second nucleosome in ﬁlaments with irregular b sequences. This type of double ﬁber can be considered to be derived from the inter ﬁber contacts between two ﬁlaments at b(5), in which every second linker is exposed on each of the ﬂat faces. If two such ribbons are placed one on top of the other with the nucleosomes being stacked, every linker in one ribbon has two symmetrically positioned crossover points with two linkers in the other ribbon (Figure 5E). If the two ribbons are staggered in parallel with the axes of the ribbons, the linkers slide along each other, one crossover moving toward the middle of the linker, the other moving toward the terminal region of the nucleosomal DNA. The nucleosomal DNA of every second nucleosome in one ﬁlament will thus have a crossover point with the same region at every second nucleosome in the other ﬁlament (Figure 5F–G). Rotation of the nucleosomes counterclockwise from b(5) makes the two ﬁlaments coil left-handed around each other, main- taining the crossovers by intercalation. The ability to form double ﬁbers by intercalation (Figure 5C,D) is not limited to ﬁlaments with closed, negative conﬁguration. In Figure 5H and I is shown an example of a left-handed double helix with a varying between 908 and 1088, and b(+3). This type of double ﬁber is formed by ﬁlaments with alternating