Figure 4.

Juxtaposition of DNA segments enforced by chromatin architecture. (A) Nucleosomes accommodate to positive helical tension (+) by adopting a meta-stable conformation in which H2A–H2B dimers and the H3–H4 tetramer reorganize and shape a right-handed path for DNA. The entry and exit DNA segments of this nucleosomal conformation might configure an ideal positive DNA crossing to be targeted by topoisomerase II. DNA transport would relax the helical tension and revert the chiral transition of the nucleosome. This scenery might explain why topoisomerase II relaxes helical tension in nucleosome arrays as efficiently as in naked DNA. (B) In absence of DNA helical tension, DNA juxtaposition could be tailored by neighbouring DNA–protein interactions. DNA transport would then result in supercoiling (or knotting). The reaction would be analogous to that of DNA gyrase. The only difference is that protein–DNA interactions enforcing the juxtaposition of a T-segment are established outside rather than inside the topoisomerase–DNA complex.