Figure 3.

Nucleosomes and R-loops share in topological relief duties. A, positive supercoiling (red) traveling ahead of the translocating RNAP (shown here localized to the linker DNA) destabilizes nucleosomes, favoring their extrusion. Nucleosome release frees one negative supercoil, mitigating the buildup of positive superhelicity. Reassociation of the nucleosome behind the polymerase is favored by the transcription-driven negative supercoiling; sequestration of one negative supercoil upon nucleosome reformation relieves that stress. B, the formation of an R-loop results in two main consequences. First, nucleosome redeposition is inhibited. Second, the accompanying relaxation of the upstream region may weaken surrounding nucleosome-DNA contacts.