Torque response of DNA loaded with (H3.3-H4)₂ tetrasomes.

<p>(A) Diagram of the eMTT configuration used in these experiments. The eMTT resembles the FOMT configuration, but additionally has two pairs of Helmholtz coils placed around the flow cell to permit the application of torque in the horizontal plane. (B) The torque stored in DNA loaded with 5 tetrasomes plotted as a function of the number of applied rotations, <i>θ</i>. The black squares represent the data for a bare DNA molecule, prior to assembly. Following assembly, the torque response of DNA loaded with tetrasomes is measured by decreasing the number of applied turns from +17 to -17 (red triangles, labeled by ‘1’). Consecutively, the torque response of DNA loaded with tetrasomes is measured in the opposite direction by increasing the number of applied turns -17 to +17 (green diamonds, labeled by ‘2’). The solid lines are segmented fits to the plateau regions (with slope 0) and to the sloped regions in which torque is built up. The widths of the plateaus for positive (red) and negative (green) rotation directions are 7.5 ± 1 and 5.9 ± 1 turns, respectively, as determined from the intersections between the segmented fits. The applied force is ~0.8 pN. (C) The DNA end-to-end length plotted as a function of the number of rotations, <i>θ</i>. The applied stretching force is 0.3 pN. The black squares show the data for a bare DNA molecule, prior to any tetrasome assembly. Following tetrasome assembly, a broad plateau (of ~8 ± 1 turns) is observed surrounding 0 turns (red circles). The solid lines are linear fits to the data (5 per trace).