Figure 3. Torsional measurements of a single chromatin fiber.

(A) Experimental configuration. One end of a chromatin fiber was torsionally constrained to the surface of a microscope coverslip while the other end was torsionally constrained to the bottom of a nanofabricated quartz cylinder that was held in the angular optical trap (AOT) (Figure S2). The chromatin fiber was then placed under a constant force of 0.5 pN and twisted in both the (+) and (−) directions. At the end of this torsional measurement, the nucleosome quality and saturation level of each chromatin fiber were assayed by returning the fiber to the zero-turns state and stretching it axially to disrupt the nucleosomes (Figures 2B and 2C; Figures S3A–S3C; Quantification and Statistical Analysis). The fiber stability was assayed from the twisting data (Figures S3D and S3E).

(B & C) The measured extension and torque versus turns added, under different levels of nucleosome array saturation, indicated by the mean number of nucleosomes on the substrate (N ~ 35 traces for each curve). Extension and torque signals were smoothed by sliding windows of 1 turn and 4 turns respectively. For naked DNA, torque increases until DNA buckles to form a plectoneme, then torque plateaus. For chromatin fibers, the data suggest that an analogous structural transition may be occurring. The (+) torsional stiffness of each curve in (C) was determined from a linear fit (not shown) to the initial (pre-”buckling”) slope upon adding (+) turns.

(D) The measured torsional modulus as a function of the number of nucleosomes in the substrate. k_BT is the thermal energy. Error bars were converted from the uncertainties in the slopes of the linear fits in (C). The dashed curve is a fit by a simple model, yielding a modulus of 10.2 nm k_BT for a chromatin fiber with all 64 NPEs occupied (Quantification and Statistical Analysis). The solid vertical line at 64 indicates the number of NPEs on the template.