Figure 3: Helicase displacement of a single nucleosome.
(a) Experimental configuration. A single dsDNA molecule was unwound by a T7 helicase as the two strands of the DNA were held under 12 pN of force by an optical trap, which assisted helicase unwinding but was insufficient to mechanically separate the dsDNA (Supplementary Fig. 1B). The nucleosomal DNA template is specified in Supplementary Fig. 2A, Supplementary Table 1 and Methods. (b) Representative helicase-unwinding traces on a nucleosomal (black) or naked (grey) template. Helicase unwinding was interrupted by discrete pauses along the DNA template. Dashed lines indicate the dyad locations of the initial positioned nucleosome and the transferred nucleosome. N=49 traces. (c) Histogram of nucleosome transfer distance. A transfer distance was obtained from the first transfer event of each trace as indicated by the arrow in Fig. 2b. The histogram was obtained by pooling data from 49 traces. The prediction (not a fit) from the DNA looping model is plotted for comparison. The resulting Pearson test gives a reduced χ2 of 0.31 with a P value of 0.95 (Methods; Supplementary Fig. 4A).
