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FIGURE 2:. DNA bending and looping by H1. (A) The extension remained stable with 12.5- and 2.1-pN holding forces in 10 nM H1. The thermal fluctuations are ∼150 nm around a fixed extension at 2.1 pN. After the force dropped to 1 pN, the extension decreased rapidly at the beginning and slowed down in ∼4 min. The force was further decreased to 0.5 pN. The extension decreased to 0 in ∼8 min. At 10 nM H1, a relatively low concentration, continuous nanometer-scale decreases are suggestive of compaction by bending. Larger, hundreds-of-nanometer drops strongly suggest loop formation. When force was subsequently increased to 12.5 pN the extension jumped to approximately the length of naked DNA. On switching from 12.5 to 0.5 pN, the extension dropped again. Finally, a force increase to 2.7 pN increased the DNA extension. (B) Expanded view of compaction at 1 pN described in A. The thermal fluctuations at 1 pN are ∼200 nm. The DNA was observed to undergo compaction by 20- to 100-nm decreases (smoothed data). Results similar to those shown in A and B were observed in five separate experiments. Image published in: Xiao B et al. (2012) © 2012 Xiao et al. This image is reproduced with permission of the journal and the copyright holder. This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike license Larger Image Printer Friendly View

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