Fig. 3.
The DNA overstretching transition at 65 pN is a melting transition. (A) Three consecutive fluorescence images of overstretched 3′-3′ attached DNA exposed to mitochondrial single-stranded binding protein (mtSSB) labeled with Alexa-555. MtSSB accumulates in two spots (white arrows) that brighten and translocate upon further extension. (B) The intensity in the moving spot scales linearly with the distance moved from the bead, showing that ssDNA accumulates during overstretching. The spot to the right remains stationary relative to the right bead and does not brighten. The inset shows the fraction of base-paired nucleotides as a function of the relative extension, L/L0. The fraction is obtained from the distance between the mtSSB spots, assuming dsDNA in between. (C) Interpretation of images of fluorescent mtSSB bound to overstretched DNA (A). Overstretching initiates on fraying DNA extremities near the beads. MtSSB binds to the melted DNA strand that is not connected to the bead and consequently not under tension.