Fig. 1: Pol II is a polar roadblock to a progressing DNA fork.

a Experimental configuration showing co-directional (CD) and head-on (HO) collision orientations between a DNA fork and Pol II. The two daughter strands are tethered between two optically trapped beads. The DNA fork is mechanically unzipped through a Pol II elongation complex at a velocity of 100 nm/s, and force-extension data reveal the strength and location of the Pol II–DNA interactions. b Representative force-extension traces of the DNA fork unzipping through a paused Pol II at A20 in both orientations. Pol II is paused 20 nt from the transcription start site (TSS). Dashed curves show predicted force-extension profiles for forks encountering Pol II after transcription of the specified number of nucleotides. c Scatter plots of the peak disruption force and Pol II sliding distance for both orientations of paused Pol II without and with RNase T1. The mean values (also as the red bars) and their SEMs are indicated. Sample sizes: CD: N = 38 (−RNase T1), N = 24 (+RNase T1); HO: N = 28 (−RNase T1), N = 39 (+RNase T1). d Representative force-extension traces of the DNA fork unzipping through an elongating Pol II in both orientations. e Scatter plots of the peak disruption force and Pol II sliding distance for both orientations of elongating Pol II without and with RNase T1. The sliding distance is measured only from traces with a force rise above the baseline. The mean values (also as the red bars) and their SEMs are indicated. Sample sizes for peak force: CD: N = 27 (−RNase T1), N = 19 ( + RNase T1); HO: N = 60 (–RNase T1), N = 21 ( + RNase T1). Sample sizes for sliding distance: CD: N = 23 (–RNase T1), N = 4 (+RNase T1); HO: N = 60 (−RNase T1), N = 21 (+RNase T1). Source data are provided as a Source data file.