(a) Geometry for the dual-trap optical tweezers experiments. (b) Representative trajectories of individual transcribing polymerases with or without the nucleosome at different ionic strengths. (c), (d), and (e) Probability of arrest or termination as a function of polymerase position on the DNA template at 300, 150, and 40 mM KCl respectively. Arrest is defined as a pause that lasts longer than 20 minutes or until the tether breaks. Data for transcription of bare DNA is in solid black, and nucleosome data is in semi-translucent colors. The shaded region represents the NPS.