Figure 2.

Dependence of DNA extension on force, salt and HU concentration. Dashed lines show extension of naked DNA under corresponding forces. Each experimental data point, presented as the mean value, was obtained from 6–10 measurements. (A) Bimodal binding of HU and single λ-DNA in 40 mM NaCl buffer. Compaction was observed for HU concentrations < 100 nM and was maximum at ∼50 nM HU. From 50 nM to higher concentrations, the extension increased gradually and could become longer than 0 nM HU, indicating the formation of rigid filaments. (B) Bimodal binding as in A, for 100 mM NaCl. Essentially the same dependence of binding mode on HU concentration was observed as for 40 mM NaCl. (C) Unimodal binding in 150 mM NaCl buffer. As the HU concentration was increased, the extension decreased with concentration to the maximum concentration studied of ∼1000 nM HU. No rigid filament stiffening effect was observed. (D) No binding was detectable through extension changes in 300 mM NaCl buffer. Extension remained unchanged relative to that of naked DNA for all HU concentrations and applied forces studied.