Figure 4.

Magnesium causes higher-order DNA compaction mediated by inter-filament interactions of Dan. (A) Single-DNA stretching experiments show that the Dan nucleoprotein filament is relatively stable up to 10 mM MgCl₂. At 10 mM MgCl₂, hysteresis between DNA relaxation and stretching suggests DNA folding. (B–E) AFM images of DNA–Dan complexes incubated at a 1:1 ratio in the presence of 1 mM MgCl₂ suggests that magnesium causes Dan filaments to interact with each other, resulting in a higher-order DNA compaction. Dan nucleoprotein filament is still observed at 1 mM MgCl₂ (white arrows), and DNA compaction is also observed (yellow arrows). (E) Occasionally, large-scale DNA aggregation is also observed. For this image, different scales are used to accommodate the large DNA–Dan complex.