FIGURE 4.
SsrBC displaces H-NS from DNA-only when bound in polymerization mode. A, H-NS stiffens DNA in the absence of MgCl2. Black squares are the extension of DNA in the buffer alone. Red circles are the extension of the same DNA in the presence of 600 nm H-NS when the force was gradually reduced. The blue triangles are extension of the same DNA when the force was increased. The red circles overlapped with the blue triangles, indicating that the extension reached equilibrium. The symbols and colors are identical in subsequent panels. B, H-NS folds DNA at 10 mm MgCl2. When the force was reduced to <0.3 pN, folding occurred over a large distance within the recording time (red circles). The folded DNA could be unfolded at forces >0.8 pN (blue triangles). C, SsrBC folds DNA in stiffening buffer (i.e. in the absence of MgCl2). The DNA shortens, even at low pN forces. Large scale folding within the recording time window occurred as the force was decreased below 1 pN (red circles). In addition, the folded DNA could not be unfolded as the force was reapplied to high tensions (>10 pN; blue triangles). D, SsrBC folds DNA in the bridging buffer of H-NS (i.e. in 10 mm MgCl2). SsrBc folds DNA in H-NS stiffening buffer, and it can also fold DNA in H-NS bridging buffer (red circles). Unlike what we observed with H-NS (B), the SsrB-folded DNA cannot be unfolded under large tensions (blue squares). E, SsrBC folds DNA in the presence of H-NS bound in the stiffening mode (no MgCl2). In the presence of 600 nm H-NS (bound to DNA in stiffening mode), addition of 300 nm SsrBC leads to DNA folding (red circles). Once the DNA is folded by SsrBC, it cannot be unfolded even under large tensions (blue triangles). F, DNA folding in the presence of 600 nm H-NS, 300 nm SsrBC, in bridging buffer (pH 7.4, 50 mm KCl, 10 mm MgCl2, 24 °C). At high MgCl2, DNA is folded (red circles) and unfolded (blue triangles) at forces observed with H-NS alone. Thus, when H-NS is bound to DNA in the bridging mode, it appears to be inert to addition of SsrBC.