Fig. 5.

BldC binds DNA to form extended protein-nucleic acid superstructural filaments. a FP-based stoichiometry binding experiments performed with fluoresceinated 22mer opt, 36mer opt (containing 4 binding repeats), and the smeA-ssfA BldC site, each experiment was conducted in technical triplicate with a representative analysis shown for each. For the experiments, DNA was present at a concentration 10-fold above the K_ds, which are 20, 20, and 60 nM, respectively. Binding was observed as increasing mPs until saturation, at which the mPs no longer increase. The inflection point of each binding isotherm represents the subunit protein concentration at saturation. This occurred at BldC: DNA duplex stoichiometries of 2:1, 4:1, and 4:1, respectively. b Structure of the BldC-smeA-ssfA promoter complex. The DNA packs pseudo-continuously. The unit corresponding to the BldC-smeA-ssfA complex is boxed. Two views are shown. Left is a view looking down the axis of the structure and right shows the view rotated 90°. The structure shows how BldC could bind promoters with longer sites than present in the smeA-ssfA promoter. c Electrostatic representation of the structure with blue and red, representing electropositive and electronegative regions. Notably, the continuous protein superstructure also forms a continuous electropositive stripe that tracks along the DNA