Extended Data Figure 5. SAXS analysis of FluPol_C.

a, Calculated solution-state SAXS profiles for the crystal structures of vRNA-FluPol_A³ (activated conformation) and apo-FluPol_C (closed conformation). A distinguishing difference between these profiles is the dip at q ~0.1 Å⁻¹ in the FluPol_C curve. b, Solution-state SAXS profile of FluPol_C, without added promoter RNA, overlaid with the calculated curve for the vRNA-FluPol_A structure³. The good match between these curves suggests that in this particular buffer (0.5 M NaCl, 25 mM HEPES-NaOH, pH 7.5, 5% (v/v) glycerol), FluPol_C adopts the same globular conformation as the RNA bound state. c, Dimensionless Kratky plot of apo-FluPol_C in the presence of 0.5 M NaCl, 25 mM HEPES-NaOH, pH 7.5 and 5% (v/v) glycerol (blue) or 100 mM KCl, 2% (w/v) sucrose and 100 mM sodium phosphate, pH 7.3, with (magenta) or without (grey) 200 mM proline. Cross-hairs denote the Guinier–Kratky point (1.732, 1.104), the peak position for an ideal, globular particle. As indicated by the upward shift of the peaks in the dimensionless Kratky plot, FluPol_C is less globular in the presence of phosphate than it is in the 0.5 M NaCl buffer. This effect can be lessened if proline is also present, potentially owing to increased molecular crowding.