Synchrotron SAXS data from solutions of Lys-Glu mutant RNAP in 20 mM phosphate buffer, 400 mM NaCl, 0.05% NaN3, pH 6.6 were collected on the EMBL P12 beam line at PETRA III (Hamburg, Germany) using a Pilatus 2M detector at a sample-detector distance of 3 m and at a wavelength of λ = 0.124 nm (I(s) vs s, where s = 4πsinθ/λ, and 2θ is the scattering angle). One solute concentration of 2.60 mg/ml was measured at 20°C. 20 successive 0.050 second frames were collected. The data were normalized to the intensity of the transmitted beam and radially averaged; the scattering of the solvent-blank was subtracted.