Abstract: E. coli aspartate transcarbamoylase (ATCase) is considered a textbook example of allostery, but the molecular mechanism by which nucleotides exert allosteric control has been long debated. This SEC-SAXS dataset is one of six collected in the presence of different nucleotide conditions at pH 7.5 in the presence of Mg2+. Corresponding cryo-EM map and model: EMD-47958, PDB: 9EEM Experimental description: Size exclusion chromatography (SEC) coupled small-angle X-ray scattering (SAXS) was performed with a 11.34 keV 250 x 250 μm X-ray beam at the Cornell High Energy Synchrotron Source (CHESS) ID7A beamline. The sample (50 μL of 56 μM ATCase in 40 mM Tris-HCl pH 7.5, 15 mM MgCl2, 1 mM TCEP, 10 mM succinate, 0.5 mM CP, 1.5 mM CTP, 1.5 mM UTP) was centrifuged (15,000 × g, 10 min, 4 °C) and loaded onto a Superdex 200 10/300 GL column (Cytiva) operated by a GE Akta Purifier at 4 °C that was pre-equilibrated in a matched buffer. The elution flowed directly into an in-vacuum X-ray sample cell held at 4 °C at 0.3 mL min−1. Scattering images (3-second exposures) were collected throughout the elution on an Eiger 4M detector covering a range of q ≈ 0.0079 – 0.453 Å−1, where q = 4π/λ sin θ, λ is the X-ray wavelength, and 2θ is the scattering angle. File description: zip file of .dat files integrated in BioXTAS RAW