Small-angle X-ray scattering

SAXS data were collected in batch mode on the EMBL/DESY synchrotron beamline P12 [52], using standard procedures [53]. In addition to LigT alone, yeast tRNA (Sigma) and a 1:1 molar mixture of LigT and tRNA were analyzed. Data were processed with the ATSAS package [54]. Distance distribution functions were analyzed using GNOM [55]. For modelling LigT alone, GASBOR [56] was used. For modelling the protein-RNA complex, we used the program MONSA [57] with the protein and RNA in different phases. The tRNA alone was modelled using DAMMIN [57]. Molecular weight was estimated through a comparison of the forward scattering intensity of the sample to that of a fresh sample of monomeric bovine serum albumin.