Small-Angle X-Ray Studies on the Structure of 16-S Ribosomal RNA and of a Complex of Ribosomal Protein S4 and 16-S Ribosomal RNA from Escherichia coli

16-S ribosomal RNA and a complex of ribosomal protein S4 and 16-S rRNA were studied in solution by small-angle X-ray scattering. Concentration series of the 16-S rRNA and the S4 - 16-S-rRNA complex were measured in 37.5 mM Tris-HCl buffer pH 7.4 at 5 degrees C. The following data were determined. The radii of gyration for the 16-S rRNA and S4 - 16-S-rRNA complex were R = 17.6 +/- 0.6 nm, respectively. The two respective values of the radii of gyration of the cross-section were Rq,1 = 8.42 +/- 0.1 nm and 8.33 +/- 0.3 nm, and Rq,2 = 0.988 +/- 0.03 nm and 0.996 +/- 0.03 nm. The largest diameters of the 16-S RNA and S4 - 16-S-RNA complex were L = 61.8 +/- 1 nm and 60.0 +/- 1 nm, respectively. Volumes of V = 1570 +/- 60 nm3 were found for both particles. In the Tris buffer used, no significant differences were found between the scattering curves of 16-S rRNA and the complex is a flat elliptical cylinder with the following dimensions: large axis 61.7 nm, small axis 35.4 nm and height 2 nm. The theoretical scattering curve fits the experimental one as long as the shape of the measured curve is due only to the overall shape of the particle. A model equivalent over the whole measured angular range is one built up from a large number of spheres that simulate the known substructure of the RNA. The outer dimensions of this model correspond to those of the flat elliptical cylinder.