[Structure and density of ribosomal RNA and its complexes with proteins in a solution]

X-ray and neutron scattering, as well as velocity sedimentation, were used to study the shape and dimensions (compactness) of isolated ribosomal (16S and 23S) RNA's and their complexes with ribosomal proteins. The neutron scattering of ribosomal particles in 42% 2H2O where the protein component is contrast-matched, were taken as a standard of comparison characterizing the dimensions and shape of the 16S and 23S RNA in situ. This comparison allowed the following conclusions: (1) The shape of the isolated 16S RNA at a sufficient Mg2+ concentration (e. g., in the reconstruction buffer) is similar to that of the 16S RNA in situ, but its compactness is somewhat less. (2) The 16S RNA in the complex with protein S4 has a shape and compactness similar to those of the isolated 16S RNA. (3) The 16S RNA in the complex with four core proteins, namely S4, S7, S8 and S15, has a shape and compactness similar to those of the isolated 16S RNA. (4). The six ribosomal proteins, S4, S7, S8, S15, S16, and S17, are necessary and sufficient for the 16S RNA to acquire a compactness similar to that in situ.