Localization of proteins and tRNA molecules in the 70S ribosome of the Escherichia coli bacteria with polarized neutron scattering

Isotopic substitution methods are widely used in neutron scattering for the determination of the in situ structure of macromolecular components in quaternary structures. The contrast created by the substitution of the hydrogen isotope H-1 (proton) by H-2 (deuteron) is the most prominent example. A further increase of contrast by a factor of three is possible by polarized neutron scattering from polarized nuclear spins. This offers the possibility of measuring small labels, such as proteins, which contribute less than 0.5% to the whole ribosomal mass, or weakly contrasted molecules, such as tRNA ligands, in the 70S ribosomes. In this study, the positions of the proteins S6 and S10 of the Esherichia coli ribosome with respect to the whole 70S ribosome have been determined by nuclear-spin contrast variation. Furthermore, the localization of two weakly contrasted tRNA molecules bound to the pre-and post-translocational 70S ribosome, respectively, is presented. So far, no other technique has allowed the determination of the in situ structures of these molecules.