DIRECT LOCALIZATION OF THE TRANSFER-RNAS WITHIN THE ELONGATING RIBOSOME BY MEANS OF NEUTRON-SCATTERING (PROTON-SPIN CONTRAST-VARIATION)

A new technique for neutron scattering, the proton-spin contrast-varia tion, improves the signal-to-noise ratio more than one order of magnit ude as compared to conventional techniques. The improved signal enable s small RNA ligands within a large deuterated ribonucleic acid-protein complex to be measured. We used this technique to determine the posit ions of the two tRNAs within the elongating ribosome before and after translocation. Using a four-sphere model for each of the L-shaped tRNA s, unequivocal solutions were found for the localization of the mass c entre of both tRNAs. The centre of gravity is located in the interface cavity separating the ribosomal subunits near the neck of the 30 S su bunit. It moves during translocation by 12(+/-4) Angstrom towards the head of the 30 S subunit and slightly towards the L1 protuberance of t he 50 S subunit. (C) 1997 Academic Press Limited.