Comparison of the structures of free and ribosome-bound tRNAPhe by using slow tritium exchange

The rate of incorporation of tritium from the solvent into the C-8 position of purines in RNA is markedly sensitive to the microenvironment. This slow tritium exchange reaction has been used to study the structure and interactions of yeast tRNAPhe bound to poly(U)-programed tight-couple 70S ribosomes of Escherichia coli. The tritium incorporation into specific sites of the tRNA was determined by enzymatic digestion and measurement of the specific activity of each of the isolated radioactive fragments. Ribosome binding leads to marked suppression in the exchange rate of a number of fragments. This delineates extensive regions of tRNA-ribosome contact. No change in exchange rates is seen for fragments from the corner of the molecule, indicating that this region of bound tRNA is readily accessible to the solvent. Ribosome binding results in an enhanced exchange rate at the T loop. This appears to be the result of a conformational change that is most likely an unfolding of the T and D loops. Additional tritium exchange reactions suggest this conformational change is induced by ribosomes and not by messenger.