Conformational changes of yeast tRNAphe as monitored by 31P NMR.

The 31P NMR spectra of tRNAs contain approximately 17 resonances resolved from the main resonance which consists of about 80% of the total resonance intensity arising from the sugar phosphate backbone. In the present paper we study the behavior of the 31P resonances of yeast tRNAPhe as a function of temperature and of solution conditions. By comparison with other melting experiments we show that three resonances (called c, e and j2) belong to phosphates in the anticodon loop, while the remaining resolved 31P resonances come from phosphates in specific conformations in the central part of the molecule imposed by the tertiary structure. These conformations are different from the normal g-,g- conformation found in A-RNA double helices. The assignments are in good agreement with those previously made on the basis of chemical and enzymatic modification experiments [P. J. M. Salemink, T. Swarthof & C. W. Hilbers (1979) Biochemistry, 18, 3477-3485]. AT high Mg2+ concentrations the anticodon loop is found to be present in two different conformations. For all solution conditions studied loss of the anticodon loop structure takes place before the tertiary structure is melted out. The melting of the tertiary structure is not strictly an all- or-none process. The lifetimes of phosphate conformations involved in the tertiary structure may differ by at least a factor of two. It can also be concluded that the range of chemical shifts observed for phosphodiesters cannot at the moment be accounted for by theoretical calculations.