Nuclear magnetic resonance studies on yeast tRNAPhe. III. Assignments of the iminoproton resonances of the tertiary structure by means of nuclear Overhauser effect experiments at 500 MHz.

Resonances of the water exchangeable iminoprotons of the tertiary structure of yeast tRNAPhe were studied by experiments involving Nuclear Overhauser Effects (NOE's). Direct NOE evidence is presented for the assignment of all resonances of iminoprotons participating in tertiary basepairing (except that of G19C56 which was assigned by an elimination procedure). The present results in conjunction with our previous assignment of secondary iminoprotons constitute for the first time a complete spectral assignment of all iminoprotons participating in basepairing in yeast tRNAPhe. In addition we have been able to assign the non(internally) hydrogen bonded N1 proton of psi 55 as well as the N3 proton of this residue, which is one of the two iminoprotons hydrogen bonded to a phosphate group according to X-ray results. No evidence could be obtained for the existence in solution of the other iminoproton-phosphate interaction: that between U33 N3H and P36 located in the anticodon loop. Remarkable is the assignment of a resonance at 12.4 - 12.5 ppm to the iminoproton of the tertiary basepair T54m1A58. The resonance positions obtained for the iminoprotons of G18 (9.8 ppm) and m2(2)G26 (10.4 ppm) are surprisingly far upfield considering that these protons are involved in hydrogen bonds according to X-ray diffraction results. As far as reported by changes in chemical shifts of iminoproton resonances the main structural event induced by Mg++ ions takes place near the tertiary interactions U8A14 and G22m7G46.