Paramagnetic ion effects on the nuclear magnetic resonance spectrum of transfer ribonucleic acid: assignment of the 15--48 tertiary resonance.

We have studied the effects of Co2+ and Mn2+ ions on the low-field nuclear magnetic resonance (NMR) spectra of pure class 1 transfer ribonucleic acid (tRNA) species. With 1.2 mM tRNA in the presence of 15 mM MgCl2 discrete paramagnetic effects were observed for Co2+ at concentrations in the range 0.02--0.1 mM and for Mn2+ in the range 0.002--0.01 mM, indicating fast exchange of these cations with tRNA. Both of these cations paramagnetically relax the s4U8--A14 resonance as well as other resonances from proximal base pairs. The Co2+ site appears to be the same site on G15 which was observed crystallographically [Jack, A., Ladner, J. E., Rhodes, D., Brown, R. S., & Klug, A. (1977) J. Mol. Biol. 111, 315-328]; the initially occupied tight Mn2+ site is the cation site involving the phosphate of U8. There are three base pairs within 10 A of both sites, namely, G15--C48, A14--s4U8, and C13--G22; this has led to the assignment of the G15--C48 and C13--G22 resonances in the NMR spectrum [Jack, A., Ladner, J. E., Rhodes, D., Brown, R. S., & Klug, A. (1977) J. Mol. Biol. 111, 315--328; Holbrook, S. R., Sussman, J. L., Warrant, R. W., Church, G. M., & Kim, Sung-Hou (1977) Nucleic Acids Res. 4, 2811--2820; Quigley, G. J., Teeter, M. M., & Rich, A. (1978) Proc. Natl. Acad. Sci. U.S.A. 75, 64--68].