Study of the phosphorescent bases of yeast phenylalanine transfer RNA with the aid of optical detection of magnetic resonance

The phosphorescence of brewers' yeast phenylalanine transfer RNA has been investigated at 77 °K and at 1.2 °K in pumped liquid helium. Although the phosphorescence at 77 °K originates almost completely from the Y base in the anticodon loop, independent of excitation wavelength, the phosphorescence originates from normal bases with 270 nm excitation at temperatures in the helium range. The low-temperature phosphorescence is assigned to the triplet state of adenosine by optical detection of magnetic resonance measurements. The adenosine phosphorescence at 1.2 °K is quenched by the binding of the codon poly(U), as well as by the removal of Mg²⁺. The former result indicates that the adenosine phosphorescence originates from the anticodon, -Gm-A-A-, while the second shows that a conformational change introduced by removing Mg²⁺ (possibly involving unstacking of the anticodon) prevents energy trapping in the anticodon triplet state. The lack of triplet energy transfer from anticodon to Y indicates that Y cannot be stacked with the anticodon in the conformation that is stable at helium temperature. The adenosine phosphorescence of transfer RNA^Phe is nearly completely quenched at 77 °K, at least partially due to energy transfer to Y. We think that the thermally activated energy transfer is associated with some mobility of the Y base at 77 °K. Our observations are in contrast with previous results on bakers' yeast tRNA^Phe where there is apparently little, if any, energy transfer to Y from the normal nucleotides at 80 °K with 265 nm excitation. Optically detected magnetic resonance measurements on the triplet state of Y base in various environments indicate that removal of Mg²⁺ causes a shift of the Y base in tRNA^Phe to a more solvent-exposed position, whereas the binding of poly(U) has little effect on the environment of Y.