A Conformational Study of Yeast Phenylalanine Transfer Ribonucleic Acid

Abstract

Abstract Yeast phenylalanine tRNA (tRNAphe) exhibits a 4½-fold enhancement of fluorescence in response to the addition of Mg2+. Concurrently, over the same concentration range (0 to 0.4 mm) of Mg2+, the circular dichroism peak at 263 nm displays a 20% increase in ellipticity and a blue shift of 2 nm. The existence of the negative circular dichroism band at 295 nm appears to be cation-dependent. This conformational change is accompanied by a 5.4% hypochromicity and is readily observable by gel filtration. NaCl inhibits competitively the enhancement of fluorescence by Mg2+. High concentrations (2.0 m) of NaCl alone are only 47% as effective as Mg2+ in producing an increase in fluorescence. A series of organic diamines exhibits a diminishing ability to enhance the fluorescence of tRNAphe as the number of carbon atoms between the two amine groups increases. Spermine and spermidine are more effective than any of the diamines examined, but spermidine, which is the most active oligoamine tested, is only 41% as effective as Mg2+. Gel filtration results indicate that the level of fluorescence enhancement obtainable with various divalent cations corresponds qualitatively to the relative extent of folding of tRNAphe inducible by these cations. Heat denaturation of tRNAphe in the presence of Mg2+ revealed a preferential melting of the fluorescence enhancement at lower temperatures as compared with the hyperchromicity of the sample. The conformational change of tRNAphe induced by Mg2+ is interpreted to represent the formation of tertiary structure. Where comparisons are possible, yeast tRNAphe bears a close similarity to bovine liver tRNAphe in its fluorescence/conformational properties.