Abstract
A tRNA Phe derivative carrying ethidium at position 37 in the anticodon loop has been used to study the effect of spermine on conformational transitions of the tRNA. As previously reported (Ehrenberg, M., Rigler, R. and Wintermeyer, W. (1979) Biochemistry 18, 4588–4599) in the tRNA derivative the ethidium is present in three states (T 1–T 3) characterized by different fluorescence decay rates. T-jump experiments show two transitions between the states, a fast one (relaxation time 10–100 ms) between T 1 and T 2, and a slow one (100–1000 ms) between T 2 and T 3. In the presence of spermine the fast transition shows a negative temperature coefficient indicating the existence of a preequilibrium with a negative reaction enthalpy. Spermine shifts the distribution of states towards T 3, as does Mg 2+, but the final ratio [ T 2] [ T 1] obtained with spermine is higher than with Mg 2+, which we tentatively interpret to mean that spermine stabilizes one particular conformation of the anticodon loop.
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