Kinetics of conformational changes in tRNA Phe (yeast) as studied by the fluorescence of the Y-base and of formycin substituted for the 3'-terminal adenine.

The kinetics of the melting transitions of tRNA Phe (yeast) were followed by the fluorescence of the Y-base and of formycin substituted for the 3'-terminal adenine. As judged from differential UV absorbance melting curves the formycin label had virtually no influence on the conformation of the tRNA. A temperature jump apparatus was modified to allow the simultaneous observation of transmission and fluorescence intensities by two independent optical channels. The design of a temperature jump cell with an all quartz center piece is given. The cell is resistant to temperatures up to 90 degrees C; it provides high optical sensitivity, low stray light intensity and the possibility of measuring fluorescence polarization. The T-jump experiments allowed to discriminate between fast unspecific fluorescence quenching (r less than 5 musec) and slow cooperative conformational changes. In the central part of the temperature range of UV-melting (midpoint temperature 30 degrees C in 0.01 M Na+ and 39 degrees C in 0.03 M Na+, pH 6.8) two resolvable relaxation processes were observed. The corresponding relaxation times were 20 msec and 800 msec at 30 degrees C in 0.01 M Na+, and 4 msec and 120 msec at 39 degrees C in 0.03 M Na+. The Y-base fluorescence shows both of the relaxation effects, which almost cancel in equilibrium fluorescence melting, because their amplitudes have opposite signs. From this finding the existence of some residual tertiary structure is inferred which persists after the unfolding of the main part of tertiary structure during early melting (midpoint temperature 24 degrees C in 0.03 M Na+). In the fluorescence signal of the formycin also the two relaxation effects appear. Both of them are connected with a decrease of the fluorescence intensity. From the results a coupled opening of the anticodon and acceptor branches is concluded.