The conformation of the tRNAPhe anticodon loop monitored by fluorescence

The intrinsic fluorescence of the Wye base was used to study the conformational change of the anticodon loop of yeast tRNAPhe brought about by the addition of magnesium. The fluorescence emission and excitation spectra show dramatic changes as magnesium is added to the solution. The rotational relaxation time changes from 6 nsec without added magnesium to 33 nsec with 10 mM magnesium at an ionic strength of 0.1 M. Stern-Volmer quenching by iodide or iodoethanol shows greater access of the base to the quencher with no added magnesium. A plausible interpretation of this data is that the base stack of the anticodon loop is altered by tilting or twisting the Wye base with respect to the adjacent bases and the base becomes parallel to its neighbors upon the addition of magnesium.