4-Thiouridine, a built-in probe for structural changes in transfer RNA.

The luminescence of an aqueous solution of 4-thiouridine was compared with its emission when forming part of the polynucleotide chain of tRNA. In both cases excitation into the last absorption band at 335 nm yields a weak emission in the 520--550 nm region. However, while in aqueous solution this emission has a lifetime of approximately 240 ns, it increases in native tRNA to tau congruent to 6.6 mus. Oxygen and Cl- ions quench the thiouridine emission efficiently in aqueous solution while Na+ and Mg2+ ions have no influence on it. On the other hand thiouridine which forms part of a tRNA molecule is quite insensitive to Cl- ions and to O2 while its emission is greatly enhanced by Na+ and Mg2+ ions. From these salt effects as well as from data on the temperature dependence of the emission yield and the decay curve, it is concluded that the site of the thiouridine residue is very well protected within the tertiary structure of tRNA. Both permanent changes in the secondary and in the tertiary structures of the polynucleotide as well as dynamic conformation changes can be observed by following the emission characteristics of its thiouridine residue.