Dimeric tyrosyl-tRNA synthetase from Bacillus stearothermophilus unfolds through a monomeric intermediate. A quantitative analysis under equilibrium conditions.

Tyrosyl-tRNA synthetase from Bacillus stearothermophilus comprises an N-terminal domain (residues 1-319), which is dimeric and forms tyrosyladenylate, and a C-terminal domain (residues 320-419), which binds the anticodon arm of tRNATyr. The N-terminal domain has the characteristic fold of the class I aminoacyl-tRNA synthetases. The unfolding of the N-terminal domain by urea at 25 degreesC under equilibrium conditions was monitored by its intensities of light emission at 330 and 350 nm, the ratio of these intensities, its ellipticity at 229 nm, and its partition coefficient, in spectrofluorometry, circular dichroism, and size-exclusion chromatography experiments, respectively. These experiments showed the existence of an equilibrium between the native dimeric state of the N-terminal domain, a monomeric intermediate state, and the unfolded state. The intermediate was compact and had secondary structure, and its tryptophan residues were partially buried. These properties of the intermediate and its inability to bind 1-anilino-8-naphthalenesulfonate showed that it was not in a molten globular state. The variation of free energy deltaG(H2O) and its coefficient m of dependence on the concentration of urea were, respectively, 13.8 +/- 0.2 kcal.mol-1 and 0.9 +/- 0.1 kcal.mol-1.M-1 for the dissociation of the native dimer and 13.9 +/- 0.6 kcal.mol-1 and 2.5 +/- 0.1 kcal.mol-1.M-1 for the unfolding of the monomeric intermediate.