Dimeric tyrosyltRNA synthetase from Bacillus stearothermophilus unfolds through a monomeric intermediate. A quan

Tyrosyl-tRNA synthetase from Bacillus stearother-mophilus 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 amino-acyl-tRNA synthetases. The unfolding of the N-terminal domain by urea at 25 °C 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 spectroflu-orometry, circular dichroism, and size-exclusion chro-matography experiments, respectively. These experi-ments showed the existence of an equilibrium between the native dimeric state of the N-terminal domain,