The tyrosyl-tRNA synthetase (EC 6.1.1.1) from Bacillus stearothermophilus is a dimer of two identical subunits. The dimer shows "half-of-the-sites" reactivity in that only one molecule of tyrosyladenylate is formed and one molecule of tRNATyr binds per dimer. To identify whether the tRNATyr binds to a single subunit in the dimer, or to both subunits, heterodimers were constructed by mixing two variant dimers together in 8 M urea. As the unfolded protein is electrophoresed into a native polyacrylamide gel, it refolds and reassociates, and heterodimers can be purified from the parental dimers. Kinetic analysis of heterodimers formed between variant enzymes with defective tyrosine activation or tRNA aminoacylation shows that a molecule of tRNATyr interacts with the N-terminal region of one subunit and the C-terminal region of the other subunit in the dimer.