The active form of the leucyl-tRNA synthetase from an extreme thermophile Aquifex aeolicus has a heterodimeric (alpha/beta type) quaternary structure that is unique among class I aminoacyl-tRNA synthetases. In an attempt to clarify the individual roles of each subunit in the function of leucyl-tRNA synthetase, several elementary activities were separately measured using each of the subunits alone or the reconstructed alpha/beta complex. It was found that the P subunit alone is capable of recognizing its cognate tRNA, while the leucyl-adenylate formation and the overall leucyl-tRNA formation are detected only when both of the subunit proteins coexisted. (C) 2002 Elsevier Science (USA). All rights reserved.