Lysyl-tRNA synthetase from Bacillus stearothermophilus (B.s. LysRS) (EC ) catalyzes aminoacylation of tRNA(Lys) with l-lysine, in which l-lysine was first activated with ATP to yield an enzyme (lysyladenylate complex), and then the lysine molecule was transferred from the complex to tRNA(Lys). B.s. LysRS is a homodimeric enzyme with a subunit that consists of two domains, an N-terminal tRNA anticodon-binding domain (TAB-ND: Ser(1)-Pro(144)) and a C-terminal Class II-specific catalytic domain (CAT-CD: Lys(151)-Lys(493)). CAT-CD alone retained catalytic activity, although at a low level; TAB-ND alone showed no activity. Size exclusion chromatography revealed that CAT-CD exists as a dimer, whereas TAB-ND was a monomer. The formation of a complex consisting of these domains was detected with the guidance of surface plasmon resonance. In accordance with this, the addition of TAB-ND to CAT-CD significantly enhanced both the l-lysine activation and the tRNA aminoacylation reactions. Kinetic analysis showed that deletion of TAB-ND resulted in a significant destabilization of the transition state of CAT-CD in the l-lysine activation reaction but had little effect on the ground state of substrate binding. A significant role of a cross-subunit interaction in the enzyme between TAB-ND and CAT-CD was proposed for the stabilization of the transition state in the l-lysine activation reaction.