Figure 1.

Schematic depiction of the base pair near the end of the acceptor stem of a lysine tRNA acting as a blocking determinant for one enzyme but not another. The class I lysyl-tRNA synthetase binds to the minor groove side of the helix, whereas the class II enzyme approaches from the major groove side. The class II lysyl-tRNA synthetase of E. coli accommodates major groove determinants encoded by G:C, but is blocked by the presence of a G:U pair. In contrast, the class I lysine enzyme of the spirochete B. burgdorferi accepts the minor groove determinants of G:C or G:U. The appearance of blocking determinants for a class II lysyl-tRNA synthetase could explain its displacement by its class I counterpart.