We have also demonstrated that the nucleotide at position 33, where only tRNA Ser CAG uniquely possesses G, modulates the leucine-accepting activity. G33 may prevent tRNA Ser CAG from excessive leucylation, which S.cerevisiae cells, but that the viability of the cells decreased substantially. This finding suggests the polysemous state may be tolerated only when the ambiguous recognizes its cognate leucine tRNA from the 3Ј-side of the anticodon loop, which is afforded by the uridine-turn translation is under a strict constraint. We consider that G33 functions as a negative modulator in the leucylation structure due to U33 ( Figure 6A ). The methyl moiety of m 1 G37 is directly recognized by LeuRS. In the case of tRNA Ser CAG, thereby controlling the relative seryl-to leucyl-tRNA Ser CAG ratio.