ODCase activity resulting from the translation of the A high degree of accuracy in tRNA aminoacylation has been considered crucial for preserving fidelity in protein URA3 gene was examined in the presence of a pyrimidine analog, 5-fluoroorotic acid (5FOA), an inhibitor in synthesis. It has been established that aminoacyl-tRNA synthetase is able to discriminate precisely its cognate pyrimidine biosynthesis. Incorporation of 5FOA with ODCase results in the formation of 5-fluorouridylate, amino acid from other structurally related amino acids at the adenylation reaction step, and its cognate tRNAs from which is harmful to cell propagation (Boeke et al., 1984) . Thus, URA3-defective strains grow normally on a medium non-cognate ones (reviewed in Parker, 1989; Kurland, 1992) . The misacylation error in this process has been containing 5FOA, whereas cells possessing the active URA3 gene are unable to grow on this medium. Cells estimated to range between 10 -4 and 10 -5 (Lin et al., 1984; Okamoto et al., 1984) . Discrimination of cognate harboring the respective plasmids were cultivated in the presence of 5FOA in addition to uracil. tRNA from non-cognate tRNAs is mediated by positive and negative identity determinants localized on the tRNA As shown in the bottom row of Figure 5B , cells harboring pCSU-CTG exhibited similar growth on the molecule (Yarus, 1988; Normanly and Abelson, 1989) . The only exception reported so far is that tRNA Gln is agar plate to those with pCSU-TCT and pUTH18, although the transformants with pCSU-CTC and pCCU were unable aminoacylated with glutamate in Gram-positive bacteria and in some organelles (Lapointe et al., 1986 ; Schön to grow. These results indicate that the CTG codon at position 45 was mainly translated as serine in C. maltosa, et al., 1988) . However, this differs from misaminoacylation in that this process is indispensable to compensate for the so as to produce the inactive ODCase. However, when the liquid medium was supplied with 5FOA, a slight lack of glutamyl-tRNA synthetase in these organisms. In general, high fidelity in the aminoacylation process is reduction in the growth rate was observed in the case of pCSU-CTG, compared with pCSU-TCT ( Figure 5C ), considered to be indispensable for translating genes into functionally active proteins with a high degree of accuracy. while very slow growth was observed in the case of pCSU-CTC used as a control. In order to detect a low