as it is the substrate for both SerRS and LeuRS. The K m value for LeuRS is 5.0 µM, which is only one order of To date, artificial manipulations of molecules participating in the translation process, such as the overproduction magnitude larger than that for SerRS (0.22 µM). In an in vitro aminoacylation experiment Ͼ30% of tRNA Ser CAG of aminoacyl-tRNA synthetase (Swanson et al., 1988) , mutations of tRNAs etc. and/or control of growth condi-subjected to the reaction could be converted to leucyl-tRNA Ser CAG using an increased amount of LeuRS and a tions, such as deprivation of amino acids in the medium (Edelmann and Gallant, 1977; O'Farrell, 1978; Parker and longer incubation time (data not shown). We observed that while the presence of SerRS and non-radioactive Precup, 1986), have been found to increase the error rate in translation (reviewed in Parker, 1989) . However, our serine reduced leucylation, complete loss of leucylation could not be achieved (data not shown), indicating that the observation is based on experiments using wild-type cells grown in a rich medium suitable for high viability. In affinity of LeuRS toward tRNA Ser CAG is relatively high. In proliferating cells of C.zeylanoides, the leucyl-these respects, the polysemous codon is a phenomenon completely different from these artificial translational tRNA Ser CAG in the cells was estimated to be 3% of the seryl-tRNA Ser CAG, which is much lower than that errors. It is known that many examples exist for alternative decoding of universal codons-initiation codons other obtained in the in vitro experiments. We consider that such a reduction in leucylation is due to the competition than AUG (Gold, 1988; Kozak, 1983) , leaky stop codons caused by nonsense suppresser or native tRNAs (Murgola, for the tRNA Ser CAG between SerRS and LeuRS in the cells. Despite this competition, the distinct detection of 1985), the UGA codon used for incorporation of selenocysteine (Leinfelder et al., 1988) and so on. However, leucylated tRNA Ser CAG in vivo supports the existence of an ambiguous aminoacylation reaction toward the single because of strong dependence on the context effects or possible secondary structures of mRNAs, these recoding tRNA Ser CAG species. The polysemous codon results from the coexistence of events are those which are programed in the mRNAs (Gesteland et al., 1992) . We have sequenced several genes tRNA identity determinants for serine and leucine in a single tRNA molecule. Construction of tRNA Ser CAG in Candida genomes, but we could not find any secondary structure around the codon CUG in these genes. Con-variants by the microsurgery method led to the finding that a single methyl moiety of m 1 G at position 37 sidering that the polysemous codon is mediated by a single tRNA, it is unlikely that a polysemous codon occurs under is involved in the leucylation process. In contrast, the tRNA Ser CAG of C.cylindracea, which has A at the same the influences of the neighboring regions in mRNAs. Alternative decoding of a polysemous codon CUG is position, is deprived of such leucine-accepting activity. Himeno and his co-workers noted that three nucleotides possible, assuming that LeuRS is overexpressed under a certain physiological condition. Depending on the of leucine tRNAs were strongly recognized by S.cerevisiae LeuRS using unmodified variants transcribed by T7 RNA increased amount of the LeuRS in cells, incorporation of leucine corresponding to codon CUG may occur fre-polymerase (Soma et al., 1996) . Although the discriminator base, A73, is the strongest recognition site among them, quently, which causes the production of polypeptides with new functions. This possibility should be examined in A35 and G37 in the anticodon loop also play roles as determinants in tRNA. They were able to compare the further experiments.