finding tRNA with the anticodon sequence potentially complementary to codon CUG other than tRNA Ser CAG In order to quantify the growth rate of the cells harboring pCSU-CTG, the viability of the cells was examined in (Yokogawa et al., 1992; Ohama et al., 1993; Suzuki et al., 1994; Ueda et al., 1994; our unpublished observation) . liquid medium without uracil. As shown in Figure 5C , whereas translation of the CTG codon as serine completely Futhermore, tRNA genes for serine and leucine from these Candida species were sequenced following the blocked cell growth in the case of pCSU-TCT, and full complementation was observed in the case of pCSU-CTC amplification by cloning and/or PCR methods, and we found that only tRNA Ser CAG is able to translate codon in which the CTC codon was read as leucine, intermediate cell growth was observed in the case of pCSU-CTG, CUG (Yokogawa et al., 1992; Ohama et al., 1993; Suzuki et al., 1994; Ueda et al., 1994 ; our unpublished observ-indicating that ODCase was expressed in an active form, albeit at a low level, when there was a slight incorporation ation). Thus, it could be concluded that only the tRNA Ser CAG species inserts leucine into polypeptide of leucine at the CTG codon. The slow growth of the cells harboring pCSU-CTG was not due to the spontaneous corresponding to codon CUG. reversion of the CTG codon to another leucine codon or due to any other mutation, because the cells harvested Discussion from the colony on the SD-plate show the same growth phenotype. These results are unlikely to reflect the different The observations presented here clearly demonstrate that, in certain living organisms, a single codon can be simul-expression levels of the URA3 gene variants because the URA3 mRNA level is not altered by mutations at position taneously assigned to two distinct amino acids. Most codons in the genetic code degenerate, but our findings 45 (Ohkuma, 1993) . Furthermore, the possibility that the URA3 gene with CTG at position 45 is translated more show that some amino acids are also able to degenerate with respect to a particular codon. Such codon ambiguity efficiently than the gene with TCT at the same site due to codon preference (Ikemura, 1982) is excluded by the is governed by a tRNA acceptable to two amino acids simultaneously, as described above. We propose to desig-fact that the TCT codon is the most preferred of all the serine codons, including the CUG codon, in C.maltosa nate a codon corresponding to multiple amino acids a 'polysemous codon'. (Sugiyama et al., 1995) .