Ueda T, Suzuki T, Yokogawa T, Nishikawa K, Watanabe K
Department of Chemistry and Biotechnology, Faculty of Engineering, University of Tokyo, Japan.
Biochimie. 1994;76(12):1217-22. doi: 10.1016/0300-9084(94)90052-3.
In an asporogenic yeast, Candida cylindracea, codon CUG is not translated as leucine but as serine. On the basis of our recent work on the determination of the genetic code using in vitro translation systems coupled with isolation of the corresponding tRNA molecules, it appears that this non-universal genetic code is unitized not only in C cylindracea but also in various Hemiascomycetes. Here we show that in addition to the species already reported, three pathogenic yeasts, C guilliermondii, C lusitaniae and C tropicalis, have tRNA(Ser)CAG, indicating that this non-universal genetic code (CUG=Ser) also exists in these species. Determination of their primary structures revealed that the uridine conserved at position 33 in usual tRNAs, is replaced by guanosine or cytidine. This suggests that the three-dimensional structures of the anticodon loop of these tRNAs differ from the conventional structure comprising the U turn in this position. Moreover, we succeeded in isolating putative ancestral serine tRNA genes whose sequences are highly homologous to tRNA(Ser)CAG in each case. These tRNA genes all have the anticodon sequence CGA corresponding to the codon UCG, indicating that tRNA(Ser)CAG might have emerged from tRNA(Ser)CGA during evolutionary change of the assignment of codon CUG.
在一种不产孢子的酵母——圆柱假丝酵母中，密码子CUG不是被翻译为亮氨酸，而是被翻译为丝氨酸。基于我们最近利用体外翻译系统结合相应tRNA分子的分离来确定遗传密码的工作，这种非通用遗传密码似乎不仅在圆柱假丝酵母中，而且在各种半子囊菌中都是统一的。在这里我们表明，除了已经报道的物种外，三种致病酵母——季也蒙毕赤酵母、葡萄牙假丝酵母和热带假丝酵母，都有tRNA(Ser)CAG，这表明这种非通用遗传密码（CUG = Ser）也存在于这些物种中。对它们一级结构的测定表明，在通常的tRNA中位于第33位保守的尿苷被鸟苷或胞苷所取代。这表明这些tRNA反密码子环的三维结构不同于在这个位置包含U型转弯的传统结构。此外，我们成功分离出了推定的祖先丝氨酸tRNA基因，其序列在每种情况下都与tRNA(Ser)CAG高度同源。这些tRNA基因都具有与密码子UCG相对应的反密码子序列CGA，这表明tRNA(Ser)CAG可能是在密码子CUG的分配发生进化变化期间从tRNA(Ser)CGA中产生的。
