SELENOCYSTEINE, THE 21ST AMINO ACID IN THE GENETIC CODE

在哺乳动物中发现了两种硒半胱氨酸tRNA同工受体 这个实验室里的细胞。这些tRNA有两个功能：1)读取 终止密码子，UGA，在蛋白质合成中，因此捐赠 硒半胱氨酸对特异性多肽链生长的反应 UGA密码子；和2)作为载体分子用于生物合成 硒半胱氨酸。硒半胱氨酸的生物合成途径是 TRNA[Ser]Sec-&gt；Seryl-tRNA[Ser]Sec-&gt；Phoseryl-tRNA[Ser]Sec 硒半胱基-tRNA[Ser]Sec.两个同种受体(指定为NCA和CMCA 基于它们反密码子序列)来自单个复制基因 它们彼此之间存在着五个嘧啶跃迁，它们发生在 分子的5‘半。因此，一个等受体必须是一个 另一个的编辑版本。这两个等受体都可以从 将硒半胱氨酸tRNA基因注入非洲爪哇卵母细胞。 非洲爪哇卵母细胞初级转录本的分离及其鉴定 重新注入会导致两种同种受体的形成。我们目前正在 探讨这些异构体的合成途径 无论一个是另一个的前身，还是两者都产生于共同的 前身。硒半胱氨酸tRNA种群的相对丰度 NCA和CMCA的分布受土壤水分含量的影响 培养的哺乳动物细胞和大鼠组织中的硒 来自大鼠的含硒和不含硒的饮食，表明tRNA 编辑发生时，编辑过程响应Selify。一个 已经在动物身上发现了能够解码UGA的硒半胱氨酸tRNA， 本实验室中的植物和原生动物(以及其他 研究人员)，因此，通用遗传密码已扩展到 包括硒半胱氨酸作为第21种自然产生的氨基酸。

Two selenocysteine tRNA isoacceptors have been identified in mammalian cells in this laboratory. These tRNAs have two functions: 1) to read the termination codon, UGA, in protein synthesis and thus donate selenocysteine to the growing polypeptide chain in response to specific UGA codons; and 2) to serve as carrier molecules for the biosynthesis of selenocysteine. The pathway for the biosynthesis of selenocysteine is tRNA[Ser]Sec --- > seryl-tRNA[Ser]Sec --- > phosphoseryl-tRNA[Ser]Sec --- > selenocysteyl-tRNA[Ser]Sec. Both isoacceptors (designated NCA and CmCA on the basis of their anticodon sequences) arise from a single copy gene and differ from each other by five pyrimidine transitions which occur in the 5' half of the molecule. Therefore, one isoacceptor must be an edited version of the other. Both isoacceptors can be generated from the selenocysteine tRNA gene after its injection into Xenopus oocytes. Isolation of the primary transcript from Xenopus oocytes and its reinjection results in formation of both isoacceptors. We are presently investigating the pathway of synthesis of these isoacceptors as to whether one is a precursor of the other or both arise from a common precursor. The relative abundance of the selenocysteine tRNA population and the distributions of NCA and CmCA are influenced by the presence of selenium in mammalian cells grown in culture and in rat tissues taken from rats on diets with and without selenium, suggesting that tRNA editing occurs and the editing process responds to selenium. A selenocysteine tRNA that decodes UGA has been identified in animals, plants and protists in this laboratory (and in bacteria by other investigators) and thus, the universal genetic code has been expanded to include selenocysteine as the 21st naturally occurring amino acid.