SELENOCYSTEINE, THE 21ST AMINO ACID IN THE GENETIC CODE

两个硒代半胱氨酸tRNA异受体，其功能是读取 终止密码子，UGA，在蛋白质合成（从而捐赠 硒代半胱氨酸的生长多肽链，以响应特定的 UGA密码子）并作为生物合成的载体分子 硒代半胱氨酸已在哺乳动物细胞中被鉴定。 的途径 硒代半胱氨酸生物合成由tRNA[Ser]Sec -> seryl- tRNA[Ser]Sec ->磷酸丝氨酰-tRNA [Ser]Sec ->硒代半胱氨酸-tRNA [Ser]- 秒 这两种同种受体均来自大鼠肝脏， 在反密码子的摆动位置上通过单个碱基彼此连接。 一 同种受体，根据其反密码子序列命名为CmCA， 含有0-甲基胞苷，另一种称为NCA，含有 在这个位置的未鉴定的（推测为修饰的尿苷）碱基。 由于这两种tRNA都来自单拷贝基因（其中基因含有一个 在摆动位置的胸苷），异受体之一必须是 物种编辑 tRNA的合成途径已经被 在显微注射基因后在非洲爪蟾卵母细胞中重构，或 RNA（由克隆到表达载体中的人类基因产生） 向量）。 该途径从tRNA[Ser]Sec基因开始->加工 初级转录本-> NCA -> CmCA。 在哺乳动物细胞中观察到， 在特定的大鼠组织中， 卵母细胞对硒有反应。 一级结构的变化 每个生物合成步骤目前的特征在于， 聚合酶链反应，指纹图谱和小碱基分析。 已经在动物中鉴定了解码UGA的硒代半胱氨酸tRNA， 植物、真菌和两种截然不同的原生生物;通用遗传密码 因此，已扩大到包括硒代半胱氨酸作为第21届 天然氨基酸。

Two selenocysteine tRNA isoacceptors which function 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 to serve as carrier molecules for the biosynthesis of selenocysteine have been identified in mammalian cells. The pathway for selenocysteine biosynthesis proceeds from tRNA[Ser]Sec ---> seryl- tRNA[Ser]Sec ---> phosphoseryl-tRNA[Ser]Sec ---> selenocysteyl-tRNA[Ser]- Sec. Both isoacceptors were sequenced from rat liver and differ from each other by a single base in the wobble position of the anticodon. One isoacceptor, designated CmCA on the basis of its anticodon sequence, contains 0-methylcytidine and the other, designated NCA, contains an unidentified (presumably a modified uridine) base in this position. Since both tRNAs arise from a single copy gene (where the gene contains a thymidine at the wobble position), one of the isoacceptors must be an edited species. The pathway of synthesis of the tRNAs has been reconstituted in Xenopus oocytes following microinjection of the gene or of the RNA (generated from the human gene cloned into an expression vector). The pathway proceeds from the tRNA[Ser]Sec gene ---> processed primary transcript ---> NCA ---> CmCA. As observed in mammalian cells in culture and in specific rat tissues, the tRNA[Ser]Sec population in oocytes is responsive to selenium. The changes in primary structure at each biosynthetic step are presently being characterized by the process of polymerase chain reaction, and by fingerprint and minor base analysis. A selenocysteine tRNA that decodes UGA has been identified in animals, plants, fungi and two very divergent protists; the universal genetic code has therefore been expanded to include selenocysteine as the 21st naturally occurring amino acid.